Basics of Spaceflight by Dr. Regina E. Schulte-Ladbeck

BASICS OF SPACEFLIGHT FOR SPACE EXPLORATION, SPACE COMMERCIALIZATION, AND SPACE COLONIZATION

D R . R E G I N A E . S C H U LT E - L A D B E C K

First edition, January 2016 2015 by Dr. Regina E. Schulte-Ladbeck

This book was written and edited by Dr. Regina E. Schulte-Ladbeck. It contains the authorâ&#x20AC;&#x2122;s original writing and photography as well as material from the internet curated by the author. Materials adopted from the internet were edited by the author for uniformity. Explicit links to source materials used are provided in the Credits and Licensing section. Text from internet sources was homogenized to the readersâ&#x20AC;&#x2122; assumed prior level of scientific knowledge and mathematical skill, considered to be high school. This book is published by RESLscience under the Creative Commons-Attribution-NonCommericalShareAlike 4.0 International license. For details, please visit http://creativecommons.org. You are free to remix, tweak, and build upon this book non-commercially, as long as you credit the author and license your new creations under the identical terms. Before reusing material from this book, please, also review the licenses that apply to the curated material and are given in the Credits and Licensing section on page 221.

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Front cover image: Star trails photographed from the International Space Station, photo by Dr. Donald Roy Pettit/NASA JSC

CONTENTS

About This Book.....................................................................iii Acknowledgements.................................................................iv

Chapter 1

Introducing Spaceflight....................................1

Part I: Astronomy Chapter 2

Outer Space.....................................................17

Chapter 3

The Solar System............................................39

Chapter 4

Space Navigation............................................69

Part II: Astrodynamics Chapter 5

Rocket Physics................................................95

Chapter 6

Lift Off!..........................................................113

Chapter 7

Spacecraft Trajectories.................................141

Part III: Applications Chapter 8

Space Exploration.........................................157

Chapter 9

Space Commercialization.............................181

Chapter 10

Space Colonization.......................................203

Credits and Licensing..........................................................221 Section Index.......................................................................237 About The Author................................................................246

ABOUT THIS BOOK Spaceflight is exciting again! Robotic space probes have recently explored the dwarf planets Ceres and Pluto; and a spacecraft has even landed on a comet. Commercial aerospace companies have successfully launched rockets to space and delivered cargo to the International Space Station. In the United States, new laws are being passed concerning the property rights on the Moon, and in anticipation of asteroid mining. In light of this surge in spaceflight activities, a basic appreciation of spaceflight issues would seem valuable to anyone wishing to better participate in discussions about this rapidly evolving scientific and technological field. This book is a college-level undergraduate textbook about spaceflight targeted at liberal arts students majoring in the humanities and the social sciences. It does not use calculus. The book would also be appropriate for readers with prior high-school knowledge of science and mathematics who have an interest in becoming more informed about spaceflight concepts and topics related to spaceflight. Following the introduction, the book has three parts. Part I: Astronomy, is an overview of the history of the Universe, and the different objects in it, and describes where they are located. Part II: Astrodynamics, explains how just four laws of physics, Newtonâ&#x20AC;&#x2122;s three laws of motion and his universal law of gravity, suffice to obtain an appreciation of how rockets lift off from Earth and travel through space. Part III: Applications, combines the scientific understanding of spaceflight gained in the previous two parts with select issues in space exploration, commercialization, and colonization. Part III reflects the state of the art in 2015; many rapid changes are expected to occur in the near future. Each chapter is accompanied with end-of-chapter review questions and problems. In parts I and II, the problems require a mathematical solution. In part III, the assignments are research-based, and require an essay. This book incorporates a wealth of materials from the internet: text, as well as images and graphs. The author edited the source material for uniformity. The Credits and Licensing section at the end of the book provides detailed references to text and imaging sources used in this book. This detailed list of sources gives readers the opportunity to follow the links for further study. The links to the Figures are also a useful resource for instructors looking for illustrations of spaceflight concepts. To honor the requests of the volunteers who contributed to the web content that was used in the creation of this book, and to allow students of spaceflight access to educational material free of charge, this book has been published online as a free eBook. Written in the United States, this book gives preference to American spaceflight issues.

iii

ACKNOWLEDGEMENTS

Thank you to • the Wikimedia Foundation for operating some of the largest collaboratively edited reference projects in the world, including Wikipedia; • the volunteers who are contributing content to Wikipedia; • NASA and its centers for their web presences and outreach materials; • the NASA employees who are developing materials for NASA’s public websites; • NASA’s Jet Propulsion Laboratory and its employees, especially principal author Dave Doody, for their “Basics of Spaceflight Learners’ Workbook,” which was the inspiration for this textbook; • the University of Pittsburgh for allowing me to offer a course in spaceflight and for granting me sabbatical leave to work on this book; • Pitt colleagues, staff and students for assistance and feedback on draft chapters; • my colleagues at George Mason University for hosting me during my sabbatical visit and for providing me with feedback on portions of this book; • Prof. Dr. Jennifer Birriel of Morehead State University for consistently giving me feedback during the writing of this book; • my husband, John B. Hughes, for his unwavering support.

Early morning view on 9 November 1967 of Pad A, Launch Complex 39, Kennedy Space Center, showing the Apollo 4 Saturn V rocket prior to launch later that day. This was the first launch of the Saturn V. (NASA via Wikipedia) v

Astronauts on an extravehicular activity are continuing the construction of the International Space Station (NASA)

INTRODUCING SPACEFLIGHT Spaceflight uses rockets to traverse tremendous distances. What is spaceflight? And why is spaceflight considered a useful human endeavor?

1.1 What Is Spaceflight? Spaceflight is flight beyond Earth’s atmosphere into and through Outer Space. Spaceflight gives us the power to leave Earth, the planet we call home, and to explore our Solar System. Maybe, someday, we will even reach the stars.

1.1.1 Reaching Space Rockets are the only practical means to leave Earth, at least for now. The first rocket to reach space was a German V-2 rocket which was launched from Germany on 3 October 1942. It was developed during the Second World War under the technical leadership of Dr. Wernher von Braun (1912–1977). The V-2 rocket was designed to carry a warhead as its payload, making it the world’s first ballistic missile. Figure 1-2. A V-2 rocket is readied on the launch pad in Peenemünde, Germany, March 1942. The V-2 was designed to attack Allied cities as a form of retaliation for the ever-increasing Allied bomber effort against German cities, and was the first long range ballistic missile to be actively used in combat. It carried an explosive warhead that was capable of flattening a city block. Beginning in September 1944, over 3,000 V-2s were launched by the German military against Allied targets during the war, mostly London and later Antwerp and Liège. (Bundesarchiv via Wikipedia) A rocket used to launch a payload into space is called the “launch vehicle” or “carrier rocket.” At the tip of the rocket is its nose cone, where the rocket’s payload is located. The “payload” is the object that the rocket is carrying into space. Once space is reached, the payload separates from the rest of the carrier rocket and performs its mission in space. Depending on the purpose of the space mission, the payload could be a warhead, a scientific or commercial satellite, or a space vehicle transporting a human crew.

Figure 1-1. Dr. Wernher von Braun during the Apollo 11 mission, decades after he was taken to the United States near the end of World War II as part of the then-secret Operation Paperclip. He was one of the leading figures in the development of rocket technology in Germany during World War II and, subsequently, in the United States. Von Braun became the foremost rocket developer and one of the most prominent spokesmen of space exploration in the US during the 1950s. He is credited as being one of the “Fathers of Rocket Science.” (NASA/KSC)

1.1.2 Types Of Spaceflight Unmanned spaceflight is spaceflight without a necessary human presence in space. Manned spaceflight is spaceflight with humans. Manned

Figure 1-3. A schematic of the V-2 rocket. The rocket body is the structural frame of the rocket, similar to the fuselage on an airplane. The nose cone contains the payload, in the case of the V-2, a warhead. The guidance system maneuvers the rocket in flight. That of the V-2 included two gyroscopes for lateral stabilization, an accelerometer to control engine cutoff at a specified velocity, and a simple analog computer that adjusted the azimuth for the rocket. Most of the body was taken up by the propulsion system. The V-2 used a liquid rocket engine consisting of fuel and oxidizer tanks, pumps, a combustion chamber with nozzle, and the associated plumbing. (Wikipedia)

spaceflight is nowadays also called “crewed spaceflight,” or “human spaceflight.” Unmanned space missions have flown fully autonomously such that the spacecraft received no instructions sent from Earth. Instead, commands were loaded into an on-board computer before launch and executed by the computer throughout the space mission. Autonomy is important for distant space probes where the signal travel time prevents rapid decisions and control from Earth. The Mars Exploration Rovers, for example, are highly autonomous and use on-board computers to operate independently for extended periods of time. In addition to being fully autonomous, some spacecraft are controlled remotely. Telecommunications is the exchange of information at a distance by technological means. In spaceflight, we use wireless communications through electromagnetic waves. Radio waves are the most common electromagnetic waves we employ, although sending and receiving information at other wavelengths is also possible. Radio antennae installed at 3

Cold War rivals, the USSR and the US, for supremacy in spaceflight capability. The Space Race had its origins in the missile-based arms race that occurred following World War II, when forces from both the US and the USSR captured advanced German rocket technology and personnel. It was a time when spaceflight became closely intertwined with political agendas. The technological superiority required for spaceflight supremacy was seen as necessary for national security, and symbolic of ideological superiority. The Space Race spawned pioneering efforts to launch satellites, then unmanned probes to the Moon, Venus, and Mars, and human space missions into low Earth orbit (LEO) and to the Moon. The Soviet Union beat the US to launching the first satellite. Sputnik 1 was launched 4 October 1957 from the former Soviet Union. It was the size of a basketball and orbited the Earth once in 98 minutes. The launch of Sputnik 1 surprised the American public and shattered the perception of the US as the technological superpower and the Soviet Union as a backward country. The Sputnik carrier rocket was designed by Sergei Pavlovich

Figure 1-4. Sputnik 1 was the first artificial satellite successfully placed in orbit around the Earth. (NASA/ GSFC) ground stations on Earth, such as the Deep Space Network, send telecommands to spacecraft. The ground station receives back telemetry from the space probe, which includes data about the spacecraft itself, such as its position, and also measurements of the celestial body the probe was sent to study. The spacecraft in turn is equipped with radio antennae as well. By pointing them toward Earth, information can be sent back and instructions received. Space missions with animals but no humans aboard are also considered unmanned missions. Animals in space served to test the survivability of spaceflight before human space missions were attempted. The first animals sent into space were fruit flies aboard a US-launched German V-2 rocket on 20 February 1947. Spacefaring animals have included mice, dogs, and monkeys. Crewed spaceflight allows human control of a space ship. A pilot on board can maneuver the space vehicle when it is not controlled remotely or by computer. A person on a spaceflight is called an “astronaut” in the United States (US) and other English-speaking countries. In countries belonging to the former Union of Soviet Socialist Republics (USSR or Soviet Union), including Russia, such a person is referred to as a “cosmonaut.”

1.1.3 The Space Race

Figure 1-5. The famous hand shake welcome. Chimpanzee Ham is greeted by the recovery ship Commander after his flight on the Mercury Redstone rocket. (NASA/HQ)

Spaceflight had a glorious run in its heyday during the Space Race. The Space Race was a 20thcentury (1955–1975) competition between two 4

Figure 1-7. Valentina Tereshkova, the first woman in space (Wikipedia)

Figure 1-6. Yuri Gagarin, the first man in space (Wikipedia)

1969. The first man to set foot on the Moon was Neil Alden Armstrong (1930–2012). As he stepped onto the surface of the Moon, he said his famous words,

Korolev (1907–1966). Before his death he was often referred to only as “Chief Designer,” because his name and his pivotal role in the Soviet space program had been held to be a state secret. The Soviet Union also received credit for sending the first human into space. On 12 April 1961, Yuri Alekseyevich Gagarin (1934–1968) made one orbit around the Earth aboard the Vostok 1 spacecraft. He was 27 years old, and a Senior Lieutenant in the Soviet Air Force. To this day, Russia and some other former USSR countries celebrate Cosmonautics Day on 12 April. In 2011 the United Nations (UN) General Assembly adopted a resolution declaring 12 April as the International Day of Human Space Flight. In another first for the Soviet space program, Valentina Vladimirovna Tereschkova (born 1937) became the first woman in space aboard Vostok 6 on 16 June 1963. “Landing a man on the Moon and returning him safely to the Earth” was defined in 1961 as the ultimate goal of the Space Race by US President John F. Kennedy. And the US scored the big win here. The first spaceflight to put humans on the surface of another celestial body happened only eight years after the first human had ever entered space. It was a flight from the US Apollo program. Apollo 11 landed on the Moon on 20 July

“That’s one small step for [a] man, one giant leap for mankind.” More men, all Americans, walked on the Moon between 1969 and 1972, but no woman. The Space Race is considered to have ended with the first multinational crewed space mission, the Apollo-Soyuz Test Project, in 1975.

1.1.4 Spaceflight In The 21st Century By now we have reached all of the planets of our Solar System with robotic space probes, and a few space probes are even on their way to leave the Solar System. This is a tremendous scientific and engineering accomplishment. However, many people who have lived through the Cold War and the Space Race believed that human spaceflight to planetary destinations would become a reality within their lifetimes. First the Moon, then Mars! And one day, maybe even the stars. After all, there is an entire literary (sub-) genre built around crewed space-

Figure 1-8. The crew of the Apollo 11 mission. Left to right are Neil Armstrong, Michael Collins, and Buzz Aldrin. Armstrong is the first man to have walked on the Moon. (NASA via Wikipedia) Government-sponsored spaceflight has flight. It is aptly called “science fiction,” because been the only avenue for space exploration, and science fact is that there is no interplanetary flight for space travel involving transporting humans bewith humans as yet. The Moon is the farthest yond LEO. Today, spaceflight is still largely fiplace from Earth any human has been; no human nanced by governments. But there are starting to has set foot on any other Solar System body. be some changes. After the Space Race, the emphasis on huDennis Anthony Tito (born 1940) has beman spaceflight decreased and so did public fundcome widely known as the first person to have ing of human spaceflight. Human spaceflight in paid for a trip into space. He is an the post-Apollo era has been American engineer and multimiltaking place very close to lionaire. He visited the ISS on 28 Earth, a mere few hundred April 2001, after having been miles up in LEO. The US launched on a Soviet Soyuz rocket. Space Shuttles, now retired, He stayed on the ISS for nearly took astronauts to LEO, but eight days. Tito paid a reported no further. The International $20 million US for his trip. He Space Station (ISS), also in was a client of Space Adventures LEO, is humanity’s continued Figure 1-9. Armstrong on the Ltd., a US-based space tourism presence in Outer Space. A huMoon company. man mission to Mars, alIn 2013, Tito founded the Inspiration Mars Foundation, with the aim to conduct a crewed Mars flyby. Two other non-profit organizations aiming for Mars are Mars One and the Mars Initiative.

though much debated in the (NASA/HQ) US, is unlikely to occur before the mid-2030s. And as of 2015, Russia and the People’s Republic of China are the only two nations actively engaging in crewed spaceflight. 6

Spaceflight has also enabled us to study the planets directly with space probes that have traversed our Solar System, sent back close-up images and measurements, and in some cases, have even analyzed soil samples. By now, a fleet of robotic space probes has visited all of the planets in our Solar System. Missions to other locations, such as our dwarf planets, are under way. Read about some of the techniques of space exploration in chapter 8.

Spaceflight in the 21st century is in transition from a government-sponsored operation, to a private enterprise. The jury is still out on whether corporate spaceflight will yield the technologies necessary for easy access to Outer Space or for lunar and planetary transfers with humans.

1.2 Why Spaceflight? Advocates of spaceflight often get asked the question, “Why should we spend money on spaceflight while there are so many problems to be solved here on Earth? ”Here are some arguments pro spaceflight. Uses for spaceflight fall into three broad categories: • scientific, • commercial, and • political. Lets’ briefly consider each one.

1.2.2 Commercial Uses Of Spaceflight The first commercial satellite was launched in 1962. Telstar 1 was a communications satellite that belonged to the American company AT&T. It started a pivotal change in communications across the Atlantic Ocean. Transatlantic phone calls, for example, used to rely on underwater cables before the advent of communications satellites. Telstar 1 was launched on a NASA carrier rocket; it was the first privately sponsored space launch.

1.2.1 Scientific Uses Of Spaceflight

Spaceflight has allowed us to study the Universe with telescopes in space. In 1966, the National Air and Space Administration (NASA) launched the first Orbiting Astronomical Observatory (OAO) mission. There have been many small and large space telescopes since. Hubble Space Telescope (HST), launched in 1990, is a well known satellite for astronomy research. Space telescopes are used to study the kinds of electromagnetic radiation impeded or blocked by Earth’s atmosphere. Other space telescopes have collected particles, such as cosmic ray particles, or will try to detect Figure 1-10. The Hubble Space Telescope, seen from space, apparently gravitational waves. floats against the background of Earth. (NASA/HST) 7

was to use ground-based and space-based systems to protect the US from attack by strategic nuclear ballistic missiles. SDI was also known as “Star Wars,” after the popular 1977 film by George Lucas. Apart from the global threats posed by warring nations with nuclear capability, there are environmental threats which concern us all. Global warming, created by the industrialized nations, may one day make our planet inhospitable. Other global threats originate not on Earth, but in Outer Space. An example is that of cosmic collisions. The most famous incident has been the massive comet/asteroid impact 65 million years ago which led to the mass extinction of some three-quarters of the plant and animal species on Earth—including all non-avian dinosaurs. The “overview effect” is a cognitive shift in political awareness reported by some astronauts and cosmonauts during spaceflight, often while viewing the Earth from orbit or from the lunar sur-

Figure 1-11. Telstar was the world’s first commercial payload in space. The Adidas Telstar soccer ball was designed to look like it, and has become the stereotypical look for a soccer ball. (Wikipedia) Aside from communications satellites, there are other lucrative launch markets, such as satellite television. These are purely commercial, although the carrier rockets have often at least partly been funded by governments. There is a growing interest in launch vehicles paid for by commercial companies. If commercial companies could work more efficiently than governments, launch costs could come down. Space launch vehicles such as SpaceX’s Falcon family of rockets have been wholly developed with private finance, and the quoted costs for launches are decreasing. Commercial spaceflight is presented in more detail in chapter 9.

1.2.3 Political Uses Of Spaceflight There are both national and geopolitical motivations prompting spaceflight. Space warfare is combat that takes place in Outer Space. Space warfare includes ground-to-space warfare, such as attacking satellites from the Earth, as well as spaceto-space warfare, such as satellites attacking satellites. Deployment of these systems was seriously considered in the mid-1980s under the banner of the Strategic Defense Initiative (SDI). SDI was proposed by US President Ronald Reagan. Its goal

Figure 1-12. A US Peacekeeper intercontinental ballistic missile is test launched from its underground silo launch facility. (US Air Force via Wikipedia) 8

Figure 1-13. Earthrise is the name given to a photograph of the Earth that was taken by astronaut William Anders on 24 December 1968 during the Apollo 8 mission. Nature photographer Galen Rowell declared it “the most influential environmental photograph ever taken.” Apollo 8’s crew was the first to travel to the Moon. (NASA/HQ)

mankind will have to leave the Solar System to ensure the continuation of the human species. Viewing our Earth as a whole suggests the need for collaborative planetary politics. A global human spaceflight enterprise aimed at colonizing space could provide us with a life insurance policy for the survival of the human species. More on space colonization in chapter 10.

face. The term and concept were coined in 1987 by Frank White, who explored them in his book “The Overview Effect — Space Exploration and Human Evolution.” It refers to the experience of seeing firsthand the reality of the Earth in space, which is immediately understood to be a tiny, fragile ball of life, “hanging in the void,” shielded and nourished by a paper-thin atmosphere. From space, astronauts claim, national boundaries vanish, the conflicts that divide people become less important, and the need to create a planetary society with the united will to protect this “pale blue dot” becomes both obvious and imperative. The threats to life on Earth due to natural disasters, or human-made ones, make leaving Earth not just a dream, but a necessity. At the latest, when our Sun will become a red giant star and engulf the Earth some 5 billion years from now,

1.3 Rockets for Spaceflight Spaceflight relies on rockets for reaching space. Who made the first rocket? We think that the rocket is a Chinese invention, although the principle behind what makes it work was also known to the ancient Greeks. The Chinese had discovered gunpowder sometime in the 3rd century and used it to make firecrackers. In 970, a man called Feng Jishen is said to have attached a hollow bamboo tube 9

stuffed with gunpowder to an arrow, creating a flying fire arrow. Gunpowder, also known as black powder, is a chemical explosive—the earliest known. It is a mixture of sulfur, charcoal, and potassium nitrate (saltpeter). The sulfur and charcoal act as fuels and the saltpeter as an oxidizer. In rocketry, fuel and oxidizer together are also referred to as “propellant.” The gunpowder-filled tube of the fire arrow is a primitive solidpropellant rocket booster. Thus, the rocket was invented well before the relevant chemistry and physics principles were spelled out by scientists.

When a rocket flies with its engines on, we call this “powered flight.” On the other hand, a spacecraft that flies with its engines off is in “free flight.” Most spaceflight happens to be free flight. Here is why: once moving, an object in space will continue to move according to Newton’s law of inertia. This does sound counterintuitive. But imagine motion in the absence of friction, as Newton did. A car on Earth would continue to move indefinitely once you cut its engines. It’s the friction between the tires and the road which slows it down. No friction, and the car, once set in motion, would continue to move in a straight line and at a constant speed. Outer Space is the closest natural approximation to a perfect vacuum. It has effectively no friction, allowing stars, planets, moons and spacecraft to move freely according to their inertia. Using free flight helps us save propellant. Propellent mass, while a necessity, is also a big impediment to the acceleration of rockets. Once in space, the motion of a spacecraft is still affected by gravity. We even use gravity, for example, by employing the gravitational slingshot technique, to send spacecraft to the farthest places of our Solar System (see chapter 7). We call “astrodynamics” the field of study that applies Newton’s laws (cf. chapter 5) to man-made objects in space.

Figure 1-14. A Chinese soldier lights a fire arrow. (NASA/MSFC)

1.3.1 The Rocket Principle A rocket works by the principle of action and reaction. According to Sir Issac Newton’s third law of motion, for every action, there is an equal and opposite reaction. The combustion of the fuel with the oxidizer in the rocket engine creates a hot gas under pressure. The hot gas is allowed to shoot out downward from nozzles at the bottom of the rocket. This is the action. In reaction, the rocket propels upward. A simple everyday demonstration of the rocket principle is a party balloon. Try it for yourself! When you blow up the balloon, filling it with air, you create a pressure force on the inside wall of the balloon. When you let go of the balloon with-

Spaceflight typically begins with a rocket launch, which provides the initial thrust to overcome the force of gravity, and propels the spacecraft upward from the surface of the Earth (see chapter 6). Sir Isaac Newton (1643–1727) discovered that gravity is an attractive force which acts between masses. Because the mass, or amount of material, in Earth is very large, the attractive force between it and any object on its surface is very strong. The force of gravity gives rise to a rocket’s weight. In rocket motion, the force that propels the rocket upward (or forward) is termed the “thrust.” In order to launch a rocket, the thrust of its engines must overcome its weight.

moving. For example, 60 miles per hour is a speed, whereas 60 miles per hour due east is a velocity. As a convention, we always abbreviate velocity with the letter v. Also, for distance, we use the letter d, and for time, t. The relationship between the three quantities can then be expressed mathematically as

Figure 1-15. A party balloon illustrates the rocket principle. (wclipart.com) out tying off its neck, the air escapes from the balloon. The balloon flies away from you. Pressure is force divided by the area it acts on. At the moment when you let go of the balloon, the forces on its inside, which were equal as long as you pinched the neck shut, became unbalanced. At the opening, the air now no longer pushes on the wall, and escapes. Allowing the air to escape in one direction, drives the balloon in the opposite direction.

d . t

This expression only holds true when the velocity is constant. Constant velocity requires constant speed in a straight line. The fastest possible speed at which energy can travel, according to special relativity, is the speed of light in a vacuum. We abbreviate the speed of light with the letter c. The speed of light in vacuum is 299,792,458 meters per second, or approximately 671,000,000 mph. Replacing v with c in the velocity-distancetime equation above results in

1.4 What’s the Biggest Obstacle for Spaceflight? The single biggest problem for spaceflight is that space is a very big place. Spaceflight requires travel over extreme distances at large velocities. The relatively small distances currently achieved in spaceflight pale in comparison to the distances between objects of interest in space. Traversing such tremendous expanses with the velocities provided by current rocket engines takes a long, long time. This is why our spacecraft have not ventured very far into Outer Space.

d , t

or also, by rearranging, t=

d , c

1.4.1 Velocity, Distance, And Time

d = c × t.

Here is a closer look at the relationship between velocity, distance, and time, and implications for spaceflight. Velocity is the rate of change of the position of an object. Velocity has an amount, or magnitude, say, 60 miles per hour, and a direction, say, to the east. Velocity is a “vector” quantity; both amount and direction are required to define it. The amount of velocity is separately called “speed.” Speed describes only how fast an object is moving, whereas velocity gives both how fast and in what direction the object is

All electromagnetic signals in space can be considered to travel at the speed of light in vacuum. When far away from large masses, they travel in a straight line. Here are some consequences of the universal speed limit of c. For us to see any object, we have to wait until the light it emits, or reflects, has had time to reach our eyes. In most everyday situations this is unimportant as we are always close to objects we can see. But take a celestial body that has an appre11

control and Apollo 8 when it became the first manned spacecraft to orbit the Moon: for every question, the ground control station had to wait nearly three seconds for the answer to arrive. We use the speed of light to define distances, such that the meter is defined as the length of the path travelled by light in vacuum during a time interval of 1/ 299,792,458 of a second. In spaceflight and in space science, we use light travel times and distances interchangeably. For example, 1 lightsecond is the distance Figure 1-16. The Goldstone Deep Space Communications Complex light travels in 1 second, that is, in the Mojave Desert in California is one of three complexes that 299,792,458 m. Within our Solar comprise NASA’s Deep Space Network (DSN). The DSN provides System, distances measure in lightradio communications for all of NASA’s interplanetary spacecraft minutes and lighthours. Considerand is also utilized for radio astronomy and radar observations ing the extremely large distances of the Solar System and the Universe. (NASA/JPL) between stars, another commonly ciable distance from us. For example, when we used length is the lightyear. One lightyear is the look at the Moon on a clear night, we see the light distance that light travels in one year. The lightthat has left its surface 1.28 seconds ago. The year is equal to just under 10 trillion kilometers, Moon is, on average, 384,400 km (238,900 mi) or about 6 trillion miles (more precisely, distant from Earth; and 1.28 s is, on average, how 9.461×1012 km or 5.879×1012 mi). The lightyear is long light travels until it reaches us. Owing to the abbreviated with the letters ly. For example, the large distances in Outer Space, anything we see nearest star system to our Solar System, the Alpha right now is not how the thing looks right now, beCentauri system, is located 4.37 ly from the Sun. cause light left it at some time in the past. Looking Proxima Centauri is the nearest component, at out into space is looking into the past. 4.22 ly from us. Similarly, for us to remain in touch with a The distance to Proxima Centauri illustrates robotic spacecraft far out in the Solar System, rawhy interstellar space travel will remain the realm dio signals must have sufficient time to travel beof science fiction for the foreseeable future. Even tween the radio antenna aboard the spacecraft if we could travel at the speed of light (which, acand a radio antenna on Earth (see chapter 4). cording to special relativity, objects having mass There is a delay from the source to the receiver, cannot do) a trip to Proxima Centauri would take which becomes more noticeable as distance inus 4.22 years. For interstellar travel to become a creases. The minimum delay for a signal from the reality, a combination of great speed (some perMars Exploration Rovers is around 4 minutes and centage of the speed of light) and huge travel time the maximum is around 24 minutes, depending (lasting from years to millennia) would be reon the locations of the planets in their respective quired. The speeds are far beyond what current orbits. Back-and-forth communication takes at methods of spacecraft propulsion can provide; least twice as long, if a response is sent immediand the resulting travel times exceed a human life ately. Take the communications between ground span. More on this in chapter 10. 12

1.4.2 SI Units, Prefixes, And Scientific Notation

Table 1.1. Common SI Prefixes

POWERS TEXT SYMBOL FACTOR By now you have noticed that disOF TEN tances, velocities, and times, are being Giga G 1,000,000,000 10 9 given in metric units as well as in imperial units. The International System of Units Mega M 1,000,000 10 6 (abbreviated SI from French: Le Système kilo k 1,000 10 3 International d’Unités) is the modern form (None) (None) 1 10 0 of the metric system and is the world’s most widely used system of measurement. centi c 0.01 10 -2 It is used in both everyday commerce and milli m 0.001 10 -3 science. International spaceflight employs the SI system as well. The US is the only inmicro µ 0.000 001 10 -6 dustrialized country where the metric sysnano n 0.000 000 001 10 -9 tem is not the predominant system of units. Let’s take a quick look at SI units, common SI prefixes, and scientific notation. The basic unit of time in the SI system is the second, designated by the unit symbol s. The Scientific notation is a way of writing numsecond is related to other time units as follows: bers of very large and very small sizes compactly. 1 minute (min) 1 hour (h) 1 day (d) 1 year (yr)

We use powers of ten to express such numbers. Basically, for large numbers, you just count the zeros that follow a number. A 1 followed by n zeros is written as

= 60 s = 3,600 s = 86,400 s = 3.156×107 s.

10n. The basic unit of length in the SI system is the meter. Its unit symbol is m. You already encountered the kilometer: 1 km

When n is positive, this indicates the number of zeros after the number, and when n is negative, this indicates the number of decimal places before the number. A special case is when n is 0; this is the number 1 itself. Here are a few examples:

= 1,000 m.

A meter is equivalent to about 1.0936 yards or 39.37 inches in the imperial system. A kilometer can be converted into miles and vice versa: 1 km 1 mi

1 km 1 nm

= 0.621371 mi = 1.60934 km.

= 1,000 m = 0.000000001 m

= 103 m = 10-9 m

300,000 km/s = 3×105 km/s. Scientific notation makes it easy to multiply and divide large numbers. Suppose, you have a ten with exponent n, and another with exponent m, then the rule for multiplying is

You have already seen SI prefixes for large quantities in this text. For example, 1,000 meters is the same as 1 kilometer (1 km). A few commonly used SI prefixes are given in Table 1.1. Consult the internet for more.

10 n × 10 m = 10 n+m.

Here is an example: 10 × 1,000 = 101 × 103 = 104. It is also easy to divide in this notation: 10 n = 10 n × 10−m = 10 n−m. 10 m So, for example: 10 101 = 3 = 101 × 10−3 = 101−3 = 10−2. 1000 10 We’ll be using a few other SI units in this text, in addition to the second and the meter. The SI unit of mass is the kilogram, abbreviated kg. In the US, the pound as a unit of mass has been officially defined to be 2.20462 pounds to a kilogram. The SI unit of temperature is the Kelvin, designated by the letter K. The Kelvin temperature scale is different from the Fahrenheit scale that is in use in the US. And its zero point is tied to the absolute zero point of temperature. Absolute zero, 0 K, is equivalent to −459.67 °F.

Figure 1-17. This is the “Pale Blue Dot” photograph of Earth taken by the Voyager 1 spacecraft on 6 July, 1990, 12 years after Voyager’s launch when it was traveling at 64,000 km/h (40,000 mph) at a distance of about 6 billion kilometers (3.7 billion miles). The Earth is the tiny speck of light about halfway across the uppermost sunbeam. (NASA via Wikipedia)

Figure 1-18. The cameras of Voyager 1 on 14 February, 1990, pointed back toward the Sun and took a series of pictures of the Sun and the planets, making the first ever portrait of our Solar System. Thirtynine wide angle frames link together six of the planets of our Solar System in this mosaic. Outermost Neptune is 30 times further from the Sun than Earth. Our Sun is seen as the bright object in the center of the circle of frames. (NASA/JPL)

Figure 1-19. The International Space Station (ISS) gives humans continued access to space at the start of the 21st century. This image of the ISS, flying at an altitude of approximately 220 miles, and the docked space shuttle Endeavour, was taken by Expedition 27 crew member Paolo Nespoli from the Soyuz TMA-20 following its undocking on 23 May 2011 (US time). The pictures taken by Nespoli are the first taken of a shuttle docked to the International Space Station from the perspective of a Russian Soyuz spacecraft. Notice also the large solar arrays which furnish the ISS with electrical power. Onboard the Soyuz were Russian cosmonaut and Expedition 27 commander Dmitry Kondratyev; Nespoli, a European Space Agency astronaut; and NASA astronaut Cady Coleman. Coleman and Nespoli were both flight engineers. The three landed in Kazakhstan later that day, completing 159 days in space. (NASA/P. Nespoli) 15

II. C ALCULATE T HE A NSWER .

1 Test Your Understanding

1. If a spacecraft could achieve a speed of 1% the speed of light, how long would it take it to journey to Proxima Centauri? Assume a distance of 4.22 ly for Proxima Centauri, and that the spacecraft is flying at constant speed in a straight line for the entire time.

I. A NSWER IN A FEW SENTENCES . 1. Which was the first rocket to reach Outer Space? Which year did this occur? 2. What is a carrier rocket? 3. What is a payload? 4. What do telecommunications, telecommand, and telemetry refer to? 5. Which was Earth’s first artificial satellite? Which year did this occur? 6. What was the Space Race? 7. What is the difference between an astronaut and a cosmonaut? 8. Who was the first human in space? Which year did this occur? 9. Who was the first human to step onto the surface of the Moon? Which year did this occur? 10.What are some of the uses of spaceflight? 11. How is space warfare defined? 12.What is the overview effect? 13.What is an example of a cosmic collision? 14.What is meant by planetary defense? 15.What is black powder? 16.Why is gravity an important force in spaceflight? 17.What is thrust? 18.What is the difference between powered flight and free flight? 19.What is meant by astrodynamics? 20.What is the rocket principle? 21.What is the difference between a velocity and and a speed? 22.What is a vector? 23.How are distance, velocity, and time related? 24.What is a lightyear? 25.What are the basic SI units for length, time, mass and temperature?

2. When the distance from the Earth to the Sun is exactly 1.496 × 1011 m, how long does it take for sunlight to travel to Earth? Convert your answer to minutes. 3. Write the following numbers in scientific notation: 500,000, 0.02, 12000000, 0.0000009. 4. A spacecraft travels 3 × 1013 km in 3 × 108 s. What is the speed of the spacecraft? Convert your answer to miles per hour. 5. A rocket was in powered flight for 8 minutes at a constant speed of 2 km/s. How far did the rocket travel? 6. When Voyager 1 took the “Pale Blue Dot” photo shown in Figure 1-14, it was traveling at the incredibly high speed of 40,000 mph. At what fraction of the speed of light did Voyager 1 travel? Express your answer in scientific notation. 7. NASA scientists shoot a radar beam at a nearEarth asteroid. The reflection arrives 6 s after the signal is sent. How far away is the asteroid? Use 300,000 km/s for the speed of light.

The layers of Earthâ&#x20AC;&#x2122;s atmosphere, and the Moon, as seen from the International Space Station (NASA/JSC)

OUTER SPACE Outer Space is vast, yet at the same time, it is extremely close to us. What do we know about where space begins, where it ends, and whatâ&#x20AC;&#x2122;s in it? How are different regions of space distinguished?

Figure 2-1. The Kármán line relative to Earth’s atmospheric layers. The portion of the atmosphere from a height of about 50 km (31 mi) to 1,000 km (620 mi) is ionized and contains a plasma which is referred to as the ionosphere. (NOAA/ Wikipedia)

2.1 Where Does Outer Space Begin? Space is defined to begin at the Kármán line. The Kármán line is at an altitude of 100 km (62 mi) above the Earth’s sea level.

2.1.1 The Kármán Line The demarcation of Outer Space is named after Dr. Theodore von Kármán (1881–1963), a Hungarian-American mathematician, aerospace engineer and physicist who was active in the fields of aeronautics and astronautics. He calculated that around an altitude of 100 km, Earth’s atmosphere becomes too thin to support aeronautical flight. Earth’s atmosphere contains about 1025 molecules per cubic meter. In contrast, Outer Space contains only a few atoms per cubic meter making it a vacuum better than any we can produce in a lab on Earth. There isn’t really an abrupt edge to Outer Space, as the word “line” in the term Kármán line might suggest. Rather, the density of our atmosphere gradually decreases with altitude. But there is a transition relevant to how airplanes fly at around 100 km in altitude. To achieve flight, airplanes utilize the atmosphere in two ways. First, airplanes use lift to offset weight. The lift force on an airplane is generated by air rushing past the wings. Without air, the wings cannot create lift. Science fiction movies often depict winged spacecraft which are whooshing through Outer Space but that wouldn’t actually work. Where there is no air, wings are meaningless. Second, airplanes use a combustion engine for propulsion just like rockets. But unlike rockets, airplanes carry on board only the fuel and not the oxidizer required for combustion. An airplanes’ jet engines rely on taking in oxygen from the outside air. Therefore, they are also called “air-breathing engines.” Without air, there is no oxygen to sustain combustion. The Kármán line is accepted by the Fédération Aéronautique Internationale (FAI), which is an international standard-setting and record-keeping body for aeronautics and astronautics. According to the FAI, a person can call themselves an astronaut when they have traveled to an altitude of 100 km. A different standard is used in the US, where 80 km (50 mi) suffices to gain astronaut wings. Following the FAI’s rule book, as of 8 June 2013, a total of 532 people from 36 countries have reached 100 km or more, of which 529 entered LEO. Of these, 24 people have traveled beyond LEO, to either lunar orbit or to the surface of the Moon. 18

Rules and agreements are bound to give rise to legal issues. The status of international agreements relating to activities in Outer Space is compiled and distributed every year by the United Nations Office for Outer Space Affairs. These agreements are also informally referred to as space law. Since reviewing space law would get us a bit off topic, it is set aside in section 2.1.2 below. Feel free to read about it now, or to refer to it later.

Figure 2-2. Dr. Theodore von Kármán migrated to the US between World Wars I and II, and assumed the directorship of the Guggenheim Aeronautical Laboratory at the California Institute of Technology. He co-founded the Jet Propulsion Laboratory. (NASA/JPL)

2.1.2 An Aside Concerning Legal Issues At The Kármán Line Where Outer Space begins is an important legal question. There are two legal areas with relevance for objects transiting above Earth’s surface, air law, and space law. Because of the international nature of air travel, countries have entered into conventions to standardize the laws regulating airlines and air travel. International Aviation Law also covers the rights of passengers. There is as yet no International Space Law. It would be desirable to agree upon where Outer Space is and to provide for regulations of objects that fly through the Outer Space above different countries. International lawyers have been unable to agree on a uniform definition of Outer Space, although most agree to the approximately 100 km set forth by the Kármán line. Beginning in 1957, nations began discussing systems to ensure the peaceful use of Outer Space. Bilateral discussions between the US and USSR in 1958 resulted in the presentation of spaceflight issues to the United Nations (UN) for debate. In 1959, the UN created the Committee on the Peaceful Uses of Outer Space (COPUOS). COPUOS is the only international forum for the development of International Space Law. Since its inception, COPUOS has achieved five sets of legal principles governing space-related activities: • The 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (the “Outer Space Treaty”). • The 1968 Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space (the “Rescue Agreement”). The 1972 Convention on International Liability for Damage Caused by Space Objects (the “Liabil• ity Convention”). • The 1975 Convention on Registration of Objects Launched into Outer Space (the “Registration Convention”). • The 1979 Agreement Governing the Activities of States on the Moon and Other Celestial Bodies (the “Moon Treaty”).

2.2 Where Does Outer Space End? The short answer to that is, we don’t really know where the Universe ends. It may, in fact, be infinite. The long answer is more complicated. We do know the size of the “observable Universe.” The edge of the observable Universe visible from Earth is about 46 billion lightyears away from us in all directions. This answer comes from the rapidly evolving scientific field of cosmology. “Cosmology” is the study of the origin, evolution, and eventual fate of the Universe as a whole.

Figure 2-3. Hubble discovered that the redshifts of galaxies are generally larger the further away they are from us. This is now also called “Hubble’s Law.” In this figure, redshift is given in km/s and the distance is given in millions of parsecs. The parsec is a distance unit used for very large distances in space, and amounts to about 3.26 lightyears. The line drawn through the data points is Hubble’s Law. In the middle of a cluster of galaxies, such as the Virgo Cluster, galaxies have additional motions about one another, in addition to participating in the expansion of space. This causes the deviations of the data points from the straight line. (Wikipedia)

2.2.1 Hubble’s Law Outer Space is changing. It is becoming bigger. The expansion of the Universe can be traced by observing the motions of galaxies. Galaxies are massive, gravitationally bound star cities consisting of billions of stars and large amounts of dark matter. You will see several pictures of galaxies throughout this chapter. The American astronomer Dr. Edwin Powell Hubble (1889–1953) first showed that all galaxies move away from us. And not only that, they move away faster the more distant they are. Astronomers now call this relationship “Hubble’s Law” and named the Hubble Space Telescope (HST) in his honor. Hubble’s recession of the galaxies is interpreted as an expansion of space, which carries the galaxies with it.

tric and magnetic fields that propagate at the speed of light through a vacuum. In the quantum theory of electromagnetism, electromagnetic radiation consists of photons, the elementary particles responsible for all electromagnetic interactions. Electromagnetic waves can be characterized by either the wavelength or frequency of their oscillations to form the electromagnetic spectrum. The wavelength, abbreviated λ, is the distance from one peak of the wave to the next, and is given in meters. The frequency, ν, tells you how many peaks of a wave pass you every second. It is given in per second, and this unit has been named a Hertz (Hz). In order of decreasing wavelength (or increasing frequency) the electromagnetic spectrum includes the following types of radiation: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. There is an inverse relationship between the energy, E, that electromagnetic radiation car-

2.2.2 Electromagnetic Radiation Primer To appreciate what Hubble discovered, it is important to understand the nature of light, or more generally, electromagnetic radiation. Electromagnetic radiation is radiant energy released by certain electromagnetic processes. Visible light is one type of electromagnetic radiation, other familiar forms are invisible electromagnetic radiations such as radio waves, infrared light and X-rays. Electromagnetic radiation can be viewed as a wave or as a particle in nature. In one view, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of elec20

Figure 2-4. The electromagnetic spectrum contains radiation ranging from long wavelengths, such as radio, to short wavelengths, such as gamma rays. Long wavelengths correspond to shorter frequencies. Radio, some infrared, and visible radiation penetrate Earth’s atmosphere, the other radiation types are blocked. Opaque objects emit most strongly at wavelengths that correspond to their temperature. (Wikipedia) Outer Space with the use of space telescopes (cf. chapter 8). All normal, atomic matter emits electromagnetic radiation when it has a temperature above absolute zero. The radiation represents a conversion of a body’s thermal energy into electromagnetic energy, and is therefore called thermal radiation. Conversely, all normal matter absorbs electromagnetic radiation to some degree. An object that absorbs all radiation falling on it, at all wavelengths, is called a black body. When a black body is at a uniform temperature, its emission has a characteristic wavelength distribution that depends on the temperature. Its emission is called black-body radiation. An almost perfect blackbody radiation spectrum is exhibited by the cosmic microwave background radiation, discussed in section 2.2.4. The wavelengths of radiation can be stretched (or compressed) when the source of the

ries and its wavelength: short wavelength radiation is more energetic than long wavelength radiation. Mathematically E

hc λ

where h is Planck’s constant, or also E

hν .

For example, short-wavelength gamma rays and X-rays are very energetic. You know this type of radiation is harmful to humans. Long-wavelength radio waves are less energetic forms of radiation. The atmosphere of Earth blocks most types of harmful, energetic radiation coming to us from Outer Space. Thus, only some types of electromagnetic radiation can be observed from Earth. All other types of radiation can only be observed from 21

for a long time. As you may imagine, the measurements become more and more difficult as distances to astronomical objects increase. Here are a few examples of how you can derive distances in Outer Space. Within the Solar System, you can measure distances to some objects by bouncing radar beams off of them and using the velocity-distance-time relationship from chapter 1. When determining the distances to nearby stars, astronomers use trigonometry. Stellar parallax is the apparent shift of position of any nearby star against the background of distant objects. Created by the different orbital positions of Earth, the extremely small observed shift is largest at time intervals of about six months, when Earth arrives at exactly opposite sides of the Sun in its orbit. The parallax angle, p, itself is considered to be half

Figure 2-5. Measuring stellar distances using trigonometry involves knowing the size of Earth’s orbit around the Sun and determining the small parallax angle, p. (NASA, ESA, and A. Feild (STScI)) radiation is moving away (or toward) the observer. A redshift happens when electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, “redder” means an increase in wavelength. Conversely, when the electromagnetic radiation from an object is decreased in wavelength, astronomers speak of a blueshift (cf. chapter 8). A redshift occurs whenever a light source moves away from an observer. Galaxies exhibit a redshift as space expands and carries them away from each other. Due to the expansion of the Universe, distant galaxies appear to be redder than nearby ones, with the redshifts of their electromagnetic radiation corresponding to the rate of increase in their distances from Earth.

2.2.3 Distance Primer Hubble showed that the redshifts of galaxies are greater the greater their distances. For that, he had to measure both the redshift and the distance of each galaxy. Distance measurements have been a major topic of astronomical research 22

Figure 2-6. The bright spot in the bottom left corner of this image is an exploding star called a supernova of type Ia. This supernova was discovered in 1994 in the outskirts of the galaxy NGC 4526. By measuring the brightness of the supernova, and by knowing how much radiant energy supernova Ia explosions produce, the distance to the supernova and hence, to galaxy NGC 4526 can be calculated using the inverse-square law of radiation. (NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team)

of the measured angular shift. Once a star’s parallax is known, its distance from Earth can be computed trigonometrically. Because stellar parallaxes and distances all involve such skinny right triangles, a convenient trigonometric approximation can be used to convert parallaxes (in seconds of arc or arcseconds, for short) to distance (in parsec). The distance is simply the reciprocal of the parallax: d 1′ = . 1 pc p For example, Proxima Centauri (the nearest star to Earth), whose parallax is 0.78′, is 1.28 pc distant. To convert parsecs into lightyears, multiply the parsecs by 3.26:

Figure 2-7. It’s hard to imagine the expansion of space, so here it is illustrated with twodimensional sheets of galaxies. Notice that the galaxies themselves do not change size, merely the space between them does. Because all galaxies are moving apart, it follows that at an earlier time they were closer together. Extrapolating Hubble’s Law backwards in time brings all galaxies closer and closer together. At the earliest time, galaxies, stars, planets, and such, did not exist at all. (Wikipedia)

1 pc = 3.26 ly. For yet more distant objects, we can employ the method of standard candles. When you know how bright an emitting object is intrinsically, and you then measure how bright it appears to you, you can calculate the object’s distances. This is based on a radiation law which states that the strength of a signal drops off with the square of the distance from the emitting source (see chapter 3.2.1). One such example for objects for which we know how much radiant energy they emit intrinsically, is a type of exploding star called a “supernova Ia.” By measuring how bright a distant supernova appears to be, its distance can be calculated.

called the “Big Bang.” Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past; this is the Big Bang singularity. We can still see the radiation of this dense, hot state of the early Universe all around us. This provides us the evidence that the Universe really did start with the Big Bang, as suggested by the extrapolation of Hubble’s Law. With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. However, with radio telescopes we detect a faint background signal, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This signal is strongest in the microwave region of the radio spectrum. It was discovered by accident in 1964 by American radio astronomers Drs. Arno Penzias and Robert Wilson and earned them the 1978 Nobel Prize. It has been named the

2.2.4 The Big Bang And The Cosmic Microwave Background Radiation By extrapolating the stretching of space backwards in time using Hubble’s Law, we infer that the Universe was smaller in the past than it is today. At the earliest times, the Universe would have been very small. Since everything would have been crammed together, the Universe would have to have been be very dense and very hot. This hot dense state from which the Universe expanded is 23

The behavior of CMB photons moving through the early Universe is analogous to the propagation of optical light through the Earth’s atmosphere. Water droplets in a cloud are very effective at scattering light, while optical light moves freely through clear air. Thus, on a cloudy day, we can look through the air out towards the clouds, but can not see through the opaque clouds. Cosmologists studying the CMB radiation can look through much of the Universe back to when it was opaque: a view back to 380,000 years after the Big Bang. This “wall of light” is called the surface of last scattering since it was the last time most of the CMB photons directly scattered off of matter. When we make maps of the CMB, we are mapping this surface of last scattering. The photons that existed at the time of photon decoupling have been propagating from the surface of last scattering ever since, though growing fainter and less energetic, since the expansion of space causes their wavelength to increase over time. The CMB we observe today corresponds to a 3 K black body, (2.7260±0.0013) K to be precise. A map of the CMB made by the WMAP Satellite Team is shown in Figure 2-8. Because our eyes are not sensitive to microwaves, the all-sky map is drawn with false color. One of the most striking features about the CMB is its uniformity. Only with very sensitive instruments can cosmologists detect fluctuations in the CMB across the sky. By studying these fluctuations, cosmologists can learn about the origin of galaxies and large scale structures of galaxies and they can measure the basic parameters of the Big Bang theory. Another one of WMAP results was a measurement of the age of the Universe, 13.772 billion years old, with an uncertainty of plus or minus 59 million years. In 2013, the European Space Agency’s Planck Spacecraft Team estimated the age of the Universe to be 13.82 billion years, slightly higher but within the uncertainties of the earlier number derived from the WMAP data. By combining the Planck data with previous missions, the best combined estimate of the age of the Universe is currently (13.798±0.037)×109 years.

Figure 2-8. This is one of the false-color images of the entire sky that was produced from microwave data obtained with the WMAP satellite, and is showing the isotropy of the remnant heat, or afterglow, from the Big Bang. It is very common in the space sciences to use false-color images to display information that is not visible to the human eye, like microwaves. The picture also uses what’s called a Mollweide projection to show the sky all around us as a flat map. Compare this to the flat map of the Earth in Fig. 4-5. (NASA/WMAP Science Team) Cosmic Microwave Background radiation, or CMB. Since microwave radiation is more easily observed from space than from Earth, several space missions have been deployed to measure it. Some significant space missions are RELIKT-1, COBE, WMAP, and Planck. Early on in its expansion from the singularity, the Universe was filled with opaque (= nontransparent) plasma of densely packed particles and with radiation. As the Universe expanded more, both the plasma and the radiation filling it grew cooler. When the Universe cooled enough, protons and electrons combined to form neutral atoms. These atoms could no longer absorb the radiation, and so the Universe became transparent instead of being an opaque plasma. Cosmologists refer to the time period when neutral atoms first formed as the recombination epoch, and the event shortly afterwards when photons started to travel freely through space rather than constantly being scattered by electrons and protons in the plasma is referred to as photon decoupling. This occurred about 380,000 years after the Big Bang.

Figure 2-9. The Universe’s timeline, from inflation to the present. The WMAP satellite is also shown, on the right. (NASA/WMAP Science Team) allowed to move at faster-than-light speed according to relativity. What followed was a long, second phase, lasting nearly half the age of the Universe, during which the Universe was exhibiting the equivalent of free flight. However, during this time, the expansion slowed down. The slowing down was caused by the gravitational attraction of the matter contained in the Universe. But eventually, the expansion began to speed up again. It is as if the Universe went back into powered flight. The cause for the accelerated expansion is thought to be the dark energy which opposes gravity. This is the third, and current phase we distinguish in the Universe’s history. While the oldest light we see was emitted about 13.7 billion years ago, the radius of the observable Universe is actually calculated to be about 46 billion lightyears. This number is based on where the variable expansion of space has taken the most distant light observed. Simply put, in doing this calculation cosmologists consider a

2.2.5 The Size Of The Observable Universe The observable Universe consists of the CMB, the galaxies, and other matter that can be observed from Earth in the present day because electromagnetic radiation from these objects has had time to reach the Earth since the beginning of the Universe’s expansion. We distinguish three phases of the Universe’s expansion. The first one caused the Big Bang that made the Universe expand from primordial “quantum fluctuations.” Quantum fluctuations are a temporary change in the amount of energy in a point in space. Our current understanding involves a very brief, but very rapid period of expansion in the early Universe, called “inflation.” This period was very brief — in fact only around 10-35 seconds — but in that time it is thought that the Universe expanded by an astonishing factor of around 1060. Since the Universe expanded so quickly during inflation, faster than the speed of light in fact, we can only see a tiny fraction of the entire Universe. Yes, space is 25

about 46 billion light years away from us in all directions, while the Universe as a whole may be larger, and may not have an actual edge. Bottom line: We don’t really know the size of Outer Space.

2.3 What Is in Outer Space? The content of Outer Space is stranger than you might think! The largest component of massenergy is in the form of dark energy. Plasma, which is ionized gas, accounts for most of the ordinary matter in Outer Space. There could also be an abundance of planets, perhaps as many as 1022. Figure 2-10. A pie chart of the content of the Universe from WMAP results (NASA/WMAP Science Team)

2.3.1 Energy Content Of The Universe When we speak of the energy content of the Universe, we imply this to mean both energy and mass. In physics, “energy” is the ability to do work, or the ability to move or elicit change in matter. Energy has a few important properties. For one, energy is always conserved, meaning that it cannot be created or destroyed. It can, however, be transferred between objects or systems by the interactions of forces. According to Dr. Albert Einstein’s theory of general relativity, energy and mass are equivalent concepts, following his famous equation

velocity-distance-time relationship in which the velocity has not been constant but time-variable, and has also exceeded the speed of light. This is why the Universes’s age of 13.7 Gyr does not correspond to a size of 13.7 Glyr, as you would calculate using the equations from chapter 1, but to 46 Glyr instead. The distance to the edge of the observable Universe is roughly the same in every direction. That is, the observable universe is a spherical volume centered on the observer, regardless of the shape of the Universe as a whole. Every location in the Universe has its own observable Universe, which may or may not overlap with the one centered on Earth. Notice that Earth is not considered to be at the center of the Universe. No evidence exists to suggest that the edge of our observable Universe constitutes a boundary on the Universe as a whole. None of the mainstream cosmological models propose that the Universe has any physical boundary. The WMAP results can be interpreted to mean that the Universe is infinite. Other interpretations are that the Universe could be finite but unbounded, like a higherdimensional analogue of the two-dimensional surface of a sphere that is finite in area but has no edge. To sum up, think of the observable Universe as a bubble having a visible edge that is

mc 2 ⇔ m =

E . c2

Until about thirty years ago, astronomers thought that the Universe was composed almost entirely of ordinary matter. Now we know that more than 95% of the matter-energy content of the Universe is in a form that has never been directly detected in the laboratory! The results from the WMAP mission showed us that ordinary matter, the protons, neutrons and electrons which make up the atoms of the periodic table of elements, is the smallest fraction of the overall content of the Universe, amounting to a mere 4.6%. Yet, ordinary matter is important to us because it is the constituent matter of human bodies as well as planets and all the celes-

matter. Dark matter is thought to be composed of one or more types of sub-atomic particles that interact very weakly with ordinary matter. Particle physicists have many plausible candidates for the dark matter, and new particle accelerator experiments are likely to generate new insight in the coming years. The biggest fraction of the current composition of the Universe by far, 71.4%, is called “dark energy.” It seems to behave like anti-gravity, and appears to be the source of the accelerated expansion of the Universe. The first observational evidence of dark energy’s role in the expansion of the Universe dates back to the 1980’s when we were trying to understand the formation of galaxy clusters. The explanation behind the observed distribution of galaxies made more sense if dark energy were present, but the evidence was highly uncertain. In the 1990’s, observations of exploding stars called supernovae were used to trace the recent expansion history of the Universe and the big surprise was that the expansion appeared to be speeding up! There was concern that the supernova data was being misinterpreted, but the result has held up to this day. In 2011, Drs. Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess earned the Nobel Prize for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.

Figure 2-11. Artist’s logarithmic scale conception of the observable Universe with the Solar System at the center, inner and outer planets, Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda galaxy, nearby galaxies, Cosmic Web, Cosmic Microwave Background Radiation and the Big Bang’s plasma on the edge. (Wikipedia)

tial objects that surround us. Imagine that for most of human history, scientists have focused on studying only 4.6% of the Universe! Ordinary matter is also referred to as “luminous matter,” since the astronomical bodies that we see to large distances are the self-luminous stars. Stars like our Sun are self-luminous because they generate energy in their cores which they emit from their surfaces. They shine! Stars are grouped together in star cities which we call galaxies, like our home, the Milky Way Galaxy. There are billions of galaxies visible to the Hubble Space Telescope, because they give off light. A much greater fraction, 24% of the Universe, is a different kind of matter that has gravity but does not emit any light. In other words, this matter is non-luminous and is therefore termed “dark matter.” While the dark matter cannot be seen directly, its presence is inferred from the gravitational pull it exerts on ordinary, luminous

2.3.2 Ordinary Matter Content Of The Universe Atoms are the smallest recognized division of the chemical elements. They have sizes of 10-10 m, or 0.1 nm, and consist of a dense central nucleus surrounded by a cloud of electrons. Atoms are electrically neutral. Add or subtract an electron, and you get an ion. Bond two or more atoms together, and you get a molecule. The celestial objects we observe, galaxies, stars, planets, and so forth, and everything we know on Earth, are made of ordinary matter. This begs the question as to what kind of atoms might be the most prevalent. The Earth is primarily 27

We do have an idea of the number of stars because the Hubble Space Telescope collected information on the number of galaxies in two tiny sectors of the Universe. An extrapolation made by assuming 100 billion average galaxies and 100 billion stars per average galaxy results in 1022 stars. Now, to the best of our best knowledge, every star in the Milky Way Galaxy hosts at least one planet, on average. If all galaxies are considered to be equal, this yields an astounding number, 1022 planets.

made of iron (32.1%), followed by oxygen (30.1%), silicon (15.1%), and magnesium (13.9%). These are the atoms that make up molecules in rocks, in our oceans, and in the atmosphere. Oxygen contributes a majority of the human body’s mass, followed by carbon. When we look at the Universe as a whole, the story is quite different. Overall, the most common chemical element is the simplest. It is the first element in the periodic table, hydrogen. Hydrogen is the most abundant element in the Universe accounting for about 74% by mass, helium is second, with 24%, followed minutely by lithium, and then a mere trace of everything else that we observe on Earth. Our understanding of this fact is that H, He, and Li formed in the first three minutes after the Big Bang. We call this “Big Bang nucleosynthesis.” Nucleosynthesis is the process that creates new atomic nuclei from pre-existing protons and neutrons. Essentially all of the elements that are past lithium in the periodic table were created much later in time. They were formed by stellar nucleosynthesis through the life cycle of the stars.

2.4 What Are the Major Regions of Outer Space? Here, we look at the organization of Outer Space, and where the different space regions are found in order of increasing distance from the Earth.

2.4.1 Geospace Geospace is the name we give to the region of Outer Space that immediately surrounds or encloses our Earth. Geospace is a giant magnetic bubble around Earth defined by the interaction of Earth’s magnetic field with the solar particle wind. It is also called Earth’s “magnetosphere.” The magnetopause forms the interface between our planet’s magnetosphere and the solar particle wind, and is the outer edge of geospace.

2.3.3. Astronomical Objects Finally, we might be interested in how much of the mass-energy content of the Universe is comprised of familiar astronomical bodies. Again, we are surprised by the evidence. The most common state of ordinary matter is “plasma.” Plasma is also called the fourth state of matter, the other three being solid, liquid, and gas states. Plasma consists of ions and free electrons. There is plasma between the galaxies, called “intergalactic medium” (IGM), there is plasma between the stars, called “interstellar medium” (ISM), and then there is plasma in the stars themselves. The percentage of each part by mass is approximately 92.4% IGM, 5.9% stars, and 1.7% ISM. Planetary bodies are a trace of the mass of all astronomical objects. Yet we care most for planets because they can harbor life. We do not yet have a good count of planets outside of our Solar System. But we can make an informed estimate.

Figure 2-12. The Earth’s magnetic field is similar to that of a bar magnet. The magnetic axis is tilted with respect to Earth’s spin axis. (NASA/ Science Learning) 28

our magnetosphere, and create a number of phenomena. The volume of geospace is compacted in the direction of the Sun by the pressure of the solar wind, giving it a typical sunward distance of 10 Earth radii from the center of our planet. However, the tail can extend outward to at least 220 and possibly 1,000 Earth radii. The Moon passes through the tail of geospace for roughly four days each month, during which time its surface is shielded from the solar wind. Inside of geospace, the Earth has two donut-shaped radiation belts which consist of particles that are trapped in our magnetic field by a “magnetic bottle” effect. The discovery of the belts is credited to Dr. James Alfred Van Allen (1914– 2006), and as a result the Earth’s belts bear his name. The Van Allen belts were detected in 1958 with a Geiger counter aboard Explorer 1, the first US satellite to orbit the Earth, and were one of the first scientific discoveries made using a satellite. The inner belt is centered around 3,000 km (1860 mi) above Earth’s surface and contains protons. The outer belt is at about 20,000 km (12,500 mi) and is populated by electrons and ions. In 2013, NASA reported that the Van Allen Probes had discovered a transient, third radiation belt, which was observed for four weeks until destroyed by a powerful, interplanetary shock wave from the Sun. Most spaceflight these days occurs in low Earth orbit (LEO), that is, at very low altitudes, and is not affected by the Van Allen belts. But some satellites achieve higher altitudes and cross the belts. Particle radiation in the belts endangers satellites, which must protect their sensitive electronic components with adequate shielding. The Van Allen belts also place astronauts crossing them at risk for cancer unless their spacecraft is shielded to protect them. Our Sun is quite active (see chapter 3.2). Sometimes there are giant particle eruptions on its surface causing geomagnetic storms. Geomagnetic storms affect two regions of geospace, the radiation belts and the ionosphere, a region of the

Figure 2-13. The shape of the Earth’s magnetosphere is the direct result of being blasted by solar wind particles. The solar wind compresses the magnetosphere’s sunward side to a distance of only 6 to 10 times the radius of the Earth. A supersonic shock wave is created here called the bow shock. Most of the solar wind particles are heated and slowed at the bow shock and detour around the Earth in the magnetosheath. The solar wind drags out the night-side magnetosphere to possibly 1,000 times Earth’s radius; its exact length is not known. This extension of the magnetosphere is known as the magnetotail. The outer boundary of Earth’s confined geomagnetic field is called the magnetopause. The Earth’s magnetosphere is a highly dynamic structure that responds dramatically to solar variations. (NASA/GSFC/A. Kaase) Earth has a dipole magnetic field. The magnetic axis is tilted about 11.5 degrees away from the Earth’s axis of rotation. Earth’s magnetic field is critical to human survival, protecting all life on Earth from being bombarded by charged particles from space. Researchers speculate that without a magnetic field, Earth could not have retained its atmosphere or its liquid water, and life could not have developed here. Thankfully, our magnetic field deflects charged particles away from Earth, protecting the atmosphere, the surface, and life. When electrically charged particles from the Sun or ions from other astronomical sources, which are collectively referred to as “cosmic rays,” interact with Earth’s magnetic field, they shape

Figure 2-14. Two giant belts of radiation surround Earth. The inner Van Allen belt is dominated by protons and the outer one by electrons and ions. Missions beyond LEO leave the protection of the geomagnetic field, and transit the Van Allen belts. Thus they need to be shielded against exposure. The Apollo missions marked the first event where humans traveled through the Van Allen belts, which was one of several radiation hazards known by mission planners. (Wikipedia)

Figure 2-15. The Aurora Borealis, or Northern Lights, as seen above Bear Lake, Alaska. (USAF/ J. Strang via Wikipedia)

upper atmosphere which is ionized by solar radiation. The storms can cause power outages on Earth, permanently damage satellite electronics, disrupt telecommunications and GPS technologies. Geomagnetic storms can be a hazard to astronauts, even in LEO. Geomagnetic storms also create aurorae, also known as the northern and southern lights, seen near the Earth’s poles. The aurorae are caused when charged particles spiral toward the magnetic poles of Earth along its magnetic field. As the particles from space collide with particles in Earth’s atmosphere, they collisionally excite atmospheric atoms, elevating their electrons to higher orbits. When the electrons return to lower orbits, the atoms give off radiation. The resulting aurora looks like a glowing curtain in the sky. The Sun’s activity is closely monitored because of its influence on Earth’s magnetosphere and ionosphere. “Space weather” is a term which has become accepted over the past few years to collectively refer to the physical processes which occur due to the Sun’s activity, ultimately affecting human enterprises on Earth and in space. Due to our spaceflight activities, geospace now also contains material left over from rocket launches and satellites that are a potential colli30

Figure 2-16. The aurora was imaged from aboard the Space Shuttle. The payload bay and the tail of the Shuttle can be seen on the left. (Wikipedia/ NASA) sion hazard to other spacecraft. Some of this debris reenters Earth’s atmosphere periodically.

2.4.2 Cislunar Space The Moon is Earth’s natural satellite. It orbits the Earth at an average distance of about 384,400 km (238,900 mi). It takes the Moon approximately one month to orbit the Earth. The orbit of the Moon around Earth is inclined by about 50 relative to the orbit of the Earth around the Sun. The mass of the Moon is only about 1/80, and its radius, less than 1/4, that of Earth. Cislunar space is the space between the Earth and the orbit of the Moon. Cislunar space overlaps with geospace for a few days every

Figure 2-17. Schematic of the Earth–Moon system, without a consistent scale. Notice also the tilt of Earth’s rotation axis with respect to its orbit around the Sun. The Moon’s orbit has a small inclination to the Earth’s orbit, and its axis also is tilted. (Wikipedia/NASA)

planned, including government as well as privately funded efforts. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes. Cislunar space includes several locations in which spacecraft can hover in the same position relative to the Earth and the Moon. These can be thought of as gravitationally neutral points. The Lagrangian points, also Lagrange points, or L points, are the five positions in an orbital configuration where a small object can maintain a some-

month, when the Moon’s orbit carries it through the tail of Earth’s magnetosphere. Many spacecraft have traversed cislunar space. The Soviet Union’s Luna program was the first to reach the Moon with robotic spacecraft in 1959. The United States’ Apollo program achieved the only crewed missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon’s origin, the formation of its internal structure, and its subsequent history. The Moon is the only celestial body other than Earth on which humans have currently set foot. After the last human mission, the Apollo 17 mission in 1972, the Moon has been visited by robotic spacecraft, only. Of these, orbital missions have dominated. Since 2004, Japan, China, India, the United States, and the European Space Agency (ESA) have each sent lunar orbiters, which have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into a type of lunar soil called regolith. The post-Apollo era has also seen two rover missions: the final Soviet Lunokhod mission in 1973, and China’s Chang’e 3 mission, which deployed its Yutu rover on 14 December 2013. Future crewed missions to the Moon have been

Figure 2-18. This diagram shows the 5 Lagrange points for two masses. Lagrange points are locations in space where gravitational forces and the orbital motion of two bodies balance each other, and they are useful to place a third, small body, like a spacecraft, relative to two orbiting large ones, like the Earth and the Moon. (Wikipedia/ NASA) 31

what stable orbital configuration with respect to two larger objects (such as a satellite with respect to the Earth and Moon). What this means in practical terms is that a spacecraft can stay in an L point with no or minimal propellant use. The use of satellites and space stations in the L points is being discussed for future human missions to the Moon (cf. chapter 10). Technically, the L2 is in translunar space, and it would be the farthest any humans have ever traveled.

2.4.3 Interplanetary Space Interplanetary space is a bubble of space around our Sun that is defined by the interaction of the solar wind with the interstellar medium. Interplanetary space contains the Sun, the planets, and further beyond, the scattered disk. It is also called Sun’s “heliosphere.” The heliopause is the region where the solar wind is stopped by the interstellar medium, and is the edge of interplanetary space. As the solar wind streams away from the Sun, it races out toward the space between the stars. We think of this space as empty but it contains traces of gas and dust – together called the interstellar medium. The solar wind blows against this material and clears out a cavity-like region. This cavity is our heliosphere. The point where the solar wind becomes slower than the speed of sound is called the termination shock; the solar wind continues to slow as it passes through the heliosheath leading to a boundary called the heliopause, where the interstellar medium and solar wind pressures balance. Our entire heliosphere is carried through the interstellar medium along with the Sun on its orbit around the center of our Galaxy. The edge of the heliosphere is about 12o Sun-Earth distances in the direction of the Sun’s motion. The average distance between the Earth and the Sun is also known as a distance of one “Astronomical Unit,” about 150 million kilometers or 93 million miles. The exact size of the heliosphere is thought to vary depending on the interaction between the Sun and the interstellar medium.

All eight planets of the Solar System have been visited by space probes, including Earth. However, only one space probe has journeyed beyond the heliosphere. On 12 September 2013, NASA announced that Voyager 1 had exited the heliosphere on 25 August 2012, when it measured a sudden increase in plasma density of about forty times. Because the heliopause marks one boundary between the Sun’s wind and the rest of the galaxy, a spacecraft such as Voyager 1 which has departed the heliosphere can be said to have reached interstellar space. However, since there is so little known about the transition to interstellar space, the question of if and when Voyager departed the Solar System is still being debated. Voyager 1 was launched on 5 September 1977. The distance from the Sun at which the Solar System ends and interstellar space begins is actually not precisely defined. There are two separate forces we consider: the Sun’s wind and the Sun’s gravity. The Sun’s wind gives rise to the heliosphere. The Sun’s “Hill sphere,” which is the ef-

Figure 2-19. An artist’s rendition of the heliosphere, shown in blue colors. The interstellar medium is shown in red. The outer border of this bubble is where the solar wind’s strength is no longer great enough to push back the interstellar medium. This is known as the heliopause. Voyager 1 and 2 are also shown at great distances from the Sun. (Adapted from NASA) 32

Figure 2-20. This artist’s concept puts Solar System distances in perspective (but the sizes of the objects shown are not to scale). The scale bar is logarithmic and in units of the Earth-Sun distance, also called “astronomical unit,” with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the average Earth-Sun distance, which is about 150 million kilometers or 93 million miles. Neptune, the most distant planet from the Sun, is at about 30 AU. Alpha Centauri, 276,500 AU from the Sun, is currently the closest star system to our Solar System. Note that Voyager 1 is not actually headed there; it’s distance is just plotted to give a sense of scale. (Wikipedia/NASA/JPL) years in diameter. It contains 100–400 billion stars. Our Sun is not located in the center, but about 27,000 lightyears away to one side, in one of the spiral arms. The Milky Way Galaxy is disk shaped, like a frisbee. From our vantage point within our Galaxy, we see a higher stellar density in the night sky in directions where we look into the plane of the disk. This creates a band of stars crossing the night sky, and is what gave the Milky Way its name. The center of the Galaxy is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. The stars in the Galaxy orbit about the center, but due to dark matter, their orbits have differences when compared with Kepler’s laws, which describe the way planets orbit stars (cf. chapter 3.7). It takes the Sun about 220 million years to orbit the Galactic Center; this is also called a “galactic year.”

fective range of its gravitational dominance, is believed to extend up to a thousand times farther than its heliosphere. Therefore, the edge of interplanetary space is not the same as the edge of the Solar System. The “Oort Cloud,” a huge repository of comets, may extend outward as far as 100,000 Astronomical Units. That’s more than one third of the way to the nearest star, Alpha Centauri. The Oort Cloud is in interstellar space and it’s considered part of the Solar System.

2.4.4 Interstellar Space Interstellar space is the space between the stars. All the stars we see in the night sky are part of our star city, the Milky Way Galaxy. The edge of interstellar space is the edge of the Milky Way Galaxy. The Milky Way is a type of galaxy called a “spiral galaxy,” some 100,000–120,000 light33

constellation of Centaurus. Actually a multiple star system, its two main stars are Alpha Centauri A (α Cen A) and Alpha Centauri B (α Cen B), usually defined to identify them as the different components of the binary α Cen AB. A third companion—Proxima Centauri (or Proxima or α Cen C)—has a separation from A and B much greater than the observed separation between A and B and is probably gravitationally associated with the AB system. Of all three stellar components, Proxima Centauri is the one that is closest to Earth. The system may also contain at least one planet, the Earth-sized Alpha Centauri Bb. If confirmed it would be the closest known “exoplanet,” a planet that orbits a different star than our Sun. It is impossible to see any detail on any of the three stars despite the fact that these stars are so nearby. The reason is that their diameters are small when compared to their distances. A planet is generally even smaller in size than a star, so see-

Figure 2-21. A view of the band of stars we call Milky Way. We are looking toward the constellation Sagittarius, which includes the Galactic Center. The photo was taken from a place not polluted by light, the Black Rock Desert in Nevada. (S. Jurvetson/Flickr/Wikipedia) The stars in the Galaxy range in mass from between about one 1/10 to 150 times the mass of our Sun. There is also a wide range of ages, with the oldest stars having formed shortly after the Big Bang, and new stars still forming out of interstellar gas and dust in the spiral arms. Stellar remnants, such as white dwarfs, neutron stars, and black holes, are objects that mark stellar death. There are also failed stars called brown dwarfs which never made it to star status during star birth. The Milky Way Galaxy also contains planetary systems surrounding other stars. The nearest star system to us is called Alpha Centauri. It’s 4.37 lightyears from the Sun. This places it 267,500 times further away from the Sun than Earth. Alpha Centauri is the name given to what appears as a single star to the naked eye and the brightest star in the southern

Figure 2-22. This close-up picture of the nearby galaxy NGC 6744 was taken at the European Southern Observatory (ESO) in Chile. The large spiral galaxy is similar to the Milky Way, making this image look like a picture postcard of our own Galaxy sent from extragalactic space. Notice the red glow coming from hydrogen gas clouds in the spiral arms, where new stars are forming. (ESO) 34

Figure 2-23. Seen from the southern skies, the Large and Small Magellanic Clouds are bright patches in the sky. These two irregular dwarf galaxies, together with our Milky Way Galaxy, belong to the socalled “Local Group” of galaxies. (ESO/S. Brunier) axies and we have ample images of those. The Milky Way has many smaller galaxy companions which are called “dwarf galaxies.”

ing any detail on the planet’s surface, like any possible continents or oceans, is also impossible.

2.4.5 Intergalactic Space No spacecraft has reached intergalactic space. Intergalactic space is the space between the galaxies. It has a weblike structure because of the way that the dark matter is distributed in the Universe. Galaxies, which clump together in clusters of galaxies, and very large filaments of hydrogen gas, are found in the same places as the dark matter. They surround enormous empty regions of space which are called the “voids.” Astronomers distinguish three types of galaxies based on their overall shapes: spirals, ellipticals and irregulars. The spiral and elliptical galaxies can be very large and massive massive. The irregular galaxies are smaller and less massive and are also called dwarf galaxies. You already know that our Milky Way is a spiral galaxy. Because we live inside of it, and we have never left it, we don’t have a photograph of how it looks as seen from outside. But there are other galaxies which are also spiral gal-

Figure 2-24. A close-up Hubble Space Telescope image of the elliptical galaxy M89. M89 is a member of the Virgo Cluster of galaxies. Current observations indicate that M89 may be nearly perfectly spherical in shape. (NASA/STScI/Wikipedia) 35

Figure 2-26. This image shows a computer simulation of one possible scenario for how intergalactic space might be organized. The details are still not settled. However astronomers know that, on the largest scales, the Universe is structured as a vast web made up of filaments, clusters of gas, galaxies, and dark matter, separated by huge, empty voids. (A. Pontzen & F. Governato/Wikipedia)

Figure 2-25. Hubble Space Telescope image of the galaxy cluster Abell 1689. Abell 1689 is one of the most massive galaxy clusters known. It’s gravitational force bends and magnifies the light of galaxies far behind it like a lens. (NASA/N. Benitez et al.)

which results in low rates of star formation, and few young stars. Elliptical galaxies are dominated by old stellar populations, giving them an overall yellow to reddish color. The motions of stars in elliptical galaxies are predominantly radial, going through the center, rather then being on orbits about the center as occurs in spirals. The Virgo Cluster is a cluster of galaxies whose center is about 54 Mly away in the constellation Virgo. It comprises approximately 1,300 (and possibly up to 2,000) member galaxies. Clusters of galaxies contain a mix of elliptical, spiral, irregular galaxies, and hot gas. The Virgo Cluster itself is part of the larger Laniakea Supercluster. The Laniakea Supercluster is 520 Mly in size. It is made up of about 100,000 galaxies, which reside in a number of galaxy clusters. On a scale of hundreds of millions of lightyears, intergalactic space is organized into a network of vast filaments of gas, galaxies, and superclusters surrounding sparsely populated regions called “voids.” This large-scale structure is also called the “Cosmic Web.”

Some of them are irregular in shape. The most famous ones can be seen by naked eye from the southern hemisphere of Earth and are called “Large and Small Magellanic Clouds.” The nearest spiral galaxy to the Milky Way is the Andromeda galaxy, or M31. M31 is about 2.5 million lightyears distant. Our Milky Way and M31, together with a large number of smaller galaxies, form our Local Group of galaxies. The Local Group is part of a larger galaxy cluster called the “Virgo Cluster.” Together with the smaller, irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe. Spiral and irregular galaxies are found in comparatively uncrowded regions of space; they are rarely found in the busy centers of galaxy clusters. The third type of galaxy is called an “elliptical galaxy.” Elliptical galaxies are characterized by several properties that make them distinct from spiral and irregular galaxies. They are spherical or ovoid masses of stars. Furthermore, there is very little interstellar matter (neither gas nor dust), 36

Figure 2-27. The heart of the Whirlpool Galaxy as imaged with the Hubble Space Telescope. This gives us an excellent, face-on view of a spiral galaxy. The red patches show ionized gas clouds and the dark patches show interstellar dust clouds. (NASA and the Hubble Heritage Team, STScI/AURA) 37

planet that circles a star somewhere in an elliptical galaxy? (Hint: draw a sketch.) 26.What are voids?

2 Test Your Understanding I. A NSWER IN A FEW SENTENCES .

II. C ALCULATE T HE A NSWER .

1. Where does Outer Space begin? 2. What is Hubble’s Law? 3. What are the regions of the electromagnetic spectrum from short to long wavelengths? 4. Which are the two methods that astronomers use to derive interstellar distances? 5. What is the Big Bang? 6. What is the source of the CMB spectrum? 7. What is a false-color image? Research falsecolor images on the internet and provide three examples. 8. What is the observable Universe? 9. How is the observable Universe similar or different for an observer located in the Andromeda galaxy, 2.5 Mly distant from us? 10.What are the three components of the overall energy content of the Universe? 11. Which chemical element is most abundant in the Universe? 12.Which chemical element is most abundant on Earth? 13.What defines geospace? 14.What processes are involved in producing the northern lights? 15.What are the Van Allen belts? 16.What is space weather, and why is it a concern for spaceflight? 17.What are the health threats from cosmic rays, and what are implications for human interplanetary spaceflight? Use the internet to research this question more. 18.What defines cislunar space? 19.What are Lagrange points? 20.Where does interplanetary space end? 21.Where does interstellar space begin and end? 22.What is the name of the nearest star other than our Sun? 23.What are the three main types of galaxies? 24.What type of galaxy is the Milky Way? 25.How might the night sky look different from Earth’s for an alien observer who lives on a

1. Imagine that you are investigating a nearby dwarf galaxy for planets. So far, you’ve estimated that the galaxy has 1 ☓ 106 stars, 50% of which harbor planets, and that, on average, there are 5 planets in each extrasolar system. What do you estimate to be the total number of planets in this galaxy? 2. Particles ejected from the Sun travel at much lower speeds than the speed of light. How long after a solar eruption is visible will the particles from the eruption arrive on Earth? Use 300,000 km/s for the speed of light, and 400 km/s for the speed of the solar particles. Give your answer in reasonable time units, such as days, hours, minutes. 3. How long would it take a spacecraft like Voyager 1 to fly to Proxima Centauri? Assume that the speed of the spacecraft is constant and equal to the maximum speed achieved by Voyager 1, namely 62,136 km/h. Also, assume that it is flying along a straight line. Use 4.22 ly for the distance to Proxima Centauri. Use 1 ly equals 9.5 ☓ 1015 m. 4. A comet in the Oort cloud just got disturbed by the gravity of a nearby star and as a consequence, is kicked out of orbit and is now moving toward the inner Solar System. Knowing that the Oort Cloud’s distance from the Sun is equivalent to 100,000 Earth-Sun distances, or astronomical units [AU], and that 1 AU is equal to 1.5 ☓ 1011 m, answer this question: How much time will elapse before astronomers on Earth can possibly see the comet coming towards us? Use 300,000 km/s for the speed of light. 38

An extreme-ultraviolet image of the Sun taken by the Solar Dynamics Observatory (NASA/Goddard/SDO AIA Team)

THE SOLAR SYSTEM The Solar System is our cosmic backyard. What kinds of objects are in the Solar System and how do they move relative to one another? How did the Solar System form? And how will it end?

3.1 Overview of the Solar System The Solar System comprises the Sun and the objects that orbit the Sun. The objects orbiting the Sun fall into three categories: planets, dwarf planets, and Small Solar System Bodies.

Figure 3-1. The Sun, planets and dwarf planets of our Solar System. The sizes are to scale but the distances are not. (Wikipedia/NASA)

3.1.1 The Heliocentric Solar System For many thousands of years, humanity did not recognize the existence of the Solar System. People believed Earth to be stationary at the center of the Universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. In the 17th-century, Galileo Galilei, Johannes Kepler and Isaac Newton, gained an understanding of the physics that led to the gradual acceptance of the idea that Earth moves around the Sun and that the planets are governed by the same physical laws that work on Earth. The invention of the telescope led to the discovery of further planets and moons. Improvements to the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena, such as mountains and craters, as well as atmospheric and seasonal meteorological phenomena, such as clouds, dust storms and ice caps, on the other planets.

3.1.2 Objects In The Solar System The principal component of the Solar System is the Sun. The Sun is a type of astronomical object we call â&#x20AC;&#x153;star.â&#x20AC;? Like other stars, the Sun is a

by objects called natural satellites, or moons (two of which are larger than the planet Mercury). The four gas giants have many moons and are also surrounded by planetary rings, thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. This is the case with Earth’s Moon. The Solar System contains a number of dwarf planets, and more are expected to be added as their discovery in the far-out Solar System continues. A “dwarf planet” is a celestial body that 1. is not massive enough to cause thermonuclear fusion, 2. is massive enough to be rounded by its own gravity, 3. has not cleared its neighboring region of planetesimals, and 4. is not a satellite. The currently known dwarf planets ordered by increasing distance from the Sun are: Ceres, Pluto, Haumea, Makemake, and Eris. The first space probes to visit dwarf planets were NASA’s Dawn and New Horizons missions. In 2015, Dawn went into orbit around Ceres and New Horizons flew by Pluto. This leaves the SSSBs. A Small Solar System Body is simply a non-stellar object in the Solar System that is neither a planet, nor a dwarf planet, nor a satellite. These currently include most of the Solar System’s asteroids, most TransNeptunian Objects, comets, and other small bodies. The SSSB’s masses are so low that self gravity cannot pull them into a round shape.

massive, luminous sphere of plasma undergoing thermonuclear fusion in its core. The hot gases are held together by the Sun’s gravity. The Sun’s mass is huge, approximately 330,000 Earth masses. The Sun, in fact, contains 99.86% of the Solar System’s mass and dominates it gravitationally. The planets, dwarf planets, and Small Solar System Bodies (SSSB) are quite different from the Sun because they are not self-luminous. You see them shine in the night sky only because they reflect the light emitted by the Sun. Their masses are much lower than the Sun’s; together, their masses account for less than 0.2% of the mass of the Solar System. The planets, dwarf planets, and SSSBs are made of cold gas, rock, and/or ice. The Solar System contains eight planets. The modern definition of what a planet is was decided upon by professional astronomers who are members of the International Astronomical Union (IAU). Accordingly, a “planet” is an astronomical object orbiting a star or stellar remnant that 1. is not massive enough to cause thermonuclear fusion, 2. is massive enough to be rounded by its own gravity, and 3. has cleared its neighboring region of planetesimals. Planetesimals are small bodies left over from the formation of a star’s planetary system. The eight planets are the largest objects in orbit around the Sun. In increasing distance from the Sun, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the most massive planet, at 318 Earth masses, whereas Mercury is the least massive one, at 0.055 Earth masses. Most of the planets in the Solar System possess secondary systems of their own, being orbited

3.1.3 Arrangement Of The Solar System The Sun is virtually at the center of the Solar System. Although the gravity of the planets moves the center of the Solar System slightly away from the center of the Sun, the Solar System’s cen41

Figure 3-2. A schematic representation of the arrangement of the Solar System (which is not drawn to scale). It illustrates the three major regions of the Solar System: the inner region up to the asteroid belt with the Sun and terrestrial planets, the near-outer region containing the gas giants, and the far-outer region with the Kuiper belt and the dwarf planet Pluto. The illustration also shows that the orbital inclinations of the planets are very small. The planets orbit the Sun very close to the ecliptic plane. The inclination of Mercury’s orbit is largest at about 7 degrees. (Wikipedia) ter always resides deep within the Sun. Therefore, while it would be most accurate to say that the Sun and the planets orbit about their common center of mass, because this center of mass is located within the Sun itself it is OK to simply state that the planets orbit the Sun. This distinction may seem a bit pedantic; however, it is through small motions of the common center of mass that extrasolar planets orbiting other stars have been discovered! (More on that in chapter 8.) The Solar System is divided into three major regions. The inner system is composed of the Sun, the terrestrial planets and their moons, close-orbiting asteroids and comets, and the main asteroid belt, which includes the dwarf planet Ceres. Objects in the inner system are almost exclusively composed of solids such as rock, with either no atmosphere or an atmosphere that comprises little of the object’s mass. The inner system’s boundary is the main asteroid belt, which separates it from the near outer system. The near outer system is composed of the gas giant planets, their moons and rings, and aster42

oids and comets that orbit between the main asteroid belt and the Kuiper belt. Objects in the near outer system may have rock, liquid, gas, and ice as significant components. The near outer system’s boundary is defined by the orbit of Neptune. The far outer system is composed of several dwarf planets, the Kuiper belt, the scattered disk, and the Oort Cloud, and comets that orbit between the Kuiper belt and the Oort cloud. Objects in the far outer system may have some rock components, but are mainly composed of ices. Most large objects in orbit around the Sun lie near the plane of Earth’s orbit, known as the “ecliptic.” The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at significantly greater angles to it. All eight planets orbit the Sun in slightly elliptical orbits, although nearly circular orbits, no planet orbits in exactly a circular orbit. The planets and most other objects orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from a long way above Earth’s north pole). There are exceptions, such as Halley’s Comet.

while much smaller than the Sun, is so much closer to Earth than the Sun that its diameter also subtends about half of a degree in our sky. This leads to the interesting phenomenon of “solar eclipses,” whereby the Moon can move into our line of sight in front of the Sun and completely block out the bright disk of the Sun. A solar eclipse allows us to see fainter regions surrounding the Sun, which are also called the Sun’s “atmosphere.” The Sun spins on its axis. When the Sun is viewed from the north (above the Earth’s northern pole) solar rotation is counterclockwise. Because the Sun is made of gas, different regions of its surface can have different rates of rotation. This is called a “differential rotation.” At the equator, it spins at a rate of about once every 25 days. The rate of rotation is observed to be fastest at the equator, and decreases as latitude increases. The rotation rates have been determined by measuring the motion of various features on the solar surface. The first and most widely used tracers are sunspots. The Sun has a magnetic field. The Sun’s magnetic field is responsible for much of the Sun’s surface activity, and contributes to the phenomena you know as space weather.

3.2 The Sun The Sun is the star at the center of the Solar System. Astronomers use the symbol ⊙ as a shorthand designation for the Sun. The Sun’s mass is M⊙ = 1.9891 × 1030 kg (330,000 times Earth). Chemically, most of the Sun’s mass, about three quarters, consists of hydrogen, whereas the rest is mainly helium. Less than 2% consists of heavier elements, including oxygen, carbon, neon and iron, among others. The Sun is very large. It’s radius is R⊙ = 6.960 × 108 m (109 times Earth). As viewed from the Earth, the Sun looks like a bright disk in the sky. The angle that the Sun’s diameter subtends in our sky is about half of a degree. As you move closer to the Sun, it looks bigger; move farther away, and it appears smaller. For example, the angular diameter of the Sun on Mercury is 2.5 times as large as it appears from Earth, but from Neptune, the Sun looks so small that it might be mistaken for just another star in the sky. In a curious coincidence, Earth’s Moon,

Figure 3-3. The structure of the Sun. Nuclear fusion in the core generates the energy which provides the Sun’s luminosity. The energy travels through the Sun to the photosphere, from where it is radiated into space. (Wikipedia) 43

3.2.1 Sunlight Sunlight is electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth’s atmosphere, and is obvious as daylight when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. The Sun is not a transparent cloud of gas, otherwise we would be able to see through it. While the Sun is gaseous throughout, the density of the gases increases toward its center. There is a visible surface of the Sun, which is called the “photosphere.” This is not a solid surface that anything could land or stand on, just a gas layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun. The parts of the Sun above the photosphere are referred to collectively as the solar “atmosphere.” Just like the solar surface, the solar atmosphere is nothing like Earth’s atmosphere, but merely thinner regions of gas that let the light through. There are some portions of the electromagnetic radiation coming from the Sun which get removed, or absorbed, in the solar atmosphere. This causes slight deviations of the solar spectrum from being a perfect black body like the CMB (see chapter 2.2.2, 8.2.4). The spectrum of the Sun as measured from Outer Space is still close enough in shape to that of a thermal radiator that it can be approximated by one. We can use the peak to find the Sun’s surface temperature. The Sun’s spectrum indicates that its surface has a temperature of about 5,800 K, a pretty hot 9,980℉. The peak of the solar spectrum occurs in the middle of the visible wavelengths between 400 t0 700 nm. So, it should come as no surprise that human vision is attuned to the visible wavelengths since the Sun’s energy peaks in this region. The Sun gives off an enormous amount of light energy every second. In physics, the name for energy per second is “power,” and power is measured in Watt. The unit was named after the Scottish engineer James Watt (1736–1819) for his con-

Figure 3-4. The solar spectrum. The top (yellow) spectrum shows a measurement of the solar spectrum from Outer Space. This spectrum is not quite that of a perfect black body, because the solar atmosphere absorbs some of the radiation coming from deeper inside, but it can be approximated as one (grey line). When sunlight travels through Earth’s atmosphere, this causes additional absorption and scattering of radiation, resulting in the bottom (red) spectrum. There are broad spectral regions where molecules absorb solar radiation. Additional light is redistributed by Rayleigh scattering, which is responsible for Earth atmosphere’s blue color. (Wikipedia) tributions to the development of the steam engine. We are pretty familiar with the Watt when buying light bulbs. In your house, you probably have a great number of 60 W light bulbs, and maybe on your desk, one as bright as 100 W. Now compare this to the radiant power output of the Sun, which is called its “luminosity.” The Sun gives off an incredible L⊙ = 3.846× 1026 W. You should never look at the Sun directly without appropriate eye protection. The Sun is just that bright! We all know that the farther away we are from a source of light, the dimmer it looks. This is because after being emitted by the source, radiation spreads out over greater and greater areas the farther away it gets from the source. A simple way to see this is through the use of a flash light. When you point a flash light towards a wall from a very 44

short distance, the circle of light on the wall will appear very bright. Now walk away from the wall, still shining the light on it. As you move further away from the wall, the circle of light on the wall will become larger and larger. But at the same time it will also appear dimmer. As the flash light is moved farther away from the wall, the light spreads out over a larger area thus reducing the strength of light per unit area on the wall. The Sun emits sunlight in all directions. As the light spreads out from the Sun, the strength of its radiation, S, decreases proportionally to the square of the distance. The inverse-square law of radiation can be written as S=

Figure 3-5. A beam of light spreads out from a source. At a distance d, it has spread to cover an area shown as a square labeled A. At twice the distance, 2d, it had spread more to cover an area of four squares of area A, at 3d, it has spread even more, over 9A. When you compare how much light the areas A at different distances capture, the farther they are, the less they get and hence, the brightness of the source falls off with the distance squared. The source looks dimmer. (Wikipedia)

L⨀ 4πd2

and tells us that an area twice the distance from the source of radiation receives only a quarter of the strength of the radiation, three times as distant, one ninth the strength, and so forth. The inverse-square law of radiation is just one inverse square law we encounter in nature. There are several others. Gravity and the electrostatic force, for instance, also fall off in strength with the square of the distance. All planets, dwarf planets, and SSSBs receive energy from the Sun. But, as a result of the inverse-square law, the amount of energy they receive is smaller at greater distances from the Sun. For example, because Neptune is 30 times more distant from the Sun than Earth is, it gets only about 1/900 the amount of sunlight that we do. The amount of energy a planet receives is important not only to make its daytime sky bright. It is also one of the quantities which determines a planet’s temperature. In general, the surface temperature of a planet decreases with increasing distance from the Sun. Venus is an exception to the rule because its dense atmosphere acts as a greenhouse and heats the surface to around 775 K or a hot 880℉. The existence of nearly all life on Earth is fueled by light from the Sun. Plants use the energy

of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as photosynthesis. These sugars are then used as building blocks and in other synthetic pathways that allow the organism to grow. Animals and humans sustain themselves using light from the Sun indirectly by consuming plants or other animals. Humans also exploit solar energy that is stored in fossil fuels, and sometimes collect it directly using solar panels (cf. chapter 10.3.4), for the many purposes that have made our technological societies possible.

3.2.2 Solar Wind The solar wind is a stream of plasma released from the upper atmosphere of the Sun. It consists of mostly energetic electrons and protons. The stream of particles varies in density, temperature, and speed over time and over solar longitude. The solar wind flows outward supersonically to great distances, filling a region known as the heliosphere. The solar wind is the major component of space weather. It is responsible for the aurora, natural light displays in the sky in the Arctic and

years. This cycle has been observed for hundreds of years. The solar cycle was discovered in 1843 by the German astronomer Samuel Heinrich Schwabe (1789–1875). After 17 years of observations he noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk. The point of highest sunspot count during the cycle is known as solar maximum; the point of lowest count is solar minimum. Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They appear dark because they are cooler than the surrounding photosphere. Sunspots expand and contract as they move across the surface of the Sun and can be as small as 16 km (10 mi) and as large as 160,000 km (100,000 mi) in diameter. Larger sunspots are visible from Earth without the aid of a telescope. Sunspots are caused by intense magnetic activity. They usually appear as pairs, with each sunspot having the opposite magnetic pole to the other. The strength of solar radiation varies by small amounts throughout the solar cycle. The solar luminosity is about 0.07 % brighter during solar maximum than during solar minimum. Since sunspots are darker than the surrounding photosphere it might be expected that more sunspots would lead to less solar radiation. However, the surrounding margins of sunspots are brighter than the average, and so are hotter; overall, more sunspots increase the Sun’s brightness. The solar particle emission also varies. Some of these variations involve very large eruptions of particles from the Sun. A coronal mass ejection (CME) is a massive burst of solar wind and magnetic fields rising into space. Near solar maximum, the Sun produces about three CMEs every day, whereas near solar minimum, there is about one CME every five days. It is now understood that the major geomagnetic storms are induced by these CMEs. CMEs are often associated with other forms of solar activity, most notably solar flares. Flares are comparatively smaller solar eruptions that

Figure 3-6. A giant sunspot group near the center of the Sun’s disk as seen on 7 January 2013 by the Solar Dynamics Observatory’s Helioseismic and Magnetic Imager instrument. (NASA/SDO) Antarctic, as well as geomagnetic storms in Earth’s magnetosphere. It is also the cause of sudden disturbances in the Earth’s ionosphere, and is responsible for making the plasma tails of comets point away from the Sun. The solar wind is divided into two components: the slow solar wind and the fast solar wind. The slow solar wind has a velocity of about 400 km/s and a temperature of 1.5×106 K. In contrast, the fast solar wind has a typical velocity of 750 km/s and a temperature of 8×105 K. The slow solar wind is twice as dense and more variable in intensity than the fast solar wind. The slow wind also has a more complex structure, with turbulent regions and large-scale structures. The slow solar wind appears to originate from a region around the Sun’s equatorial belt. The fast solar wind is thought to originate from coronal holes, which are funnel-like regions of open field lines in the Sun’s magnetic field. Such open lines are particularly prevalent around the Sun’s magnetic poles.

3.2.3 Solar Activity The amount of radiation and particles coming from the Sun changes periodically during the “solar cycle” which has an average duration of 11 46

Figure 3-7. A historical record of sunspot number counts for the last 400 years. The period between 1645 and 1715, a time during which very few sunspots were observed, is a real feature, as opposed to an artifact due to missing data. This epoch is now known as the Maunder minimum, after Edward Walter Maunder, who extensively researched this peculiar event. (Wikipedia) served because some of the matter of the fusing nuclei is converted to gamma-ray photons. The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission. This means that fusion generally occurs for lighter elements only, and likewise, that fission normally occurs only for heavier elements. While we have many nuclear fission reactors on Earth, we have not yet been able to control nuclear fusion and harvest its energy on a large scale. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. Fusion was accomplished in 1951 with the Greenhouse Item nuclear test. Nuclear fusion on a large scale in an explosion was first carried out on 1 November 1952, in the Ivy Mike Hydrogen bomb test. Research into developing controlled thermonuclear fusion for civil purposes also began in earnest in the 1950s, and it continues to this day. The aim of researching controlled fusion for the production of electricity has not yet resulted in a viable reactor, because fusion requires two conditions that are difficult to achieve in the lab: high temperatures and close confinement of the material.

also hurl particles away from the Sun. Generally, weak flares do not have associated CMEs, whereas powerful ones do. Most ejections originate from active regions on the Sun’s surface, such as groupings of sunspots. Other forms of solar activity associated with CMEs are prominences. Prominences are arc-shaped plasma ejections that are contained by the Sun’s magnetic field. The cover image of this chapter shows a prominence, in the upper left corner, that was captured on video by NASA’s Solar Dynamics Observatory in 2010. All of these events, prominences, flares, and CMEs, are thought to be the result of a large-scale restructuring of the Sun’s magnetic field. The Sun’s magnetic field changes its polarity approximately every 11 years. This happens at the peak of a solar cycle.

3.2.4 Energy Generation The Sun generates energy in its center. This energy travels through the Sun toward the surface; and when it reaches the photosphere, it streams away from the Sun as sunlight. The source of energy that the Sun taps into is thermonuclear fusion. Nuclear fusion is a nuclear reaction in which two or more atomic nuclei collide at a very high speed and join to form a new type of atomic nucleus. During this process, matter is not con47

The origin of the energy released in fusion of light elements is due to an interplay of two opposing forces, the nuclear force which combines together protons and neutrons, and the electrostatic or “Coulomb force” which causes protons to repel each other. The protons are positively charged and repel each other but they nonetheless stick together, due to nuclear attraction. This force, called the “strong nuclear force,” overcomes electric repulsion at very close range. The effect of this force is not observed outside the nucleus, hence the force has a strong dependence on distance, making it a short-range force. The electrostatic force, on the other hand, is an inverse-square force, acting over longer distances. It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen. This is because all nuclei have a positive charge due to their protons, and as like charges repel, nuclei strongly resist being put close together. Accelerated to high speeds, they can overcome this electrostatic repulsion and be forced close enough for the attractive nuclear force to be sufficiently strong to achieve fusion. The fusion of lighter nuclei, which creates a heavier nucleus and often a free neutron or proton, generally releases more energy than it takes to force the nuclei together; this is an exothermic

process that can produce selfsustaining reactions. Inside of the Sun and other stars, the conditions of high particle speeds are accomplished because the centers of stars are very hot. The hotter the temperature the faster particles move. But a high temperature also implies a high pressure. The pressurized plasma

expands and some force is necessary to continue to hold the particles in close proximity with one another. The Sun and other stars have no difficulty providing confinement for their plasma core. Their enormous masses hold the hot gases together through the force of gravity.

Figure 3-8. The proton-proton fusion reaction which powers the Sun. The proton and neutron are nuclear particles. A positron is a negatively charged electron. And a neutrino is an electrically neutral particle created in this reaction. Gamma rays are very energetic forms of electromagnetic radiation. (Wikipedia) 48

ciency of close to 100%, meaning all of the mass involved goes into energy. An antimatter engine, if it could be built, would make a fabulous rocket engine!

The Sun uses a nuclear fusion process called the “proton-proton chain.” This was worked out by Hans Bethe (1906 – 2005), and is part of the body of work in stellar nucleosynthesis for which he won the 1967 Nobel Prize in Physics. The overall process involves the fusion of four hydrogen nuclei, or protons, into one helium nucleus plus energy. Comparing the mass of the final helium atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost. This mass has been converted into energy, following Einstein’s famous equation E

3.3 The Planets We know a great deal about the planets, from centuries of telescopic observations and decades of exploration with unmanned space probes. Here, we look at just some highlights of information for each planet. It is very easy to learn more using the wealth of information NASA is providing on the internet.

3.3.1 Mercury

mc 2.

The energy is in the form of gamma rays and neutrinos released during each of the individual reactions. The neutrinos interact very little with matter and immediately leave the Sun. The gamma rays interact many times, getting absorbed, and reemitted, and absorbed, and reemitted, until they leave the Sun as photons of all kinds of lowerenergy electromagnetic radiation. Overall, approximately 3.6×1038 protons are converted into helium nuclei every second releasing energy at a rate of 3.846×1026 W, the solar luminosity. For a long time before Bethe worked out that the Sun is powered by nuclear fusion, people wondered what energy could make it shine. According to geological records, the Sun has been shining for 4.6 billion years; what kind of power source could do that? At first, scientists considered combustion, but energy released from chemical reactions would not last long enough. Chemical reactions are not as powerful in terms of energy released per unit mass as several other processes. Nuclear fusion turns out to have a several million times higher efficiency than combustion. With an efficiency of a measly 0.7%, it has been shown to be what powers the Sun. Only direct conversion of mass into energy, such as that caused by the annihilation of matter and antimatter, is more energetic per unit of mass than nuclear fusion. Matter-antimatter annihilation has an effi49

Mercury’s surface resembles that of Earth’s Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. Mercury is the closest planet to the Sun at a distance of about 58 million km (36 million mi) or 0.39 AU. Recall an astronomical unit, or AU in short, is

Figure 3-9. Mercury bears resemblance to the Moon. This is one of the first images to be returned from MESSENGER’s second flyby of Mercury in 2008. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Only two spacecraft have visited this rocky planet: Mariner 10 in 1974-5 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011. MESSENGER observations have shown that there is water ice present on Mercury, but only in regions of permanent shadow. Those regions are cold enough to preserve water ice, despite the extreme high temperatures experienced by sunlit parts of the planet. More on MESSENGER in chapter 8.4.3. No evidence for life has been found on Mercury. Daytime temperatures can reach 800 degrees Fahrenheit and drop to -290 degrees Fahrenheit at night. It is unlikely that life (as we know it) could survive on this planet.

3.3.2 Venus

Figure 3-10. The forecast for Venus is cloudy, cloudy, cloudy. The thick atmosphere was photographed in ultraviolet light in 1979 by the Pioneer Venus Orbiter. (NSSDC Photo Gallery)

Venus and Earth are similar in size, mass, density, composition, and gravity. This, however, is where the similarities end. Venus is covered by a thick, rapidly spinning atmosphere, creating a scorched world with temperatures hot enough to melt lead and surface pressure 90 times that of Earth (similar to the bottom of a swimming pool 1.5 miles deep). Because of its proximity to Earth and the way its clouds reflect sunlight, Venus is the brightest planet in our sky. Venus is the second closest planet to the Sun at a distance of about 108 million km (67 million mi) or 0.72 AU. One day on Venus lasts as long as 243 Earth days. Venus makes a complete orbit around the Sun (a year in Venusian time) in 225 Earth days. Venus spins backwards (retrograde rotation) when compared to Earth. This means that the Sun rises in the west and sets in the east on Venus. Venus is only a little smaller than Earth. It is a rocky planet. Venus’ solid surface is a cratered and volcanic landscape. Venus is a dim world of intense heat and volcanic activity. Venus’ thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets. Venus has no moons or rings.

the average Earth-Sun distance, so Mercury’s average distance from the Sun is only 39% Earth’s. One day on Mercury (the time it takes for Mercury to rotate or spin once) takes about 59 Earth days. Mercury makes a complete orbit, or revolution, around the Sun (a year in Mercury time) in just under 88 Earth days. Mercury is the smallest planet in our Solar System. It is only slightly larger than the Earth’s Moon. Mercury is a rocky planet, also known as a terrestrial planet. It has a solid, cratered surface, superficially resembling Earth’s Moon. Mercury’s thin exosphere is composed mostly of molecular oxygen (O2), sodium (Na), molecular hydrogen (H2), and also helium (He), calcium, (Ca), magnesium (Mg, discovered with the MESSENGER space mission), and potassium (K). An exosphere is different from a stable atmosphere. Atoms that are blasted off the surface by the solar wind and micrometeoroid impacts create Mercury’s exosphere. Because of solar radiation pressure, the atoms in the exosphere quickly escape into space and form a tail of neutral particles. Mercury has no moons or rings.

Figure 3-11. This Blue Marble Earth montage was created in 2012 from photographs taken by the Visible/Infrared Imager Radiometer Suite instrument on board the new Suomi NPP satellite. It shows many stunning details of our home planet. The Suomi NPP satellite was named after Verner Suomi, commonly deemed the father of satellite meteorology. The composite was created from the data collected during four orbits and digitally projected onto the globe. Many features of North America and the Western Hemisphere are particularly visible on a high resolution version of the image. (NASA) eor(th)e and ertha (Old English) and Erde (German). Earth, our home planet, is the only planet in our Solar System known to harbor life, life that is incredibly diverse. All the things we need to survive exist under a thin layer of atmosphere that separates us from the cold, airless void of space. Earth is the third planet from the Sun at a distance of about 150 million km (93 million mi) or one AU. One day on Earth takes 24 hours, and Earth makes a complete orbit around the Sun in 365 and 1/4 days. Earth is a rocky planet, with a solid and dynamic surface of mountains, valleys, canyons, plains and so much more. What makes Earth different from the other terrestrial planets is that it is also an ocean planet: 70% of the Earth’s surface is covered in oceans. The Earth’s atmosphere is made up of 78% molecular nitrogen (N2), 21% molecular oxygen (O2) and 1% other ingredients. Many planets have

More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98% of the planet’s surface. In 2005, the European Space Agency launched Venus Express to study the atmosphere and surface. The orbiter reached Venus in April 2006, and studied the planet through 2014. Japan’s Akatsuki orbiter, after a troubled history, arrived at Venus in late 2015. Combining the Venus Express and Akatsuki datasets should greatly enhance our knowledge of the planet. No evidence for life has been found on Venus. The planet’s extremely high temperature of almost 900 degrees Fahrenheit make it seem an unlikely place for life as we know it.

3.3.3 Earth All of the planets, except for Earth, were named after Greek and Roman gods and goddesses. However, the name Earth is an English/ German word, which simply means the ground:

over 24 hours. Mars makes a complete orbit around the Sun in 687 Earth days. Mars is a rocky, terrestrial planet. Mars’ solid surface has been altered by volcanoes, impacts, crustal movement, and atmospheric effects such as dust storms. Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), molecular nitrogen (N2) and argon (Ar). It does not possess a global magnetic field. Mars has two moons named Phobos and Deimos. There are no rings around Mars. More than 40 spacecraft have been launched for Mars, from flybys and orbiters to rovers and landers that touched down on the surface of the Red Planet. The first true Mars mission success was Mariner 4 in 1965. While Mars’ surface cannot support life as we know it right now, a key science goal is determining Mars’ past and future potential for life. In 2008, NASA’s Phoenix Mars lander was the first mission to touch water ice in the Martian arctic. Phoenix also observed precipitation, snow falling from clouds, as confirmed by Mars Reconnaissance Orbiter. Soil chemistry experiments led scientists to speculate that the Phoenix landing site had a wetter and warmer climate in the recent past, the last few million years. NASA’s Mars Science Laboratory mission, with its large rover Curiosity, is examining Martian rocks and soil at Gale Crater, looking for minerals that formed in water, signs of subsurface water, and carbon-based molecules called organics, the chemical building blocks of life. That information will reveal more about the present and past habitability of Mars, as well as whether humans could survive on Mars some day.

atmospheres, but only Earth’s is breathable for human beings. Earth has one moon, but no rings. Some facts about the Moon are given ins chapter 2.4.2. The Moon was likely formed after a Mars-sized body collided with Earth early in the Solar System’s history; the debris formed into the most prominent feature in our night sky. Many orbiting satellites study Earth from above as a whole system and together aid in understanding our home.

3.3.4 Mars Mars is known as the Red Planet because iron minerals in the Martian soil are oxidized, or rusty, causing the soil and the dusty atmosphere to look red. Mars is a cold desert world. It is half the diameter of Earth but only a tenth of Earth’s mass. Like Earth, Mars has seasons, polar ice caps, volcanoes, canyons and weather, but its atmosphere is too thin for liquid water to exist for long on the surface. There are signs of ancient floods on Mars, but evidence for water now exists mainly in icy soil and thin clouds. Mars is the fourth planet from the Sun at a distance of about 228 million km (142 million mi) or 1.52 AU. One day on Mars takes just a little

3.3.5 Jupiter The most massive planet in our Solar System, with dozens of moons and an enormous magnetic field, Jupiter forms a kind of miniature Solar System. It resembles a star in composition, but did not grow quite massive enough when it formed to ignite thermonuclear fusion in its core. The planet’s swirling cloud stripes are punctuated by massive storms such as the Great Red Spot,

Figure 3-12. This picture is a composite of images acquired in 2006 by Mars Global Surveyor’s camera. (NASA/JPL-Caltech/MSSS) 52

Figure 3-13. This true-color simulated view of Jupiter is composed of four images taken by NASA’s Cassini spacecraft in 2000. These images were combined and the cylindrical map projected onto a globe in order to illustrate what Jupiter would look like if the cameras used to image this planet had a field-ofview large enough to capture the entire planet. Jupiter’s moon Europa is casting the shadow on the planet. (NASA/JPL/University of Arizona) contains a torus of trapped particles that originate from volcanic ejecta of its moon Io. Jupiter’s four largest moons, Io, Europa, Ganymede, and Callisto, were first observed by the astronomer Galileo Galilei in 1610 using an early version of the telescope. These four moons are known today as the Galilean satellites. In December 1995, NASA’s Galileo spacecraft dropped a probe into one of the dry, hot spots of Jupiter’s atmosphere. The probe made the first direct measurements of the planet’s composition and winds. The probe also studied the moons, and found some surprising facts. Io is the most volcanically active body in the Solar System; Ganymede is the largest moon in the Solar System and the only moon known to have its own magnetic field; and a liquid-water ocean with the ingredients for life may lie beneath the frozen crust of Europa, making it a tempting place to explore further. A total of eight spacecraft have studied Jupiter. NASA’s Juno mission is scheduled to arrive at

which is bigger than Earth and has raged for hundreds of years. Jupiter cannot support life as we know it. However, some of Jupiter’s moons have oceans underneath their crusts that might support life. Jupiter is the fifth planet from the Sun at a distance of about 778 million km (484 million mi) or 5.2 AU. One day on Jupiter takes about 10 hours, while Jupiter makes a complete orbit around the Sun in about 12 Earth years. Jupiter is a gas-giant planet and therefore does not have a solid surface. However, it is predicted that Jupiter has an inner, solid core about the size of the Earth. Jupiter’s atmosphere is made up mostly of molecular hydrogen (H2) and helium (He). Jupiter has 50 known moons, with an additional 17 moons awaiting confirmation of their discovery. It also has a faint ring system that was discovered in 1979 by the Voyager 1 mission. Jupiter is surrounded by a strong magnetosphere which

largest moon in the solar system; only Jupiter’s moon Ganymede is bigger. Titan is shrouded in a thick, nitrogen-rich atmosphere that might be similar to what Earth’s was like long ago. Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth. Saturn also has many smaller icy satellites. From Enceladus, which has an ocean beneath its water ice crust, to Iapetus, with one hemisphere darker than asphalt and the other as bright as snow, each of Saturn’s satellites is unique. Saturn has the most extensive ring system of all our Solar System’s planets. It is made up of seven rings with several gaps and divisions between them. Five missions have been sent to Saturn. Since 2004, Cassini-Huygens has been exploring Saturn, its moons and rings. It is the first mission to orbit Saturn. The Huygens probe descended through Titan’s atmosphere in January 2005, collecting data on the atmosphere and surface. Cassini studied the rings during Saturn’s autumnal equinox, when the Sun was shining directly on the equator, through 2010. The mission is expected to operate until September 2017.

Figure 3-14. A swing high above Saturn by NASA’s Cassini spacecraft revealed this view of Saturn and its main rings. The image is in natural color, as human eyes would have seen it. This mosaic was made from 36 images in three color filters obtained by Cassini’s imaging science subsystem on 10 October 2013. (NASA/JPL) Jupiter in 2016. The orbiter will use massive solar panels to power its suite of science instruments. Juno will be the first solar-powered spacecraft designed to operate at such a great distance from the Sun, and the second spacecraft to orbit Jupiter.

3.3.6 Saturn All four gas giant planets have rings, made of chunks of ice and rock, but none are as spectacular or as complicated as Saturn’s. Like the other gas giants, Saturn is mostly a massive ball of hydrogen and helium. Saturn cannot support life as we know it. However, some of Saturn’s moons have conditions that might support life. Saturn is the sixth planet from the Sun at a distance of about 1.4 billion km (886 million mi) or 9.55 AU. One day on Saturn takes 10.7 hours. One year in Saturnian time equals 29.5 Earth years. Saturn is a gas-giant planet. Like Jupiter, it is thought to possibly have an Earth-sized solid core. It’s atmosphere is made up mostly of molecular hydrogen (H2) and helium (He). Saturn has 53 known moons with an additional 9 moons awaiting confirmation of their discovery. Saturn’s largest satellite, Titan, is a bit bigger than the planet Mercury. Titan is the second-

3.3.7 Uranus Uranus is the only giant planet whose equator is nearly at right angles to its orbit. A collision with an Earth-sized object may explain the unique

Figure 3-15. NASA’s Voyager 2 spacecraft flew closely past distant Uranus, the seventh planet from the Sun, in January 1986. (NASA)

was near its southern summer solstice, with the southern hemisphere bathed in continuous sunlight and the northern hemisphere radiating heat into the blackness of space. Hubble Space Telescope images helped to discover two delicate rings far from the planet and two new moons.

3.3.8 Neptune

Figure 3-16. Image of Neptune by Voyager 2. (NASA) tilt. Nearly a twin in size to Neptune, Uranus has more methane in its mainly hydrogen and helium atmosphere than Jupiter or Saturn. Methane gives Uranus its blue tint. Uranus is the seventh planet from the Sun at a distance of about 2.9 billion km (1.8 billion mi) or 19.19 AU. One day on Uranus takes about 17 hours; Uranus makes a complete orbit around the Sun in about 84 Earth years. Like Venus, Uranus has a retrograde rotation (east to west). Uranus is an ice giant. About 80% or more of the planet’s mass is made up of a fluid of icy materials such as water (H2O), methane (CH4). and ammonia (NH3) above a small rocky core. Uranus has an atmosphere which is mostly made up of molecular hydrogen (H2) and helium (He), with a small amount of methane (CH4). It cannot support life as we know it. Uranus has 27 moons. The moons are named after characters from the works of William Shakespeare and Alexander Pope. Uranus has faint rings. The inner rings are narrow and dark and the outer rings are brightly colored. Voyager 2, the only spacecraft to visit Uranus, imaged a bland-looking sphere in 1986. When Voyager flew by, the south pole of Uranus pointed almost directly at the Sun because Uranus 55

The giant Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. In 2011 Neptune completed its first orbit since its discovery in 1846. Neptune is the eighth planet from the Sun at a distance of about 4.5 billion km (2.8 billion mi) or 30.07 AU. At times during the course of Neptune’s orbit, dwarf planet Pluto is actually closer to the Sun, and us, than Neptune. This is due to the highly elliptical shape of Pluto’s orbit. One day on Neptune takes about 16 hours. Neptune makes a complete orbit around the Sun in about 165 Earth years. Neptune is a sister giant to Uranus. Neptune is mostly made of a combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core. Neptune’s atmosphere is made up mostly of molecular hydrogen (H2), helium (He) and methane (CH4). Neptune’s moons are named after various sea gods and nymphs in Greek mythology. Neptune has 13 confirmed moons, six of which were discovered by Voyager 2, and 1 more awaiting official confirmation of discovery. The moon Triton is extremely cold, with surface temperatures of about about 38 K (-391℉). Despite this deep freeze at Triton, Voyager 2 discovered geysers spewing icy material upward more than 8 km (5 mi). Triton’s thin atmosphere, also discovered by Voyager, has been detected from Earth several times since, and is growing warmer. Scientists do not yet know why. Neptune has six rings. Voyager 2 is the only spacecraft to have visited Neptune. Like the other giant planets, Neptune cannot support life as we know it.

Figure 3-17. This illustration helps put the sizes of dwarf planets and asteroids in perspective. Earth’s Moon and the state of California’s sizes are provided as an easy visual reference. Ceres and Pluto are dwarf planets. Mathilde, Lutetia and Vesta are asteroids in the main asteroid belt. (NASA)

their location in the Solar System. Pluto, Haumea and Eris have moons. There are no known rings around dwarf planets. Dwarf planets Pluto and Eris have thin atmospheres that expand when they come closer to the Sun and collapse as they move farther away. It is possible that dwarf planet Ceres has an atmosphere. On 22 January 2014, scientists of the European Space Agency reported the detection, for the first time, of water vapor on Ceres, the largest object and only dwarf planet in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. NASA’s Dawn spacecraft entered into orbit around Ceres on 6 March 2015. Its New Horizons space probe flew by Pluto on 14 July 2015 (cf. also, chapter 8.3.4). Expect many new results on Ceres and Pluto in the coming years.

3.4 The Dwarf Planets Pluto was considered a planet until 2006. The discovery of similar-sized worlds deeper in the distant Kuiper belt sparked a debate that resulted in a new official definition of a planet that did not include Pluto. The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun. It was brought about by an increase in discoveries of transNeptunian objects (objects that are farther away from the Sun than Neptune) that rivaled Pluto in size, and by the discovery of an even more massive object, Eris. It is estimated that there are hundreds to thousands of dwarf planets in the Solar System. The IAU currently recognizes five: Ceres, Pluto, Haumea, Makemake, and Eris. Ceres orbits the Sun at 2.77 AU in the main asteroid belt. Pluto (39.48 AU), Haumea (43.13 AU), and Makemake (45.79 AU) orbit in the Kuiper belt. Eris is located 67.67 AU away from the Sun in the scattered disk, needing 557 years for one orbit. The orbits of the dwarf planets are tilted away more from the ecliptic than those of the planets. Most dwarf planets spin on their axes in a direction that is the same as that of Earth. However, like Venus and Uranus, Pluto exhibits retrograde rotation. Dwarf planets are solid rocky and/or icy bodies. The amount of rock vs. ice depends on

3.5 Small Solar System Bodies The SSSBs were introduced in 2006 by the IAU. The SSSBs include the asteroids and comets. Some of the larger SSSBs may be reclassified in the future as dwarf planets, pending further examination. It is also not presently clear whether a lower size bound will be established as part of the definition of SSSBs in the future, or if it will encompass all material down to the level of meteoroids, the smallest macroscopic bodies in orbit 56

tendency to otherwise fly out of the orbit. The Jupiter Trojans form the most significant population of Trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune Trojans, and NASA announced the discovery of an Earth Trojan in 2011. Near-Earth asteroids are objects with orbits that pass close by that of Earth. Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth’s orbit, and near-Earth asteroids that approach Earth’s orbital distance to within about 45 million km (28 million mi) and may pose an impact danger. Radar is a valuable tool in detecting and monitoring potential impact hazards. By reflecting transmitted signals off objects, distances and other information can be derived from the echoes. As of 19 June 2013, 10,003 near-Earth asteroids are known. About one and a

Figure 3-18. NASA’s Dawn spacecraft obtained this image of the giant asteroid Vesta with its framing camera on 24 July 2011. It was taken from a distance of about 5,200 km (3,200 mi). (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA) around the Sun. On a microscopic level there are even smaller objects such as interplanetary dust, particles of solar wind and free particles of hydrogen. Asteroids are solid, rocky and irregular bodies. The three broad composition classes of asteroids are C-, S-, and M-types. The C-type (chondrite) asteroids are most common, probably consist of clay and silicate rocks, and are dark in appearance. The S-types (stony) are made up of silicate materials and nickel-iron. The M-types are metallic (nickel-iron). The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter, and millions of smaller ones. The “Trojan” asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points, see Figure 2-14). There, the gravitational pull from the Sun and the planet are balanced by a Trojan’s

Figure 3-19. Comet Hale-Bopp as seen in the sky over Pazin, Croatia, in 1997 (Philipp Salzgeber/ Wikipedia) 57

comet’s ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and solar wind can blow the coma dust and gas away from the Sun, sometimes forming a long, bright tail. Comets actually have two tails, a dust tail and a gas tail. Comets are thought to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disk, which are two linked flat disks of icy debris beyond Neptune’s Figure 3-20. Comet 67P/Churyumov-Gerasimenko was imorbit at 30 AU and jointly extending out aged by Rosetta’s OSIRIS narrow-angle camera on 3 beyond 100 AU from the Sun. Long-period August 2014 from a distance of 285 km. (ESA/Rosetta/ comets, such as comet Hale-Bopp, whose MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/ UPM/DASP/IDA) orbits last for thousands of years, are thought to originate in the Oort cloud. The half thousand are classified as potentially hazardOort cloud also produces comets that come toous asteroids, those that could pose a threat to ward the Sun only once. Earth. The orbits within the Kuiper belt are relaOver a dozen spacecraft have explored astertively stable, and so very few comets are thought oids. NEAR Shoemaker even landed on an asterto originate there. The scattered disk, however, is oid (Eros). The Dawn mission is the first mission dynamically active, and is far more likely to be the to orbit a main belt asteroid, Vesta. It orbited place of origin for comets. Because of its unstable Vesta from 2011 to 2012, when it departed for nature, astronomers now consider the scattered Ceres. disk to be the place of origin for most periodic Comets are cosmic snowballs of frozen comets in the Solar System, with the centaurs, a gases, rock and dust. In the distant past, people population of icy bodies between Jupiter and Nepwere both awed and alarmed by comets, perceivtune, being the intermediate stage in an object's ing them as long-haired stars that appeared in the migration from the disk to the inner Solar System. sky unannounced and unpredictably. Chinese asEventually, perturbations from the giant planets tronomers kept extensive records for centuries, send such objects towards the Sun, transforming including illustrations of characteristic types of them into periodic comets. The Oort cloud is concomet tails, times of cometary appearances and sidered spherical in shape, and only loosely bound disappearances, and celestial positions. These histo the Solar System, and thus is easily affected by toric comet annals have proven to be a valuable the gravitational pull both of passing stars and of resource for later astronomers. the Milky Way itself. These forces occasionally disEach comet has a tiny frozen part, called a lodge comets from their orbits within the cloud nucleus, often no larger than a few kilometers and send them towards the inner Solar System. across. The nucleus contains icy chunks, frozen Various international spacecraft have flown by gases with bits of embedded dust. A comet warms comets. On 21 November 2014, ESA’s Philae lanup as it nears the Sun and develops an atmosphere, or “coma.” The Sun’s heat causes the 58

Figure 3-21. Planetary configurations (ESO) • the greatest eastern elongation, when the planet is as far east of the Sun as possible and the angles are the same as those mentioned above. For an outer, or “superior” planet like Mars, Jupiter etc., the following configurations occur: • opposition, when the planet is on the opposite side of the Earth from the Sun; • western quadrature, when the planet is to the west of the Sun and the Sun-Earth-planet angle is exactly 90°; • conjunction, when the planet is on the opposite side of the Sun from the Earth, and • eastern quadrature, when the planet is to the east of the Sun and the Sun-Earth-planet angle is again 90°. Here is what all of this means for viewing planets. As we observe planets in the sky from Earth, the inferior planets Mercury and Venus stay close to the Sun, so we always see them near sunrise or sunset. The maximum elongation angle for Mercury is about 28°, that for Venus about

der, part of the Rosetta mission, made the first ever landing on a comet.

3.6 Planetary Configurations The heliocentric Solar System enables us to understand how we, on Earth, see the planets in our sky. The positions of planets in their orbits with respect to the Earth and the Sun are the socalled “planetary configurations.” For Mercury and Venus which are closer to the Sun than Earth and are “inferior planets,” the following configurations occur: • inferior conjunction, when the planet passes between the Sun and the Earth; • greatest western elongation, when the planet is to the west of the Sun so that the angle Sun-Earth-planet is as large as possible (about 28° in the case of Mercury and 45° for Venus); • superior conjunction, when the planet is on the opposite side of the Sun from the Earth; and

48°. In contrast, the superior planets can have eastern or western elongation angles up to 180° east or west of the Sun. What this means is that we can see them in the middle of the night. The planets all shine by reflected sunlight, and half of a planet is always sunlit while the other half is dark. The fraction of the sunlit hemisphere seen from the Earth, however, varies with the planetary configuration. The planets go through phase cycles similar to our Moon. The variety of phases exhibited by inferior planets differ markedly from those of superior planets. The new phase can only occur at inferior conjunction and so can never be seen on the superior planets. The superior planets can never be observed in crescent phase, when less than half of the observable hemisphere is sunlit, whereas inferior planets can. Planetary configurations are important for planning space missions. For example, two types of mission trajectories, namely, conjunction-class and opposition-class, have been discussed in the context of human expeditions to Mars (cf. chapter 10.4.3).

Figure 3-22. Conic sections (Wikipedia)

counts for gravity being due to curvature of spacetime, with orbits following geodesics. For ease of calculation, relativity is commonly approximated by the force-based theory of universal gravitation based on Kepler’s laws of planetary motion. Kepler’s three laws are:

3.7 Kepler’s Laws Kepler’s laws of planetary motion are three scientific laws describing the motion of planets around the Sun. Most planetary orbits are almost circles, so it is not apparent that they are actually ellipses. Calculations of the orbit of the planet Mars first indicated to Kepler its elliptical shape, and he inferred that other heavenly bodies, including those farther away from the Sun, have elliptical orbits also. Kepler’s work improved the heliocentric theory of Nicolaus Copernicus, explaining how the planets’ speeds varied. Isaac Newton showed in 1687 that relationships like Kepler’s would apply in the Solar System to a good approximation, as consequences of his own laws of motion and law of universal gravitation. So Kepler’s laws are part of the foundation of modern astronomy and physics. Current understanding of the mechanics of orbital motion is based on Albert Einstein’s general theory of relativity, which ac-

1. The orbits of the planets are ellipses with the Sun at one focus. 2. The radius vector from a planet to the Sun sweeps out equal areas in equal time. 3. The square of the orbital period of a planet is proportional to the cube of the semimajor axis of its orbit. Kepler’s first law describes the shape of an orbit. An ellipse is a mathematical figure that is part of the “conic sections,” figures that result from slicing a cone with a plane. The conic sections are the ellipse, the parabola, and the hyperbola. They are distinguished by their eccentricity. Those with eccentricity less than 1 are ellipses, those with eccentricity equal to 1 are parabolas, 60

and those with eccentricity greater than 1 are hyperbolas. The circle is a special case of the ellipse with an eccentricity of zero. All of the conic sections are possible shapes for orbits. While real, natural orbits have small natural distortions from being perfect mathematical figures, describing their paths with mathematical figures has great predictive power. It answers questions such as where do you expect a planet to be at a future time in its orbit? The ellipse and the circle produce “closed orbits.” This means, an object on such an orbit will continue to go around and around. Examples for objects on closed orbits are planets, moons, asteroid, and periodic comets. The parabola and hyperbola produce open orbits. These apply to objects that come into the inner Solar System once, and then disappear after swinging around the Sun only once. Examples for objects on parabolic or hyperbolic orbits are single-apparition comets. As two objects orbit each other, the “periapsis” is that point at which the two objects are closest to each other and the “apoapsis” is that point at which they are the farthest from each other. More specific terms are used for specific bodies. For example, “perihelion” and “aphelion” are the closest and farthest points of an orbit around the Sun, while “perigee” and “apogee” are the lowest and highest parts of an orbit around Earth. Kepler’s second law has implications for the orbital speed of a planet. When a planet is near the Sun, near its perihelion, it travels faster, while when it is farther away from the Sun, near its aphelion, it travels slower along its orbital path. Kepler’s third law is a relationship between the size of the semi-major axis of a planet’s orbit and its period. A circle is described by its radius. Ellipses have two mutually perpendicular axes about which the ellipse is symmetric. These axes intersect at the center of the ellipse due to this symmetry. The larger of the two axes is called the “major axis” and the short one is called the “minor axes.” The semi-major axis (denoted by a) and the semi-minor axis (denoted by b) are one half of the major and minor axes, respectively. The relative 61

Figure 3-23. Example eccentricities illustrating different conic sections. The grey figure is a circle, the red one an ellipse, the green one is a parabola, and the blue one, a hyperbola. The orbits of the planets are ellipses with the Sun in one focus, F. The other focus of the ellipse is empty. (Wikipedia) sizes of a and b determine the eccentricity of an ellipse. Using the letter P for period, the 3rd law can be expressed mathematically in the following way: P 2 ∝ a 3. Kepler’s third law compares the motions of planets in orbits of different sizes. A planet that is farther from the Sun not only has a longer path than a closer planet, but it also travels slower. Therefore, the larger a planet’s orbit, the longer the planet takes to complete it. What the law also means is that for every planet in our Solar System, the ratio of their period squared to their semi major axis cubed is the same constant value. We often use this law to express orbital periods or orbital sizes of other Solar System objects relative to Earth’s, such that

P2 (1

year)2

a3 (1

AU )3

manuel Kant, and Pierre-Simon Laplace. Its subsequent development has interwoven a variety of scientific disciplines including astronomy, physics, geology, and planetary science. Since the dawn of the space age in the 1950s and the discovery of extrasolar planets in the 1990s, the model has been both challenged and refined to account for new observations.

So now, if you know the orbital period of another planet in years, you can calculate its semi-major axis in AU, and vice versa. Or you could calculate the orbits of an artificial spacecraft around the Sun.

3.8.1 Sun’s Formation

3.8 The Origin of the Solar System How did our Solar System come into being? We already know that our Sun was not a first generation star that formed shortly after the Big Bang, but one that formed later, when chemical elements we find in our Solar System had been produced by thermonuclear fusion in the lives and deaths of other stars. Our current idea of how the Solar System may have formed is called the “solar nebula hypothesis.” The Sun, like other stars, is thought to have formed from interstellar gas and dust. The formation of the Solar System is estimated to have begun 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small bodies formed. A meteorite is a solid piece of debris, from such sources as asteroids or comets, that originates in Outer Space and survives its impact with the Earth’s surface. You can sometimes see them enter Earth’s atmosphere as “shooting stars.” Meteorites raining down on Earth give us free samples of material from the Solar System. The oldest inclusions found in meteorites, thought to trace the first solid material to form in the pre-solar nebula, are 4.5682 billion years old, which is one definition of the age of the Solar System. The widely accepted model, known as the solar nebular hypothesis, was first developed in the 18th century by Emanuel Swedenborg, Im62

The solar nebular hypothesis maintains that the Solar System formed from the gravitational collapse of a fragment of a giant molecular cloud. The cloud itself had a size of about 70 ly, while the fragments were roughly 3 ly across. The further collapse of the fragments led to the formation of dense cores 2,000–20,000 AU in size. One of these collapsing fragments would form what became the Solar System. The composition of this region with a mass just over that of the Sun was about the same as that of the Sun today, with hydrogen, along with helium and trace amounts of lithium produced by Big Bang nucleosynthesis, forming about 98% of its mass. The remaining 2% of the mass consisted of heavier elements that were created by nucleosynthesis in earlier generations of stars. Because of a principle called “conservation of angular momentum,” the nebula rotated faster as it collapsed. An example for the conservation of angular momentum is an ice skater. When she performs a pirouette, she pulls in her arms, making her body spin up. The solar nebula started with a small amount of spin, and as it contracted, its center spun up. As the material within the nebula condensed, the atoms within it began to collide with one another more and more. The center, where most of the mass collected, became increasingly denser and hotter. Over about 100,000 years, the competing forces of gravity, gas pressure, magnetic fields, and rotation caused the contracting nebula to flatten into a spinning protoplanetary disk with a diameter of about 200 AU and form a hot, dense protostar at the center.

Figure 3-24. The winter constellation Orion, the celestial hunter, with his belt and sword. Even with a small telescope you can see the Orion Nebula, which is a place 1,500 ly away were new stars are currently forming. (NASA/STScI)

Figure 3-25. A close-up of the Orion Nebula made by combining images from NASA’s Hubble and Spitzer Space Telescopes. It is estimated that this cloud contains 1,000 stars which are in the process of formation. Swirls of green in Hubble’s ultraviolet and visible-light view reveal hydrogen and sulfur gases that have been heated and ionized by intense ultraviolet radiation from the Trapezium’s stars. The Trapezium stars are the yellow smudge near the center of the image. Meanwhile, Spitzer’s infrared view exposes carbon-rich molecules. These organic carbon molecules have been illuminated by the Trapezium’s stars, and are shown in the composite as wisps of red and orange. (NASA/JPL-Caltech/STScI) At this point in its evolution, the Sun had not yet started its thermonuclear fusion. It is thought to have been a type of star called a “T Tauri star.” Within 50 million years, the temperature and pressure at the core of the Sun became so great that its hydrogen began to fuse, creating an internal source of energy that countered gravitational contraction until hydrostatic equilibrium was achieved. “Hydrostatic equilibrium” means that the inward pull of self-gravity is balanced by the outward push of the gas pressure. As a result, the Sun stabilized. It stopped to contract. Thermonuclear fusion marked the Sun’s entry into the prime phase of its life, known as the main sequence. “Main sequence” stars derive energy from the fusion of hydrogen into helium in their cores. The Sun remains a main sequence star today. 63

pounds with high melting points, such as metals (like iron, nickel, and aluminum) and rocky silicates. These rocky bodies would become the terrestrial planets (Mercury, Venus, Earth, and Mars). These compounds are quite rare in the Universe, comprising only 0.6% of the mass of the nebula, so the terrestrial planets could not grow very large. The terrestrial embryos grew to about 0.05 Earth masses and ceased accumulating matter about 100,000 years after the formation of the Sun; subsequent collisions and mergers between these planet-sized bodies allowed terrestrial planets to grow to their present sizes. The giants (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the “frost line,” the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the Jovian planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements. Planetesimals beyond the frost line accumulated up to four Earth masses within about 3 million years. Theorists think it is no accident that Jupiter lies just beyond the frost line. The frost line acted as a barrier that caused material to accumulate rapidly at ~5 AU from the Sun. This excess material coalesced into a large embryo (or core) on the order of 10 Earth masses, which began to accumulate an envelope via accretion of gas from the surrounding disk at an ever increasing rate. Finally topping out at 318 Earth masses, this massive body became Jupiter. Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume. Uranus and Neptune, the icy giants, are thought to have formed after Jupiter and Saturn did, when the strong solar wind had blown away much of the disk material. The solar wind ended the formation of the planets. After between three and ten million years, the young Sun’s solar wind would have cleared all

Figure 3-26. The protoplanetary disk around the T-Tauri star HL Tauri, located 450 ly away in the constellation Taurus, was detected in radio light with the Atacama Large Millimeter/submillimeter Array (ALMA), and is shown here in false colors so that we can see it. These new ALMA observations reveal substructures within the disk that have never been seen before and even show the possible positions of planets forming in the dark patches within the system. (ALMA/ESO/NAOJ/ NRAO)

3.8.2 Formation Of The Planets The planets are thought to have formed from the disk-shaped cloud of gas and dust left over from the Sun’s formation. The currently accepted method by which the planets formed is known as “accretion,” in which the planets began as dust grains in orbit around the central protostar. Through direct contact, these grains formed into clumps up to 200 m in diameter, which in turn collided to form larger bodies, called “planetesimals” of ~10 km in size. These gradually increased through further collisions, growing at the rate of centimeters per year over the course of the next few million years. The inner region of the Solar System inside of 4 AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals that formed there could only form from com64

Figure 3-27. A schematic summary of the steps in the formation of the Solar System (NASA/Space Place) future of the Sun. First, we have a large amount of observational data on other stars which are similar to our Sun. Second, we understand gravity, properties of plasma, and thermonuclear fusion processes well enough that we can make computer models of how the Sun works, and run simulations into the past and into the future. This has allowed us to piece together an understanding of the lifecycle of stars like our Sun. The Sun is about halfway through its mainsequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. The Sun will spend a total of approximately 10 billion years as a main-sequence star. When the Sun has used up hydrogen in its core, it will begin to change. It will exit the main sequence in approximately 5.4 billion years and start to turn into a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of the Solar Systemâ&#x20AC;&#x2122;s inner planets, including Earth. Even before it becomes a red giant, the luminosity of the Sun will have nearly doubled. Once the core hydrogen is exhausted, the Sun will ex-

the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets. The planets were originally thought to have formed in or near their current orbits. However, this view underwent radical change during the late 20th and early 21st centuries. Currently, it is thought that the Solar System looked very differently after its initial formation: several objects at least as massive as Mercury were present in the inner Solar System, the outer Solar System was much more compact than it is now, and the Kuiper belt was much closer to the Sun. For a long time, our Solar System was the only one we were able to study. With the discovery of many other extrasolar planetary systems, we are in the process of learning much more about how planetary systems form and evolve.

3.9 The Future of the Solar System The future of our Solar System is determined by what will happen with our Sun. There are two lines of inquiry which help us predict the 65

Figure 3-28. A schematic of the lifecycle of our Sun (Wikipedia)

500,000 years – the Sun will only have about half of its current mass. The post AGB evolution is even faster. The ejected mass is becoming ionized as the exposed core reaches 30,000 K. The remaining core material shrinks and heats up, but not enough to fuse carbon. The final naked core temperature will be over 100,000 K; the object is now called a “white dwarf.” The planetary nebula will disperse in

pand into a “subgiant” phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous. This then starts the “red giant branch” (RGB) phase where the Sun will spend around a billion years and lose around a third of its mass. After the RGB phase the Sun now has only about 120 million years of active life left, but they are highly eventful. First the core ignites violently in the “helium flash.” Helium begins to fuse into carbon, and the Sun shrinks back to around 10 times its current size with 50 times the luminosity, with a temperature a little lower than today. When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted, except that this time it all happens faster, and the Sun becomes larger and more luminous. This is the “asymptotic giant branch” (AGB) phase, and the Sun is alternately burning hydrogen in a shell and helium in a deeper shell. After about 20 million years on the early AGB, the Sun becomes increasingly unstable, with rapid mass loss and thermal pulses that increase the size and luminosity for a few hundred years every 100,000 years or so. For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make a “planetary nebula.” By the end of that phase – lasting approximately

Figure 3-29. The size of the current Sun, now in the main sequence phase, compared to its estimated size during its red giant phase in the future. It is predicted to swell to a diameter that is twice as large as Earth’s current orbit. (Wikipedia) 66

about 10,000 years, but the white dwarf will survive for trillions of years before fading to black as fusion has burned out and it cools.

3.9.1 Fate Of The Earth The first to go are Earth’s water and most of its atmosphere. During the Sun’s life on the main sequence, the Sun is becoming more luminous (about 10% every 1 billion years, at the present time). The surface temperature of the Sun is almost constant. The increase of luminosity is essentially due to a slow increase in the solar radius. The increase in solar luminosity is such that in about another billion years Earth’s water will evaporate and escape into space, rendering it inhospitable to all known terrestrial life. Later on, Earth is not expected to survive the Sun’s transition into a red giant. At its largest, the Sun will have a maximum radius beyond Earth’s current orbit. By the time the Sun has entered the asymptotic giant branch, the orbits of the planets will have drifted outwards due to a loss of roughly 30% of the Sun’s present mass. Most of this mass will be lost as the solar wind increases. Also, tidal forces, specifically, tidal acceleration will help boost Earth to a higher orbit (similar to what Earth does to the Moon). If it were only for this, Earth would probably remain outside the Sun. However, current research suggests that after the Sun becomes a red giant, Earth will be pulled in owing to tidal deceleration.

Figure 3-30. The Helix Nebula is a planetary nebula in the constellation Aquarius. It is one of the most nearby planetary nebulae, only about 700 ly away from us. The composite picture is a seamless blend of ultra-sharp images from NASA’s Hubble Space Telescope combined with the wide view of the Mosaic Camera on the National Science Foundation’s 0.9-meter telescope at Kitt Peak National Observatory near Tucson, Arizona. A planetary nebula is the glowing gas around a dying, Sun-like star. (NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T. A. Rector (NRAO)) 67

25.Why will the Sun leave the main-sequence phase of its life in 5.4 billion years? 26.What is the size of the Sun in its RGB phase, and what does that mean for Earth?

3 Test Your Understanding I. A NSWER IN A FEW SENTENCES . 1. What is the heliocentric Solar System? 2. What types of objects are found in the Solar System? 3. What is the difference between a planet and a dwarf planet? 4. What are the three major regions of the Solar System? 5. Why is it not possible to look through the Sun? 6. What is the difference between the Sun’s photosphere and its atmosphere? 7. What is luminosity? 8. How does an inverse-square law work? 9. What are sunspots, and how do they change through the solar cycle? 10.Where does the energy released in nuclear fusion come from? 11. How can stars achieve nuclear fusion when it is so difficult to make it work in a lab? 12.Do all planets of the Solar System have an atmosphere? 13.What is the difference between the shape of a planet and that of a SSSB? 14.What causes the tail of a comet? 15.What is the phase of an outer planet when you see it at opposition? 16.What are the conic sections? 17.Which Solar System bodies can be on hyperbolic orbits and how do they get to be on such orbits? 18.What are the names for the closest and farthest points in an elliptical orbit, and how are these points called for a solar orbit, an Earth orbit? 19.What are Kepler’s three laws of motion? 20.What is the implication of Kepler’s first law for the distance between the Earth and the Sun? 21.What does Kepler’s 2nd law imply for the motions of the planets around the Sun? 22.Why is Kepler’s 3rd law useful? 23.What is hydrostatic equilibrium? 24.How did the planets form?

II. C ALCULATE T HE A NSWER . 1. An asteroid orbits the Sun at 1/3 AU. How much more or less light from the Sun does the asteroid receive per unit area as compared with the Earth? 2. A comet receives 10,000 times less solar radiation per unit area than the Earth does. How far away from the Sun is the comet’s orbit? 3. You are 1 m away from a 100 W lamp. Your friend is 10 m away from the lamp. How bright will the lamp appear to your friend? 4. An asteroid orbits the Sun at a distance (semimajor axis) of 4 AU. What is the orbital period of the asteroid? 5. A SSSB has an orbital period of 27 years. What is the distance of the SSSB from the Sun? 6. Calculate the ratio of the square of the orbital period over the cube of the semi-major axis for the planets Mercury and Jupiter. How do their ratios compare to the Earth’s ratio?

Coordinates and orientation of the Apollo 8 Command Module (NASA)

SPACE NAVIGATION Everything in space moves. The Sun, the planets, space probes are all in motion through space. How can we describe where objects are located relative to one another, and with respect to Earth?

4.1 Guidance, Navigation and Control There are no highways in space! Starting from rocket launch, to the arrival of your space probe at, say, the Moon, how do you make sure the spacecraft goes exactly where you want it to go? You’d need to know how to navigate in space. Navigation includes being able to determine, at any given time, the vehicle’s position and velocity, which is the so-called “state vector,” as well as its “attitude,” which describes its orientation. Navigation can be performed by human pilots. Additionally, in modern aviation as well as in spaceflight, navigation is automated, employing a computerized system called an “autopilot” to perform navigation, guidance, and to even control the vehicle. Guidance, navigation, and control (GNC) systems are an important subsystem of any spacecraft. GNC systems consist of three essential parts: navigation, which tracks current location, guidance, which leverFigure 4-1. Schematic of the inside of the Apollo Command Module ages navigation data and with instruments such as the space sextant, the scanning telescope, target information to diand the inertial measurement unit, used for navigating the spacerect flight control where craft. (NASA) to go, and control, which accepts guidance commands to effect changes in course, say, by firing thrusters. There are four basic forms of navigation: “Pilotage” is navigation used by pilots. It relies on recognizing landmarks to know where you are. You must either be familiar with those visual references or be able to discover them from a map or aeronautical chart. Pilotage requires a human pilot and is useful in human spaceflight. There are famous examples of astronauts overriding computerized flight systems, such as, Neil Armstrong manually landing Apollo 11’s Moon lander “Eagle” after he noticed that the automated landing would take the craft to the inside of a crater. “Dead reckoning” relies on knowing where you started from plus some form of heading information and some estimate of speed. Dead reckoning begins with a known position, or fix, which is then advanced, mathematically or directly on a map, by means of recorded heading, speed, and 70

Figure 4-2. Two US Navy sailors practicing celestial navigation with a sextant on the aircraft carrier USS Carl Vinson (CVN 70). A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the object’s altitude. Using this measurement is known as “sighting the object” and it is an essential part of celestial navigation. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart. Common uses of the sextant include sighting the Sun at solar noon and sighting Polaris at night (in the northern hemisphere), to find one’s latitude. A sextant can also be used to measure the lunar distance between the Moon and another celestial object (e.g., star, planet) in order to determine Greenwich Mean Time which is important because it can then be used to determine the longitude. (Wikipedia/US Navy) time. The basic formula for dead reckoning is our well-known distance-velocity-time relationship. Dead reckoning can give the best available information on position, but is subject to significant errors due to many factors as both speed and direction must be accurately known at all instants for position to be determined accurately. A special case of dead reckoning uses “inertial navigation systems” (INS). The operation of an INS depends upon Newton’s laws. For example, gyroscopes and accelerometers on board determine the orientation and speed of the spacecraft (cd. chapter 5.2.1). For most of our history, we humans have used the stars to find our way. “Celestial navigation” involves measuring the time and the angles between the horizon and known celestial objects

(e.g., the Sun, Moon, a planet, or a star). It traditionally employed a device called a sextant with which sailors, pilots, and astronauts could measure angles, and a clock. Sights on two celestial bodies result in lines intersecting at the observer’s geographic position. Celestial navigation has been adapted for interplanetary navigating using sensors called “star trackers.” There is a famous feat of celestial navigation executed by the astronauts on the Apollo 13 mission. Apollo 13 had an explosion on board, remembered by Jack Swigert’s famous words, “Houston, we’ve had a problem.” Apollo 13 aborted its lunar mission and returned to Earth, but the craft’s trajectory would have lead to a shallow reentry into Earth’s atmosphere; and it would have bounced back out into space. Due to debris from the explosion, the Apollo 13 astro-

for which the coordinates are used. We sometimes give an object’s position relative to the Earth’s surface. This is used when we want to know which landmasses a satellite will fly over, for example. But in other cases, it can be inconvenient because Earth spins on its axis so positions referenced relative to a surface point change rapidly with time. We sometimes give positions relative to the center of the Earth. This would be used in describing the orbit of an Earth-orbiting satellite. But since Earth orbits the Sun, these coordinates change throughout the year. In some cases, for example, for deep space probes, we reference coordinates relative to the center of the Sun. Similar considerations apply when we want to define coordinates for use by a space probe. For example, the computer on a space probe needs to know where the Earth is. Not only that, it also needs to know what it’s orientation within its flight path is relative to Earth. This is also called the “attitude” of the spacecraft. The spacecraft needs to know this in order to, for example, point its antenna toward the Earth so we can communicate. The choice of reference location, surface of the Earth, center of the Sun, or center of the spacecraft, depends on what the application is. When defining coordinates in Outer Space, we often make use of the fact that we know an object’s distance (cf. 2.2.3), or at least direction, and use that as one spatial coordinate. The other two spatial coordinates then are used to give the position of the object on the sky. The many different ways in which we can give spatial coordinates are detailed in chapter 4.3. When describing a moving object, such as a satellite as seen from Earth, or an object’s position relative to a moving reference, such as the spinning Earth, we must also consider time as the fourth coordinate. The astronomical motions of objects have been used to define time, and they also determine larger intervals of time which we encode in a calendar. Time and calendar keeping are discussed further in chapter 4.4. Spacecraft time is discussed in chapter 4.4.4.

nauts lost the ability to sight any stars as was their navigational custom. Instead, they held course by fixing on Earth’s terminator. There is a great film, “Apollo 13” with Tom Hanks, based on this story. “Radio navigation” relies on radio transmitters with known locations. The basic principles are measurements to or from these radio beacons, which can yield a vessel’s direction, distance, and velocity relative to the beacon. Radio navigation is very much used in spaceflight. But on Earth, landbased radio navigation is nowadays used less because of satellite based GPS (chapter 4.5). The Global Positioning System, better known simply as GPS, sends several signals that are used to decode the position and distance of the satellite. Comparing the signals from several satellites allows you to find your geographic position . GPS has better accuracy than any previous land-based system, is available at almost all locations on the Earth, can be implemented cheaply with modern electronics, and requires only a few dozen satellites to provide worldwide coverage. Russia and the European Union also operate satellite navigation systems similar to the US’s GPS. The four different forms of navigation, pilotage, dead reckoning, celestial navigation and radio navigation, can be used in combination, and often are. For example, a space probe might have an INS, but also use a star tracker to reduce positional errors.

4.2 Coordinates We need to agree on some way of how to describe where objects are located in Outer Space. Spatial coordinates and timing conventions are adopted in order to consistently identify locations and motions of spacecraft orbiting planets or traversing interplanetary space. Without these conventions it would be impossible to navigate the Solar System. There are three dimensions of space: forward/back, left/right, and up/down. And there is one dimension of time: past/future. Together, they are often labeled x, y, z, and t. When giving the coordinates of an object in space relative to Earth, it depends on the purpose 72

4.3 Coordinate Systems Spatial coordinate systems are used to specify positions in space. Rectangular or Cartesian coordinates employ three perpendicular axes labeled x, y, and z. The fundamental plane is the x, y plane and the primary direction is the z-axis. Spherical coordinates are an alternative to giving the coordinates of an object, by noting its radial distance from the origin and two angles.

Figure 4-3. Here is an example of employing rectangular coordinates and angular coordinates. For simplicity, we use only two dimensions. The location of point p relative to the origin is the same in both coordinate systems. We just describe that location differently depending on the coordinate system that we use. One way to specify the location of point p is to measure out from the yaxis, parallel to the x-axis, to obtain a distance xp. And then we measure up from the x-axis, parallel to the y-axis to obtain yp. The pair of coordinates (xp, yp) describe the location of point p relative to the origin. Another way to specify the location of point p would be to directly measure the radial distance r between the origin and point p. We then pick a reference line that goes through the origin and measure the angle theta formed by the reference line and a line going through point p. On the figure, we have made the reference line lie along the x-axis. The coordinate pair (r, theta) also uniquely describes the location of point p. We can define functions that let us switch between the two descriptions. Those are termed “coordinate transformations.” (NASA)

4.3.1 Geographic Coordinate System Geographic coordinates are used to give locations on the surface of Earth. Geographic latitudes and longitudes are angles measured from 73

Figure 4-4. Schematic showing the definition of latitude (ϕ) and longitude (λ) of a location on the globe. The latitude of a location is counted from the Equator, the longitude is counted from the Greenwich, or Prime Meridian. (Wikipedia) the center of Earth. They are constructed with respect to the rotation axis of Earth as the z-axis. The primary reference points are the North and South Pole where the axis of rotation of the Earth intersects the surface. The plane through the center of the Earth and orthogonal (or perpendicular) to the rotation axis is the x-y plane. It intersects the surface in a great circle called the Equator. A great circle is the intersection of a sphere and a plane which passes through the center point of the sphere. The Equator divides the globe into northern and southern hemispheres. The latitude of a point on the surface is the angle between the equatorial plane and the radius to that point. The Equator has a latitude of 0°, the North Pole has a latitude of 90° north (written 90° N or +90°), and the South Pole has a latitude of 90° south (written 90° S or −90°). Circles parallel to the equatorial plane intersect the surface in circles of constant latitude; these are the “parallels.” Latitude is abbreviated Lat, ϕ , or phi. The longitude, abbreviated Long, λ, or lambda, of a point on the Earth’s surface is the an-

Figure 4-5. A flat map of Earth using the Mollweide projection. (Wikipedia/NASA) gle east or west from a reference “meridian” to another meridian that passes through that point. Meridians are great circles which converge at the North and South Poles. A great circle which passes through the Royal Observatory, Greenwich (a suburb of London, UK), was chosen as the international zero-longitude reference line, the Prime Meridian. Places to the east are in the eastern hemisphere, and places to the west are in the western hemisphere. The antipodal meridian of Greenwich (the other half of the same great circle that the Prime Meridian is on) is both 180°W and 180°E. It marks the international date line. As an example, the geographic location of Pittsburgh can be written 40° 26′ 30″ N, 80° 0′ 0″ W. Latitude and longitude are usually expressed in that sequence, latitude before longitude. Angles are given in degrees, minutes, and seconds. Decimal degrees are an alternative to using degrees, minutes, and seconds. To convert, you need to know that 1 degree is equal to 60 minutes, and 1 minute is equal to 60 seconds. For Pittsburgh, this gives 40.4417° N, 80.0000° W. Latitudes and longitudes can be defined for celestial bodies other than Earth in a similar fashion, by using the rotation axis as reference axis. The combination of latitude and longitude specifies the position of any location on a planet, but does not consider altitude nor depth. If you are interested in the height of a location above sea level of Earth, for instance, you can add altitude or elevation from the surface as the third spatial dimension. The radial distance from Earth’s center is also used both for very deep positions and for positions of Earth-orbiting satellites. Also, every sur-

Figure 4-6. A flat map of Earth between 82°S and 82°N using the Mercator projection. (Wikipedia/ NASA) face point that is expressed in spherical coordinates ϕ, λ, can be expressed as an x, y, z coordinate as well. The conventional setup uses the right-hand rule: z-axis along the rotation axis, positive northward, x- and y-axis in the plane of the equator, x-axis positive toward 0° longitude (a prime meridian) and y-axis positive toward 90° east longitude. Since it would be inconvenient to carry a spherical globe around, maps use projections of the spherical coordinates into a plane. Different projections are possible. The map of the CMB (Figure 2-8) uses a Mollweide projection of the

Figure 4-7. Terrestrial coordinates North and South Pole, Equator, and Prime Meridian, shown on a flat-map schematic of the Earth. (NASA/ JPL) 74

spherical sky onto a plane. Figure 4-5 shows the Earth globe in this projection. A more common map projection is the Mercator projection, shown in Figure 4-6. Imagine a celestial body located directly over one point on the Earth’s surface. Consider a line connecting the center of the object and the center of the Earth. The point where this line crosses the surface of the Earth is the geographical position of this object. A spacecraft in orbit can therefore have a geographical position. You can construct it by drawing a line from the center of the spacecraft to the center of Earth. This creates the so-called “subsatellite point.” The subsatellite point is identical with the satellite’s geographic position. As the Earth turns on its axis and the spacecraft orbits overhead, a line is created by the vehicle’s flight path over the ground.

Figure 4-9. A launch sequence of the Japan Aerospace Exploration Agency’s (JAXA) H-IIA rocket. After the rocket is launched, it flies out over the Pacific ocean. The ground track shows exactly which geographic locations are directly under the spacecraft’s flight path. (JAXA) This is the satellite’s “ground track.” It consists of the series of subsatellite points, connected.

4.3.2 Horizontal Coordinate System

Figure 4-8. The local vertical connects the center of the spacecraft with the center of the planet it is orbiting. Imagine the point where this line intercepts the planet’s surface. This is the subsatellite point. In this arrangement, the plus-z axis is along the vertical line towards the planetary center, the plus-x axis is in the direction of orbital motion parallel to the local horizontal and the plus-y axis is perpendicular to the orbital plane. When the spacecraft advances in its orbit, subsequent subsatellite points trace out the ground path, or ground track of the satellite. This will cause the spacecraft’s geographic position to change over time. (NASA) 75

The horizontal coordinate system is a topocentric system for giving coordinates on the sky. A “topocentric” position is one relative to an observer on the surface of the Earth. The “celestial sphere” is a tool used to visualize these coordinates. It is an imaginary sphere surrounding the observer. All objects in the observer’s sky can be thought of as projected on the inside surface of the celestial sphere. Just like the coordinate grid we placed on the surface on the Earth, we can draw coordinates on the celestial sphere. Coordinates on the celestial sphere ignore the object’s distance, just like geographic coordinates ignore elevation. The horizontal coordinate system is a celestial coordinate system that uses the observer’s local horizon as the fundamental plane. This coordinate system divides the celestial sphere into the upper hemisphere, which is a dome above the ob-

but from south to east in astronomy. It goes from 0° to 360°. At the North Pole all directions are south, and at the South Pole all directions are north, so the azimuth is undefined in both locations. The zenith distance is the distance from directly overhead (i.e. the zenith). It is sometimes used instead of altitude in some calculations. The zenith distance is the complement of altitude (i.e. 90°-altitude). To any observer, regardless of location, these markers stay in the same positions relative to the observer. The zenith is always directly overhead, the horizon is always level, and so on. Observers standing at different places on Earth have a different horizon and hence, a different view of the sky. An observer in Singapore might see the Sun at the zenith while another observer in New York would not see the Sun at all. In other words, because the horizontal system is defined by the observer’s local horizon, the same object viewed from different locations on Earth at the same time will have different values of altitude and azimuth. There are reference points that are fixed in the sky rather than relative to the observer. These fixed reference points don’t move with respect to the stars, yet different observers see them in different positions relative to their local horizons. They are the basis for the fixed coordinate systems that we discuss later. For now, we will identify only the two most useful of these — the celestial poles and the celestial equator. The celestial equator is an extension of the Earth’s equator onto the celestial sphere. If you stand on Earth’s equator, the celestial equator will always be directly overhead and pass through the zenith. It runs from east on the horizon up to the zenith and down to west on the horizon. Anywhere you stand on Earth, the celestial equator will intersect the east and west points on the horizon. The nearer you are to the Equator, the nearer the celestial equator comes to the zenith. At the Equator, the north and south celestial poles lie on the observer’s horizon. At the North Pole or the

Figure 4-10. The celestial sphere is shown centered on the observer. The point directly overhead is the zenith. The cardinal directions lie along the horizon. The position of an object on the celestial sphere using the horizontal coordinate system is given by its azimuth and its altitude. (Wikipedia) server where objects are visible, and the lower hemisphere where objects cannot be seen since the Earth is in the way. The point directly above the observer in the upper hemisphere is called the “zenith.” The point directly below the observer in the lower hemisphere is called the “nadir.” The four cardinal directions or cardinal points are the directions of north, east, south, and west along the horizon, commonly denoted by their initials: N, E, S, W. North and south are the cardinal directions toward the North and South Poles of Earth. East and west are at right angles to north and south, with east being in the clockwise direction of rotation from north and west being directly opposite east. The great circle through the cardinal directions north and south, the zenith, and the nadir defines the observer’s celestial meridian. Altitude (Alt), sometimes referred to as elevation, is the angle between the object in the sky and the observer’s local horizon. For visible objects it is an angle between 0° and 90°. Azimuth (Az) is the angle of the object around the horizon, usually measured from the north increasing towards the east in navigation, 76

Figure 4-11. Finding Polaris, the North Star, is easy as long as you can locate the “Big Dipper,” which is part of the constellation Ursa Major. Polaris, the North Star, is found by imagining a line from the stars Merak (β) to Dubhe (α) and then extending it for five times the distance between the two pointer stars. (Wikipedia)

Figure 4-12. The north celestial pole has some altitude above the horizon for an observer located on the northern hemisphere of Earth. This altitude is equal to the observer’s latitude. Finding Polaris helps you figure out where N is and its altitude tells you your geographic latitude. (Wikipedia)

South Pole, the celestial equator lines up with the observer’s horizon. An important navigational aide for navigators on the surface of the Earth is the “North Star,” also called “Polaris.” The north celestial pole is currently (but not permanently) very close to the bright star Polaris. Thus, Polaris marks the geographic position of the North Pole on the sky. Polaris is only visible to inhabitants of the northern hemisphere. The asterism “Big Dipper” can be used to find Polaris. The two corner stars of the “pan” (those opposite from the handle) point above the top of the “pan” to Polaris. Here is what makes Polaris so important as a navigational aid: The altitude of Polaris is equal to the geographic latitude of the observer. For example, if a navigator measures the angle to Polaris and finds it to be 40.4417° above the horizon, then they are about 40.4417° north of the equator, on a parallel along which Pittsburgh is located. In the northern hemisphere, only the north celestial pole is visible because the south celestial pole is below the horizon. In the southern hemisphere, only the south celestial pole is visible. There is no corresponding star marking the South Pole on the sky.

As the Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other points appear to revolve around them, completing one circuit per day. Earth spins on its axis from west to east, thus objects rise in the east and set in the west. That is, with exception to what happens for an observer standing exactly at the North or South Pole. Here, the stars just go round and round, never to rise or set. For an observer located at the Equator of Earth, all stars will rise perpendicular to the horizon, reach a highest point overhead, and set. For observers at intermediate geographic latitudes, stars rise and set at an angle to the horizon. The positioning of a celestial object by the horizontal system varies with time, but is a useful coordinate system for locating and tracking objects for observers on Earth. For instance, horizontal coordinates are very useful for determining the rise and set times of an object in the sky. When an object’s altitude is 0°, it is on the horizon. If at that moment its altitude is increasing, it is rising, but if its altitude is decreasing it is setting. A space77

has an inclination of 51.6o. This means that, as it orbits, the farthest north and south of the Equator it will ever go is 51.6o latitude. If you live between ±51.6o latitude, like in Pittsburgh, the ISS can pass directly over your head, through your zenith. If you live north or south of 51.6o, the ISS will never go directly over your head. This includes places like Alaska. Figure 4-14 shows an example of a predicted overhead pass of the ISS from NASA’s “Spot The Station” website. On this particular night, and for this observer, the ISS will be visible for four minutes. This is the optimum viewing period as the Sun reflects off the space station and it contrasts against the darker sky. Figure 4-13. If an observer located near the North Pole of Earth tracked star positions for several hours, they would see star trails as a result of Earth’s diurnal motion. Polaris is overhead. With its small zenith distance, it traces out portions of a small circle around the pole. Stars of progressively larger zenith distances travel around and around the north celestial pole in ever larger circles. Errai and Kocab will not rise or set though the night because of their small zenith distances. They are “circumpolar” stars. (Wikipedia)

4.3.3 Equatorial Coordinate System An object’s coordinates in the equatorial coordinate system, unlike those in the horizontal coordinate system, are independent from the location of the observer on the surface of Earth. The equatorial coordinate system is a widely used celestial coordinate system with the origin at the center of the Earth. The origin at the center of the Earth means the coordinates are geocentric, that is, as seen from the center of the Earth as if it were transparent. The coordinate system, while aligned with the Earth’s rotation axis as the z-axis, does not rotate with the Earth, but remains relatively fixed against the background stars.

craft in orbit around the Earth will also rise and set relative to the observer’s local horizon. It’s visibility from different geographic locations and the path along which it moves across the observer’s sky will depend on its orbit. For example, the International Space Station (ISS) is in an orbit that Figure 4-14. The visibility of the ISS in any observer’s horizontal coordinate system can be computed from the ISS’s state vector. In order to be visible, the station must be above the observer’s local horizon and it must be struck by sunlight. In this example, the word “Appears” marks the altitude and azimuth where the ISS will first be visible. It will travel to reach its greatest height in the sky due N, then disappear at an altitude of 31o when sunlight no longer strikes it. (NASA) 78

• The fundamental plane is in the plane of the Earth’s equator. • The the x-axis is oriented toward the vernal equinox, that is, the place where the Sun crosses the celestial equator in a northward direction in its annual apparent circuit around the ecliptic. • A right-handed convention is used, specifying a y-axis 90° to the east in the fundamental plane. Recall from chapter 3.1.3 that most large Solar System objects in orbit around the Sun lie near the plane of Earth’s orbit, known as the “ecliptic.” When you project the Earth’s orbit onto the celestial sphere, you get a great circle that also goes by the name ecliptic. It marks the apparent path of the Sun in the sky throughout the year. We use the word ecliptic for the plane of Earth’s orbit, as well as for the apparent path of the Sun on the celestial sphere. As the Earth orbits the Sun, the Sun appears to move in front of different stars and you can’t see them because they are in daylight. The opposite constellations are visible in the night sky, and throughout the year, which constellations can be seen in the

Figure 4-15. The equatorial coordinate system in rectangular coordinates. The origin is in the center of the Earth. The z- axis is along Earth’s rotation axis. The x axis is aligned with the vernal equinox and the y axis is at right angles following a right-handed convention. The reference frame does not rotate with the Earth. This means the sky, and the coordinates of objects on the celestial sphere, remain fixed while the Earth spins on its axis underneath. (Wikipedia)

The equatorial coordinate system is set up as follows: • The origin is at the center of the Earth. • The z-axis is along the Earth’s rotation axis. Figure 4-16. The Earth in its orbit around the Sun causes the Sun’s sky position to advance across the constellations along the ecliptic, shown by the red line. The ecliptic is tilted with respect to the celestial equator, marked by the blue-white line. The constellations around the ecliptic are the constellations of the zodiac. (Wikipedia)

Figure 4-17. The equatorial coordinate system in spherical coordinates. The position of a star is given in declination, similar to latitude on Earth, and right ascension, similar to longitudes on Earth. The hour circle which goes through the vernal equinox is the 0 h right ascension circle. It’s purpose is the same as that of the Prime Meridian. (Wikipedia)

night sky varies. The constellations near the ecliptic are the well-known constellations of the “zodiac.” The zodiacal signs are prominent in “astrology,” the belief that the stars influence people’s fate. This is distinct from “astronomy,” the natural science which is the study of celestial objects. The Earth’s rotation axis is not perpendicular to its orbit. It is tilted by about 23.50. Therefore, when you project the ecliptic onto the celestial sphere it makes an angle of 23.50 with the celestial equator. This is also called the “obliquity of the ecliptic.” The celestial equator crosses the ecliptic at two points known as the “equinoxes.” The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south. The crossing from south to north is known as the vernal equinox, or spring equinox, also known as the first point of Aries and the ascending node of the ecliptic on the celestial equator. The crossing from north to south is the autumnal equinox or fall equinox, or descending node. Rectangular equatorial coordinates are sometimes used for giving positions of artificial satellites relative to Earth. They are also called an Earth-centered inertial (ECI) coordinate frame. It is then customary for the x, y and z axes to be designated I, J and K. Astronomers and celestial navigators prefer to use spherical equatorial coordinates. The coordinates are then called “declination” and “right ascension,” and are similar to latitudes and longitudes on the globe. Declination is analogous to terrestrial latitude. Declination, abbreviated dec or δ, measures the angular distance of an object perpendicular to the celestial equator, positive to the north, negative to the south. For example, the north celestial pole has a declination of +90°. Right ascensions is analogous to Earth’s longitude. A projection of the Greenwich Meridian onto the celestial sphere would not work to fix the x-axis of the equatorial coordinate system because it moves with Earth’s spin. The direction to the vernal equinox is used instead. Right ascension, abbreviated RA or α, measures the angular distance of an object eastward along the celestial equator from the vernal equinox to the hour circle passing through the object. Hour circles are great circles that converge at the north and south celestial poles. Right ascension is usually measured in hours, minutes and seconds instead of degrees. There are (360° / 24h) = 15° in one hour of right ascension, 24h of right ascension around the

Figure 4-18. Astronaut John Glenn, the first American to orbit Earth, studies a model of the celestial sphere in this 1962 photograph taken shortly before his historic spaceflight. NASA called this the “Celestial Training Device.” Many schools own model celestial spheres to better illustrate and explain celestial coordinates and motions for students. (NASA)

entire celestial equator. When used together, right ascension and declination are usually abbreviated RA/Dec. The description of the orientation of the reference frame so far was somewhat simplified. In reality the orientation is not quite fixed. A slow motion of Earth’s axis, “precession,” causes a slow, continuous turning of the coordinate system westward about the poles of the ecliptic, completing one circuit in about 26,000 years. The precession of the equinoxes requires the equatorial coordinate system to use a time as well. Astronomer often use the equinox of a particular date, known as an “epoch,” when giving a position. A common epoch date is the start of the year 2000. Here is an example equatorial coordinate set for the North Star, Polaris, for epoch 2000: RA: 02h 31m 49s Dec: +89° 15′ 51″. The equatorial coordinates of bright stars are important navigational aids in spaceflight. In human spaceflight, astronauts can make sightings of stars and measure

Figure 4-19. Star charts marking Apollo 15’s navigation stars. (NASA) 81

Figure 4-20. A space probe takes an image of a star field and matches it to a star-field database it has stored on board. Once the stars in the field are identified, the spacecraft can use their RA and Dec to pin down its attitude relative to them. (NASA)

Ecliptic latitude, b, measures the angular distance of an object from the ecliptic towards the north (positive) or south (negative) ecliptic pole. For example, the north ecliptic pole has a latitude of +90°. Ecliptic longitude, l, measures the angular distance of an object along the ecliptic. Like right ascension in the equatorial coordinate system, 0° ecliptic longitude points from the Earth towards the Sun at the vernal equinox of the northern hemisphere. It is measured positive eastwards in the ecliptic from 0° to 360°. For a complete position, a distance parameter is also necessary. Different distance units are used for different objects. Within the Solar System, Astronomical Units are used, and for objects near the Earth, Earth radii or kilometers are used. A corresponding right-handed rectangular coordinate system is also used occasionally, with the x-axis directed toward the vernal equinox, the y-axis 90° to the east, and the z-axis toward the north ecliptic pole.

angles between them. The Apollo astronauts were trained to find these stars; their computer had stored coordinates. Figure 4-19 shows star maps from the checklist of Apollo 15, which the astronauts had with them to help them identify the correct stars. Robotic spacecraft are equipped with “star trackers.” A star tracker is an optical device that measures the positions of navigation stars using photocells or a camera. For more complex missions, entire star-field databases are used to determine spacecraft orientation. These types of star catalogs can have thousands of stars stored in computer memory on board the spacecraft.

4.3.4 Ecliptic Coordinate System So far the coordinates we have considered were Earth-based, either topocentric or geocentric. But as a space probe travels further and further away from Earth, it may be advantageous to choose yet a different reference. For example, for spaceflight to the planets, a heliocentric coordinate system, one that has its origin in the center of the Sun, is more suitable. The ecliptic coordinate system is a celestial coordinate system commonly used for representing the positions and orbits of Solar System objects. Because many bodies in the Solar System have orbits with small inclinations to the ecliptic, it is convenient to use the ecliptic as the fundamental plane. The coordinates of an object can then be given in ecliptic longitude and latitude.

4.4 Time and Calendar Time and the calendar are about defining and telling intervals of time. In day-to-day life, a clock is consulted for periods less than a day, while the calendar tracks periods longer than a day. Time and calendar definitions have their origin in natural celestial phenomena. The spin of Earth on it’s axis defines the day, the orbit of the 82

Moon around the Earth defines the month, and the orbit of the Earth around the Sun defines the year. The complexity of calendars arises because these cycles of revolution do not comprise an integral number of days, and because astronomical cycles are neither constant nor perfectly commensurable with each other. Celestial phenomena are not strictly periodic, meaning the time intervals do not repeat with the regularity that we need for our modern technological society to function well. Throughout history, we’ve therefore gone from measuring time using astronomical phenomena, to mechanical clocks, and, most recently, to atomic clocks. This history has left us with a legacy of different definitions of time. The basic unit of time, the second, was first discussed in chapter 1.4.2. Early on in history the second was defined by astronomical phenomena as 1/86,400 of a “mean solar day.” The SI second was defined in terms of vibrations of the cesium atom in 1967, and in 1971 the name International Atomic Time (TAI) was assigned to a time scale based on SI seconds. Time-keeping is so critical to the functioning of modern societies that it is coordinated at an international level. TAI as a time scale is a weighted average of the time kept by over 400 atomic clocks in over 60 national laboratories worldwide. Due to the averaging it is far more stable than any one clock would be alone. The clocks at different institutions are regularly compared against each other. The International Bureau of Weights and Measures (BIPM, France), combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible. Coordinated Universal Time (UTC) is based on TAI, and also uses the SI second. But it is adjusted by leap seconds to account for the difference between the definition of the SI second and the actual rotation of Earth. UTC follows TAI with an exact offset of an integer number of seconds, changing only when a leap second is added to keep clock time synchronized with the rotation of

Figure 4-21. Time zones and the international date line, as if you were looking down at Earth from above the North Pole. This example depicts the situation at 04:00 h UTC Tuesday. (Times are approximate, since time zone boundaries generally do not exactly coincide with meridians. Night and day is illustrative only; daylight hours depend on latitude and time of year.) (Wikipedia) the Earth. Like TAI, UTC can only be calculated after all the data have been received. So no clock actually keeps TAI or UTC; they are adjusted and agreed upon after the fact. The difference between TAI and UTC amounts to TAI being more than 35 seconds ahead of UTC. UTC is the basis for civil time and time zones worldwide. The time-of-day expressed by UTC is the time at the Prime Meridian (0° longitude) located near Greenwich, England. There are 24 time zones loosely divided by longitude, where the same local time is kept. The time in different time zones can be expressed as an offset from UTC. For instance, in the United States, eastern standard time (EST) is five hours behind UTC and can be expressed as UTC - 5 h. So you want to know what time it is? The official United States time is determined by the Master Clock at the US Naval Observatory. Go to their website, and you will see the time on the Mas83

Different cultures organize their calendars differently. Many cultures use different baselines for their calendars’ starting years. For example, the year in Japan is based on the reign of the current emperor. Some cultures have based their calendar on the cycle of the phases of the Moon, others use a combined lunisolar calendar.

4.4.1 The Day By convention a new day starts at midnight and is a 24-hour time interval containing 86,400 SI seconds. The word “day” is also used when an observer’s location is in daylight, that is, when the Sun is above the horizon. This distinguishes the approximately 12h of daylight from the roughly 12h of night. The exact number of hours of daylight in a day changes seasonally and with geographic location; this will be discussed in section 4.4.3. When trying to measure the rotation rate of Earth, you need to ask yourself with respect to what you are going to measure this? There are several possibilities that astronomers have come up with. You can measure it with respect to the Sun. This is what sundials, which served as our first clocks, used to do. The shortest shadow that the Sun casts from one daylight period to the next gives the length of the day from noon to noon in the observer’s, or horizontal, system. Astronomers, instead of using sundials, can also measure the interval from one passage of the Sun to the next on the Prime Meridian. When measurements of solar day intervals are averaged over the year to account for variations in Earth’s orbital motion around the Sun and the obliquity of the ecliptic, the Earth’s rotation relative to the Sun defines a time called a “mean solar day.” Nowadays the rotation of the Earth is monitored by the International Earth Rotation Service. It bases the rotation rate on a radio astronomy technique “Very Long Baseline Interferometry” and records the rotation rate of the Earth relative to a very distant type of radio galaxy called a “Quasar.” Eventually, using this information an astro-

Figure 4-22. Earth’s spin on its axis defines the day. The Earth rotates from the west towards the east. As viewed from above the North Pole, the Earth turns counter-clockwise. The time it takes the Earth to spin once around with respect to the Sun defines the solar day. (Wikipedia) ter Clock in UTC. There is also a page for getting the time in your US time zone. Calendars in widespread use today include the Gregorian calendar, which is the de facto international standard, and is used almost everywhere in the world for civil purposes. Due to the Gregorian calendar’s obvious connotations of Western Christianity, non-Christians and even some Christians sometimes replace the traditional era notations “AD” and “BC” (“Anno Domini” and “Before Christ”) with “CE” and “BCE” (“Common Era” and “Before Common Era”). Year zero does not exist. The year 1 BCE is followed by 1 CE. The Gregorian calendar is a solar calendar based in the revolution of Earth around the Sun. A regular Gregorian year consists of 365 days and in a leap year, an intercalary or leap day is added as 29 February making the year 366 days. Normally a leap year occurs every 4 years, but the Gregorian calendar omits 3 leap days every 400 years. The Gregorian year is divided into twelve months, with an irregular month-length pattern. A date is fully specified by the year, the month (identified by name or number), and the day of the month (numbered sequentially starting at 1). 84

nomical time scale called UT1 is computed which reflects Earth’s rotation rate. A day defined by the spin of Earth on its axis does not have a constant length. When the rotation rate of Earth is compared with atomic clocks, it becomes apparent that it is slightly variable. It sometimes speeds up, and sometimes slows down by several microseconds. When averaged over long intervals the trend indicates that it is gradually slowing because of gravitational forces between the Earth and the Moon. This gradual decrease in the rotational rate is causing the duration of the mean solar second to gradually increase with respect to the atomic SI second. Therefore, UTC is occasionally adjusted by one second increments to ensure that the difference between a uniform time scale defined by atomic clocks, TAI, does not differ from the Earth’s rotational time, UT1, by more than 0.9 seconds. Twice yearly, during the last minute of the day of June 30 and December 31, adjustments may be made to ensure that the accumulated difference between UTC and UT1 will not exceed 0.9 s. Therefore, the last minute of the UTC time scale, on the day when an adjustment is made, will have 59 or 61 seconds. Why do we care? We do want to keep our clocks and calendars synchronized to the celestial events that occur in nature. We also want our clocks to give us a time that is regular while at the same time coinciding with where the Sun is in the sky. For instance, our current experience is that noon is always during daylight, roughly in the middle, when the Sun is highest in the sky. If we didn’t adjust UTC, then noon would slowly drift until it became the middle of the night. The spin of Earth on its axis leads to another parameter of interest for spaceflight: the speed of a point on the surface of the Earth. Using the distance-velocity-time relationship, we can understand that a surface point on the Equator has a larger speed than a surface point at an intermediate latitude. The speed of the Earth’s surface at the Equator is about 465 m/s, or about 1,040 mph.

4.4.2 The Month The month is based on the revolution of the Moon around the Earth. Again, there are many ways in which this can be measured. One way to do so is from one new phase to the next new phase of the Moon. One full cycle of Moon phases is called a “lunation” and is approximately 29.53 Earth days long. The lunar phase is the shape of the illuminated (sunlit) portion of the Moon as seen by an observer on Earth. When the Sun and Moon are aligned on the same side of the Earth, the Moon is “new,” and the surface of the Moon visible from Earth is not illuminated by the Sun. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing positions of the Moon and Sun relative to the Earth. Therefore, the portion of the Moon’s hemisphere that is visibly illuminated to an observer on Earth can vary from about 100% (full moon) to 0% (new moon). The four major lunar phases, new, first quarter, full, and third quarter, succeed each other by approximately 7 days. The Moon also spins on its axis. It spins on its axis at the same rate that it orbits the Earth. In other words, a day on the Moon lasts as long as a month on Earth. The side facing Earth is variously sunlit depending on the position of the Moon in its orbit. The lunar “terminator” is the boundary between the illuminated and darkened portions, between day an night. An observer on the surface of the Moon would experience around 14.77 Earth days of sunlight followed by 14.77 days of night. The Moon and the Earth are tidally locked due to gravitational interaction. Therefore the same lunar surface always faces Earth. Only with spaceflight have humans been able to see the far side of the Moon. The lunar phase cycle is not an integer, or full, number of days. A calendar month based on the lunar cycle could alternately be defined to be 29 or 30 days long. In addition, while 12 lunar phase cycles fit into one year, there is a difference in the total number of days between the two. Twelve lunar cycles add up to about 354.37 days, 85

Figure 4-23. The lunar phase depends on the Moon’s position in orbit around the Earth and the Earth’s position in orbit around the Sun. This diagram looks down on Earth from above its North Pole. Earth’s rotation and the Moon’s orbit are both counter-clockwise here. Sunlight is coming in from the right, as indicated by the yellow arrows. From this diagram we can see, for example, that the full moon will always rise at sunset and that the waning crescent moon is high overhead around 9:00 am local time. (Wikipedia)

to the months of the Gregorian calendar. The third type of calendar, the lunisolar calendar, has a sequence of months based on the lunar phase cycle; but every few years a whole month is intercalated to bring the calendar back in phase with the seasons. The Hebrew and Chinese calendars are examples of this type of calendar. The Gregorian calendar which we use as our civil calendar was introduced by Pope Gregor in 1582. It employs a convention for the length of the calendar months that goes back to a calendar reform made by the Roman emperor Julius Caesar. Days were added to some of the months to bring the total days in the year from 354 based on lunation up to 365 days, so the months now were out of phase with the cycles of the Moon. There

but a year is close to 365.24 days long. After three years, a strict lunar calendar would have diverged from the solar calendar by 33 days, or more than one lunation. Three distinct types of calendars have resulted from this situation. A solar calendar, of which the Gregorian calendar is an example, is designed to maintain synchrony with the seasons. To do so, days are intercalated (forming leap years) to increase the average length of the calendar year. Therefore the timing of the Moon’s phases shifts by an average of almost one day for each successive month. The lunar calendar, such as the Islamic calendar, follows the lunar phase cycle without regard for the seasons. Thus the months of the Islamic calendar systematically shift with respect 86

see in the next section, the year still differed from the solar, or tropical, year of 365.24219 days and the year would drift with respect to the seasons. This was fixed with the rules of the Gregorian calendar, which, in addition, provide a method for calculating the date of Easter, a religious day which is tied to the first full moon after the spring equinox. The length of the lunar phase cycle is increasing because the Moon is slowly moving further away from the Earth. The reason is the same as that for the long-term slow-down of Earth’s rotation, namely the tidal force between the Earth and the Moon. The gravitational interaction between the Earth and the Moon, and, to a lesser extent, the Sun, cause the rise and fall of sea levels on Earth, the tides. The gravitational attraction between the Earth and the Moon actually also affects the Earth’s crust, deforming it, though this is not as easily seen as the water tidal movements. The force between the Moon and the Earth varies across of the Earth, stretching the Earth and the ocean waters along the Earth-Moon line. This creates two bulges, or two high tides a day. Now, the tidal bulges do not exactly line up between the

Figure 4-24. The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted by one body on another is not constant across it; the nearest side is attracted more strongly than the farthest side. Thus, the tidal force is differential. The top image illustrates the effect of gravity between the Earth and the Moon on the Earth. Because the pull of gravity becomes stronger as distance decreases, the Moon pulls a little harder at point “C” (closest point to the Moon) than it does at point “O” (in the center of the Earth), and the pull is weaker still at point “F” (farthest point from the Moon). If it were not for the Earth’s gravity, the planet would be pulled apart. Yet also because of the Earth’s gravity which pulls everything toward the center of the planet we can, mathematically subtract the Moon’s pull at the center of the Earth from the Moon’s pull at both point “C” and “F”. When this vector-based subtraction occurs we are left with two smaller forces; one toward the Moon and one on the opposite side pointing away from the Moon (bottom image) producing two bulges. As the Earth makes one rotation in 24 hours, we pass under these areas where the tidal force pulls water away from the Earth’s surface and experience two high tides and two low tides. (NOAA) was also still about a quarter day difference between the true length of the year and the 365 days assumed for the Julian calendar. Thus, February was given an additional day every 4 years (leap years) and the average length of the year with leap years added was 365.25 days. However, as we will

Figure 4-25. This reflector for the Lunar Laser Ranging Experiment was left on the Moon by the Apollo 11 astronauts. (Wikipedia/NASA) 87

Earth and the Moon, as suggested by Figure 4-24. Since the Earth rotates, and there is friction between the water and the ocean floors, the bulges are swept forward a bit along with the rotation. A portion of the gravitational pull between the Earth’s tidal bulges and the Moon is sideways compared to the Earth-Moon line. The end result is that the tidal forces boost the Moon in its orbit, and slow the rotation of Earth. The motion of the Moon can be followed with an accuracy of a few centimeters by lunar laser ranging. Laser pulses are bounced off mirrors on the surface of the Moon. These mirrors were left there during the Apollo missions of 1969 to 1972 and by Lunokhod 2 in 1973. Measuring the round-trip light time of the pulses yields a very accurate measure of the lunar distance. The results show that the Moon recedes from the Earth at a rate of about 30 mm/yr (about 1.5 inches per year).

Figure 4-26. The tilt of Earth’s axis with respect to the plane of its orbit is the reason for the season. The tilt also causes the length of daylight hours to vary with geographic latitude from season to season. (NOAA/NASA)

4.4.3 The Year The revolution of Earth around the Sun defines the year. Of the many ways astronomers have come up with for measuring the length of the year, we’ll go straight to the tropical year, which is used for the Gregorian calendar. The tropical year, also known as the solar year, is defined as the period of time for the ecliptic longitude of the Sun to increase by 3600. Since the Sun’s ecliptic longitude is measured with respect to the vernal equinox, the tropical year comprises a complete cycle of the seasons. Because of the biological and socio-economic importance of the seasons, the tropical year is the basis of the calendar. The mean tropical year on 1 January 2000 was 365.24219 days. The rules of the Gregorian calendar are set up to keep the seasons synchronized with the calendar for millennia. If we let the seasons and the calendar get out of synchrony, we could one day experience snow in what used to be summer on our calendar. The seasons, spring, summer, fall, and winter, result from the Earth’s axis of rotation being tilted with respect to its orbital plane by an angle

of approximately 23.5o. The northern and southern hemispheres always experience opposite seasons. This is because during summer or winter, one part of the planet is more directly exposed to the rays of the Sun than the other, and this exposure alternates as the Earth revolves in its orbit. For approximately half of the year (from the spring or vernal equinox around March 21 to the fall or autumnal equinox around September 22), the northern hemisphere tips toward the Sun, with the maximum amount occurring during the summer solstice on about June 21. For the other half of the year, the southern hemisphere instead of the northern one tips toward the Sun, with the maximum around December 21, the winter solstice. At an equinox, the Sun is at one of the two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. The oldest meaning of the word equinox is the day when dayand nighttime are of approximately equal duration. A solstice is another astronomical event that occurs twice each year as the Sun reaches its high88

stands still in declination; that is, the seasonal movement of the Sun’s path (as seen from Earth) comes to a stop before reversing direction. For an observer at a northern latitude, when the North Pole is tilted toward the Sun the day lasts longer and the Sun appears higher in the sky. When there are more daylight hours and when the Suns’ rays strike the Earth’s surface from a higher angle, these northern geographic locations experience warmer average temperatures. When the North Pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the arctic circles, an extreme case is reached where there is no daylight at all for part of the year. A polar night occurs when the night lasts for more than 24 hours. The opposite phenomenon, the polar day, or midnight Sun, occurs when the Sun stays above the horizon for more than 24 hours. The Earth’s orbit about the Sun is not a perfectly regular time period. The Earth’s movement is perturbed by the gravity of every other planet. It has been predicted that by the year 4,000 changes in Earth’s rotation and revolution will result in the calendar to fall behind the seasons by at least 0.8 but less than 1.1 days. There may arise a need for another calendar reform at some time in the future. The orbit of Earth around the Sun leads to another parameter of interest for spaceflight, namely its orbital speed. Due to its elliptical orbit, the Earth’s orbital speed varies. Earth’s orbital speed averages about 30 km/s (67,000 mph). To put this in perspective, this speed is fast enough to cover the planet’s diameter in seven minutes and the distance to the Moon in four hours.

Figure 4-27. The illumination of the Earth during an equinox (top) results in equal amounts of daylight and nighttime. The two annual equinoxes are the only times when the subsolar point—the place on Earth’s surface where the center of the Sun is exactly overhead—is on the Equator, and, conversely, the Sun is at the zenith for an observer located on the Equator. For the day of the June solstice, the northern hemisphere has longer hours of daylight and shorter nighttime, while the southern hemisphere has shorter hours of daylight and longer nighttimes. The Tropic of Cancer is the northern-most latitude at which an observer can see the Sun directly overhead, at noon on the June solstice, when the Sun is at its northern-most position in the sky. Because the Earth’s spin axis is tilted by 23.5º relative to its orbit, this latitude corresponds to 23.5º N latitude. (Wikipedia)

4.4.4 Spacecraft Time A clock is an essential tool for spacecraft navigation, especially with radio navigation. Spacecraft have used a variety of methods to keep time. In the early days spacecraft clocks were some type of counter which simply incremented forward. It is now becoming possible to fly atomic clocks on space vehicles. A counter or clock on a spacecraft

est or lowest excursion relative to the celestial equator on the celestial sphere. The word solstice is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the Sun

corded. When the spacecraft receives the ranging pulse, it returns the pulse back to Earth, where the ERT is recorded. The time it takes the spacecraft to turn the pulse around within its electronics is known from pre-launch testing. The round-trip light time (RTLT) of the signal is used to compute the distance of the spacecraft using the distancevelocity-time relationship. The SCLK that the craft sends along when it returns the pulse, reveals the drift of its internal clock. The angle that the radio telescope on Earth is pointing when it receives the signal tells the direction of the probe, yielding its coordinates in, say RA and Dec. Signals returned from the spacecraft can also be used to determine its velocity. The physical process used is called the “Doppler effect” and will be further discussed in chapter 8. The principal is identical to the one used when a police officer points a radar gun at your speeding car. Once you know the current position and velocity of a spacecraft, you can send it instructions to control the flight path. Since it takes time for a radio transmission to reach a spacecraft from Earth, the usual operation of a spacecraft is controlled with an uploaded command script containing time markers to ensure a certain timeline of events. Because of the delay between the sending of instructions from Earth and their receipt and execution by the spacecraft, real-time commanding of robotic spacecraft is done rarely: usually only in response to an emergency event, when changes in spacecraft operations must be made as soon as possible.

tracks time in spacecraft clock (SCLK) time. Spacecraft in deep space navigate by sending timed radio signals back and forth to Earth. US space probes, for instance, exchange signals with antennae in the Deep Space Network (Figure 1-16). Just like any two clocks, spacecraft and Earth clocks tend to “drift” with respect to one another. Even if you synchronize the clock on the spacecraft with UTC on Earth upon launch, over time however, the clock reading SCLK will no longer be in synch with an Earth clock. This may be due to the nature of the clock itself, or due to its environment, such as from gravitational or thermal changes. Clock drift can cause problems for spacecraft operations. This is another concern that engineers and scientists address with the exchange of timing signals when they calculate spacecraft event time (SCET) for guidance, navigation and control. Spacecraft event time is the UTC time on the spacecraft. SCET is used to identify when specific events occur on the spacecraft relative to Earth time. SCET is derived using the radio signal which the spacecraft uses to communicate with control stations on Earth. Determining SCET involves taking the time at Earth and adding or subtracting the signal travel time, depending on whether the signal is being sent to or received from the spacecraft. One-Way Light Time (OWLT) is the elapsed time it takes for light, or a radio signal, to reach a spacecraft from Earth (or vice versa). For events transmitted from Earth to the spacecraft, the calculation for SCET is TRM (transmission time) plus OWLT. For events transmitted from the spacecraft to Earth, the SCET of an event on the spacecraft is equal to the ERT (EarthReceived Time) minus the OWLT. For example, if a signal were received on Earth at exactly 11:00h UTC from a spacecraft showing that it had just completed a maneuvering thrust, but the spacecraft was four light-hours away from Earth, the SCET time of the thrust maneuver would have been four hours earlier, at 07:00h UTC. Signal timing is used to determine the location and velocity of a space probe. A timed radio pulse is sent to the space probe and its TRM is re-

4.5 What is GPS? Like the Internet, the Global Positioning System (GPS) is an essential element of the global information infrastructure. The free, open, and dependable nature of GPS has led to the development of hundreds of applications affecting every aspect of modern life. GPS technology is now in everything from cell phones and wristwatches to bulldozers, shipping containers, and ATMs. GPS boosts productivity across a wide swath of the economy, to include farming, con90

Figure 4-28. GPS satellites broadcast radio signals providing their locations, status, and precise time from on-board atomic clocks. The GPS radio signals travel through space at the speed of light. A GPS device receives the radio signals, noting their exact time of arrival, and uses these to calculate its distance from each satellite in view. Once a GPS device knows its distance from at least four satellites, it can use geometry to determine its location on Earth in three dimensions. (GPS.gov/NASA) struction, mining, surveying, package delivery, and logistical supply chain management. Major communications networks, banking systems, financial markets, and power grids depend heavily on GPS for precise time synchronization. Some wireless services cannot operate without it. GPS saves lives by preventing transportation accidents, aiding search and rescue efforts, and speeding the delivery of emergency services and disaster relief. GPS is vital to the Next Generation Air Transportation System (NextGen) that will enhance flight safety while increasing airspace capacity. GPS also advances scientific aims such as weather forecasting, earthquake monitoring, and environmental protection. Finally, GPS remains critical to US national security, and its applications are integrated into virtually every facet of US military operations. Nearly all new military assets â&#x20AC;&#x201D; from vehicles to munitions â&#x20AC;&#x201D; come equipped with GPS. GPS is a US-owned utility that provides users with positioning, navigation, and timing services. This system consists of three segments: the space segment, the control segment, and the user segment. The US Air Force develops, maintains, and operates the space and control segments. 91

The GPS space segment consists of a constellation of satellites transmitting radio signals to users. The United States is committed to maintaining the availability of at least 24 operational GPS satellites, 95% of the time. To ensure this commitment, the Air Force has been flying 31 operational GPS satellites for the past few years. GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,350 km (12,640 miles). Each satellite circles the Earth twice a day. The satellites in the GPS constellation are arranged into six equally-spaced orbital planes surrounding the Earth. Each plane contains four â&#x20AC;&#x153;slotsâ&#x20AC;? occupied by satellites. This 24slot arrangement ensures users can view at least four satellites from virtually any point on the planet. The GPS control segment consists of a global network of ground facilities that use radio antennae to track the GPS satellites, monitor their orbits, perform analyses, and send commands and data to the constellation. The current operational control segment includes a master control station in Colorado, an alternate master control station, 12 command and control antennas, and 16 monitoring sites. The user segment consists of the GPS receiver equipment, which receives signals from the GPS satellites and uses the information to calculate the

Figure 4-29. GPS works using trilateration. Each satellite is at the center of a sphere and where they all intersect is the position of the GPS receiver. (tmjbeary) userâ&#x20AC;&#x2122;s three-dimensional position and time. In geometry, trilateration is the process of determining absolute or relative locations of points by measurement of distances, and using the geometry of circles and spheres (or triangles). In two-dimensional geometry, it is known that if a point lies on two circles, then the circle centers and the two radii provide sufficient information to narrow the possible locations down to two. Additional information may narrow the possibilities down to one unique location. In three-dimensional geometry, when it is known that a point lies on the surfaces of 92

three spheres, then the centers of the three spheres along with their radii provide sufficient information to narrow the possible locations down to no more than two. In GPS, each of three satellites becomes the centre for a sphere. The appropriate point of intersection is the location of the receiver. Earth itself can act as a fourth sphere, and then only one of the two possible points will actually be on the surface of the planet, giving you your sought-after geographic position. GPS receivers generally look to four satellites, however, to improve accuracy and provide altitude information.

Figure 4-30.

In the early 1960s, techniques for Earth-based tracking of Moon-bound spacecraft were in their infancy. It was considered that navigation would be a crew task. By the time Apollo 8 came to launch, experience had been gained in ground-based radio and radar tracking with the flights of the Ranger, Surveyor and Lunar Orbiter probes. Tracking from Earth became the prime navigation method. However, navigation sightings carried out by the crew would provide a crosscheck of the trajectory and provide a backup in case the crew lost radio contact with the ground. Apollo 8 was NASAâ&#x20AC;&#x2122;s first opportunity to prove that onboard celestial navigation works. Imagine the Earth-Moon system with an Apollo spacecraft one day out from Earth. The spacecraft is coasting along on a trajectory which depends on the engine burns that propelled it earlier in the flight. At any particular moment in time, the spacecraft will be in a certain position and it will have a certain velocity. Measuring its position is accomplished by measuring the angle between a star and the horizon of the Earth. The exact value of this angle at a particular moment in time is entirely dependent on their trajectory. Were the trajectory to be substantially different, the angle would also be different. Since the trajectory is defined by the state vector (i.e. their position and velocity at a particular time) the computer can use the Earth/star angle to calculate a current state vector. Multiple sightings are used to refine the vector by averaging out the errors inherent in the measurement. (NASA)

26.What is a solstice; how is the Sun’s motion changing on the celestial sphere? 27.Why are summers warmer than winters on the northern hemisphere? 28.Where on Earth must an observer be located to see the entire sky over the course of a year? 29.Suppose astronomers discovered a planet that orbits its star with its rotation axis perpendicular to its orbit. What would the days and the seasons be like on this planet? 30.An observer in Quito, Ecuador, which has a latitude very close to 0o, sees the Sun at the zenith at noon on which day(s)? 31.The time that your wristwatch keeps is which time? 32.How is SCET calculated from TRM and OWLT? 33.What is clock drift? 34.What kinds of orbits are the GPS satellites in? 35.What is trilateration?

4 Test Your Understanding I. A NSWER IN A FEW SENTENCES . 1. What are the four different types of navigation? 2. How many coordinates are needed to describe the location of an object? 3. How are coordinates on Earth defined? 4. What is a ground track? 5. A satellite has an orbit with an inclination of 0° 00′ 00″. Will this satellite ever be visible from Pittsburgh? 6. What is the celestial sphere? 7. What are the cardinal directions? 8. Where is the zenith? 9. How are the coordinates in the horizontal system defined? 10.What is a great circle? 11. How is the altitude of the north celestial pole related to geographic latitude? 12.In which cardinal direction does the Sun rise for an observer on the southern hemisphere? 13.How are coordinates in the equatorial system defined? 14.What is the difference between astrology and astronomy? 15.Where does the celestial equator intersect the ecliptic? 16.What is an ascending node? 17.One evening, just after sunset, you see Mars, Jupiter, and Saturn spread out across the sky. How could you trace out the rough position of the ecliptic in the sky? 18.What is at the origin of the ecliptic coordinate system? 19.How can any person, not just astronomers, measure the length of a solar day? 20.How do we define the length of a month? 21.If the Moon was new a week ago, what is its phase now? 22.How long is a day for astronauts on the Moon? 23.When does the new moon rise and set? 24.What causes the ocean tides? 25.How does the Lunar Laser Ranging Experiment work?

II. CALCULATE THE A NSWER . 1. Convert the following position to decimal degrees: -42° 16′ 00″, 15° 12′ 00″. 2. An observer measures the altitude of a star to be 32° 45′ 11″ degrees. What is the zenith distance of the star? 3. Calculate the speed of the Earth’s surface at the Equator. Assume an equatorial Earth radius of 6,378 km. Express your answer in both, km/h and mph. 4. Calculate the speed of Earth’s surface in Pittsburgh (40.4417° N, 80.0000° W). Assume an equatorial Earth radius of 6,378 km. Express your answer in both, km/h and mph. 5. Two stars, A and B, are located on the celestial equator. The RA of star A is RA: 02h 0m 00s and that of star B is RA: 07h 00m 00s. What is the angular separation between star A and B?

Stephen Hawking (center), who is widely considered the most famous scientist of the 21st century, enjoys Newtonâ&#x20AC;&#x2122;s laws during a flight aboard a modified Boeing 727 aircraft owned by Zero Gravity Corporation. (NASA)

ROCKET PHYSICS From launches to Earth orbit to transits to the planets, rockets are critical to spaceflight. Four basic laws of physics are all you need to know in order to get a basic idea of how rocket motion works.

5.1 Why Do Rockets Fly the Way They Do? A rocket in flight is subjected to four forces; weight, thrust, and the aerodynamic forces, lift and drag. The magnitude of the weight depends on the mass of all of the parts of the rocket. The weight force is always directed towards the center of the Earth and acts through the center of gravity of the rocket. The magnitude of the thrust depends on the mass flow rate through the engine and the exhaust velocity. The thrust force normally acts along the longitudinal axis of the rocket and through the center of gravity. Rockets can pivot, or gimbal, their exit nozzles to produce a force which is not aligned with the longitudinal axis. The resulting torque about the center of gravity can be used to maneuver the rocket. The magnitude of the aerodynamic forces depends on the shape, size, and velocity of the rocket and on properties of the atmosphere. The aerodynamic forces act through the center of pressure. Aerodynamic forces are not as important on rockets as they are on airplanes. In flight the magnitude, and sometimes the direction, of the four forces is constantly changing. The response of the rocket depends Figure 5-1. The four forces acting on the relative amounts and directions of the on a rocket in flight. Thrust proforces. If we add up the forces, being careful pels the rocket. Weight is due to the force of gravity which acts beto account for their directions, we obtain the tween the center of gravity of the net force on the rocket. rocket and the center of Earth. On The resulting motion of the rocket in the Moon or another planet the response to the net force is described by Newweight of the rocket is different ton’s three laws of motion and Newton’s law from what it is on Earth. There are also aerodynamic forces when of gravity. While Newtonian physics has been the rocket flies over a body which superseded in the 20th century by Einstein’s has an atmosphere. Drag is due to theories of relativity, it is still useful as an friction and opposes thrust; it acts easy approximation. When scientists and enon the rocket’s center of pressure. gineers consider some satellite problems, Lift is perpendicular to the direction of travel. (NASA) such as, for example, the timing signals from a GPS satellite, they do account for Einsteinian relativistic effects. Without relativistic compensations to the satellites’ clock times, GPS would cause navigational errors that accumulate faster than 10 km per day. Considering the motion of a rocket or a satellite using relativistic physics is pretty complex mathematically. Newton’s laws are sufficient to gain a good conceptual understanding of rocket motion.

5.2 Newton’s First Law Newton’s first law of motion reads as follows: Objects at rest will stay at rest and objects in motion will stay in motion in a straight line unless acted upon by a nonzero net force. This law of motion may seem just an obvious statement of fact, but to know what it means, it is necessary to understand the terms rest, motion, and net or unbalanced force. Rest and motion are ascertained by attaching a frame of reference to a body and measuring its change in position relative to that frame. Rest is the state of an object when it is not changing position relative to a point of reference in its immediate surroundings. If you are sitting still in a chair, you can be said to be at rest. This term, however, is relative. Your chair may actually be one of many seats on a speeding airplane. The important thing to remember here is that you are not moving in relation to your immediate surroundings. If rest were defined as a total absence of motion, it would not exist in nature. Even if you were sitting in your chair at home, you would still be moving, because your chair is actually sitting on the surface of a spinning planet that is orbiting a star; the star is moving through a rotating galaxy

that is, itself, moving through a galaxy cluster and so on. While sitting “still,” you are, in fact, traveling at a speed of hundreds of kilometers per second. Motion is also a relative term. All matter in the Universe is moving all the time, but in the first law, motion here means changing position in relation to surroundings. A ball is at rest if it is sitting on the ground. The ball is in motion if it is rolling. A rolling ball changes its position in relation to its surroundings. When you are sitting on a chair in an airplane, you are at rest, but if you get up and walk down the aisle, you are in motion. A rocket blasting off the launch pad changes from a state of rest to a state of motion. The third term important to understanding this law is a net or unbalanced force. Force is a vector quantity which has an amount and a direction. If you hold a ball in your hand and keep it still, the ball is at rest. All the time the ball is held there though, it is being acted upon by forces. The force of gravity is trying to pull the ball downward, while at the same time your hand is pushing against the ball to hold it up. The upward force that opposes gravity is termed the “normal force.” The forces acting on the ball are balanced. They have opposite directions but the same amount, which is a zero-sum game. Let the ball go, or move your hand upward, and the forces become unbalanced; 97

there is now a net force acting on the ball. The ball then changes from a state of rest to a state of motion. In rocket flight, forces become balanced and unbalanced all the time. A rocket on the launch pad is balanced. The surface of the pad pushes the rocket up while gravity tries to pull it down and the vector sum of forces is zero. As the engines are ignited, the thrust from the rocket unbalances the forces, and the rocket travels upward. Later, when the rocket runs out of fuel, it might slow down, stop at the highest point of its flight, then fall back to Earth. Objects in space also react to forces. A spacecraft moving through the Solar System is in constant motion. The spacecraft will travel in a straight line if the forces on it are in balance. This happens only when the spacecraft is very far from any large gravity source such as the Sun, Earth, or any of the other

Figure 5-2. A mechanical gyroscope consists of a rotor, which is a disk or a wheel that spins about an axis. It is gimbal mounted so that it can pivot. Due to inertia the rotor has a tendency to keep its spin axis pointed in the same direction. (Wikipedia)

rocket in the opposite direction from its movement, accelerates the spacecraft. Now that the three major terms of the first law have been explained, it is possible to restate this law. If an object, such as a rocket, is at rest, it takes a net unbalanced force to make it move. If the object is already moving, it takes a net unbalanced force to stop it, change its direction from a straight line path, or alter its speed. An object’s tendency to remain at rest or in motion in a straight line it also known as the object’s inertia. The first law is therefore also called the law of inertia.

Figure 5-3. A vehicle can move about its three translation axes and rotate about three rotation axes called yaw, pitch and roll. (Space Shuttle Wiki/NASA)

5.2.1 Accelerometers And Gyroscopes Knowing about the property of mass to have inertia helps us make more sense of Inertial Navigation Systems, introduced in chapter 4.1. Accelerometers and gyroscopes are motion sensors that detect the acceleration of a vehicle in its own body reference frame by using suspended masses. For instance, you know that if your car is subjected to acceleration in the forward direction, you are forced back in the seat. If your car comes to a sudden stop, you are thrown forward. If you replace the human in the car with a mass suspended in an elastic mounting system, any acceleration of the car will cause movement of the mass inside the vehicle. The direction the mass moves is always opposite to the direction of the vehicle’s acceleration. The amount of displacement is proportional to the force causing the acceleration. This is how an accelerometer works. A gyroscope is a spinning wheel, see Figure 5-2. A wheel that is set spinning has a tendency to keep spinning in its original orientation. It’s inertia makes it resist any change in its spin axis. When you place a gyroscope inside of a vehicle, suspended in a gimbal mount so its axis is independent of the vehicle, then the gyroscope’s axis will keep pointing in the same direction no matter how much the vehicle itself turns. Because the gyroscope maintains the original direction of the

planets and their moons. If the spacecraft comes near a large body in space, the gravity of that body will unbalance the forces and curve the path of the spacecraft. This happens, in particular, when a satellite is sent by a rocket on a path that is parallel to Earth’s surface. If the rocket shoots the spacecraft fast enough, the spacecraft will orbit Earth, instead of falling back to Earth (cf. chapter 6). When you roll a ball across the floor, it is an object in motion that should stay in motion; but in reality, the friction between the ball and the floor will make the ball slow down. The force of friction is what’s called a “nonconservative force” because it drains away energy from the object. Newton was able to imagine what an object’s motion would be like in the absence of friction. Newton considered what would happen to an object in motion that was not in contact with a surface or air molecules or any other source of friction. He envisioned that in this case, an object in motion would stay in motion. Now consider a spacecraft that was placed in Outer Space which is a very good vacuum. Even after the rocket is shut off, the spacecraft keeps on moving. It is now in free flight and will continue moving in the same direction forever as long as no other unbalanced force, such as friction with gas molecules or gravity of a planet or the firing of a

crafts’s motion, it can can be used to keep track of a craft’s orientation changes. Together, accelerometers and gyroscopes can track a vehicle’s translational and rotational accelerations. They allow a spacecraft’s onboard computer to continuously calculate via dead reckoning its position, orientation, and velocity without the need for external references.

5.3 Newton’s Second Law The simplest form of Newton’s second law considers an object that has a constant mass, in other words, a non-varying mass, and undergoes a constant acceleration. In this case, the law states:

Figure 5-4. A German stamp commemorating Newton’s contributions to classical mechanics and optics issued on the occasion of his 350th birthday. The equation in the top left corner is a version of Newton’s second law. (Wikipedia)

The force required to change the motion of an object depends both on the mass of the object and the desired acceleration. Mathematically:

that position, you are giving the car an acceleration with a constant amount. What happens when you apply constant acceleration? Your car continues to speed up as it moves faster and faster. The unit of mass is the kg, and the unit of acceleration is m/s2. Multiplying the two gives the unit of force. This unit of force is also called the “Newton,” in honor of Sir Isaac Newton; and its mathematical symbol is N. One Newton is the amount of force that is capable of accelerating a mass of one kg by one m/s2. In equation form:

m × a.

Recall that a force is something like a push or a pull. It is a vector, having both amount and direction. And mass is the amount of material in an object. Acceleration, a, is change in velocity. Since acceleration is a vector quantity, this change can be in amount, or direction. For instance, when you step on the gas pedal of your car, you accelerate it, say, from rest, or 0 mph, to a higher speed, say, 60 mph. You do the reverse when you hit the breaks, to slow your car from 60 mph back to a stop. In physics, both the increase in speed and the decrease in speed are called acceleration. This is because acceleration is a vector quantity and so can be positive, for going faster, or negative, for going slower. Acceleration also occurs when your car goes around a curve. Because of your inertia, you might need to hang on to the steering wheel harder as the car turns but your body tends to still move straight. When you push your foot on your gas pedal moving it down but after a while you let up because the car was moving too fast, this is the equivalent of changing acceleration. But when after you press down on the gas pedal you then hold

1N = 1kg × 1

m . s2

There are different ways you can apply Newton’s force law. For example, consider a given mass, m, and the force required to move it. A small force will give it a small acceleration while a large force will give it a large acceleration. Not only that, but you can get a quantitative answer on how much force gives how much acceleration. If you double the force, you double the acceleration, triple the force, and you triple the acceleration, and so forth. Or you can compare the effects of a force on a small mass, m, with that on a big mass, M. Applying the same amount of force to both will result in the small mass to experience a large acceleration and the large mass to experience a small acceleration. For a quantitative comparison, if the 99

small mass has half the mass of the big mass and you are applying the same amount of force to both you will give the small mass twice the acceleration of the big mass. Instead of considering acceleration, rocketeers prefer to talk in terms of “delta v,”Δv. Assume a rocket is first at rest with speed zero, and then is accelerated upwards in a straight line to reach a larger velocity. Say at initial time, ti, the position of the rocket is zi and its velocity is vi. At a later time, the final time tf, the position is zf and the velocity is vf. The rocket experienced a change in velocity, Δv = vf − vi, over the time interval, Δt = tf − ti , while it was displaced in position by an

When you apply Newton’s law in this form to rocket propulsion, it tells you that the amount of thrust produced by the rocket depends on the mass flow rate through the engine and the exit velocity of the exhaust. It also tells you that rocket designers can make thrust large by designing the rocket engine to eject mass as rapidly as possible and with high speed. Yet another way to look at the force law is to factor out the numerator as follows:

amount Δz = zf − zi . Since acceleration is the

change in velocity with time, you can write it as a=

Δv . Δt

This also gives you another expression for Δv: Δv = a × Δt.

Δv . Δt

Rockets burn fuel and oxidizer and expel or exhaust the combustion products, so their masses change. We replace m in the above equation with a change in mass, so that Δm = mf − mi over the time interval we are considering. Notice that for a rocket this gives a negative value since the initial mass is larger than the final mass. Also, in rocketry, Δm/Δt is the mass flow rate. It is the amount of exhaust mass per time that comes out of the rocket and also the rate by which the rocket’s mass decreases. Δt is termed the “burn” time of the rocket, which is the duration that the rocket engine fires and thrust is being applied. The force law can now also be written as

Δ(mv) . Δt

In this way the second law says a force is equal to the change in “momentum” per unit time. The momentum is defined to be the mass of an object times its velocity. Mathematically, linear momentum is written by the symbol p, and it is equal to p = mv.

Newton’s second law can now be further modified to read F=m×

Δm × Δv Δm = × Δv. Δt Δt

Momentum is another interesting property of moving objects. Applications for which the concept of momentum is particularly useful is when you want to contemplate the effect of collisions between two moving bodies. Imagine a friend spits a cherry pit at you. That would be annoying but in all likelihood not end up with an injury on your part. Now imagine that same “friend” loads the cherry pit in a rifle and shoots it at you. The same mass, the cherry pit, is now given a high velocity, and its impact with your body will cause you harm. The force law now reads F=

Δp Δt

and in words this says that the net force acting on an object equals the change in the object’s linear momentum divided by the elapsed time. In this 100

way the force law also tells you that if the net force on an object is zero, then the object’s momentum doesn’t change. Physicists also say that under these circumstances the momentum of the object is conserved. The equation also implies that changing an object’s momentum requires the continuous application of a force over an interval of time. If you multiply the force law by Δt, you can put it in yet another form FΔt = Δp. The product of the force with the time interval is also called the “impulse,” I. The goal of rocketry is to create an impulse. Impulse is therefore also an important quantity when comparing different rocket engines. The force law now looks like this: I = FΔt = Δp. The equation states that the impulse acting on an object equals its change in momentum. The result is also called the “impulse-momentum theorem.” Changing the momentum of a body requires an impulse equal to the change in momentum. But since impulse is defined as the product of force and the time interval over which it acts, there are interesting consequences. What it means is that the same change in momentum can be accomplished either by a large force acting over a short time (which is the case for chemical rockets launching from Earth), or by a small force acting over a long time (which happens in the ion drive used by some interplanetary space probes, cf. chapter 9.6.1).

5.3.1 Drag And Kinetic Energy Kinetic energy and aerodynamic drag are two quantities that also depend on the velocity of a moving object. Let’s take a brief look at either one. Drag is an aerodynamic force and is only important when a rocket flies through a medium. A rocket flying through Earth’s atmosphere, for example, will experience a resistance to its for-

ward motion due to air drag. The drag is opposite the thrust in direction and slows the rocket down. This means in the presence of drag, you need more propellant to accelerate the rocket than when the rocket is in the vacuum of Outer Space. The drag depends on the rocket’s velocity, its shape or form, and the amount of friction between the air and the surface of the rocket. You can’t do much about the velocity; it needs to be what you need it to be to fly a particular mission. Other sources of drag can be controlled to some extent through the design of the rocket. The first point of contact of the rocket with the airflow is its nose cone. In order to minimize the drag due to form, rockets have their streamlined, pointy shape. This causes the air to part around the rocket. The drag also depends on the diameter of the rocket. Rockets with a larger diameter have more drag because there is a larger cross-sectional area pushing through the air. Making a rocket as narrow as possible is a way to reduce this source of drag. Finally, to reduce surface drag, designers reduce the roughness of the skin as much as possible, such as, for instance, by eliminating protruding rivets, gaps, or bulges in the skin. Recall from chapter 2 that energy is a property of objects which can be transferred and converted into different forms but not created or destroyed. There are many forms of energy. In chapter 3, you encountered nuclear fusion energy inside the Sun which was transformed to radiant energy of sunlight by the time the energy escaped from the Sun’s surface. The propellant of a rocket, such as gunpowder used in the first primitive rockets you heard about in chapter 1, stores chemical energy which can be transformed into energy of motion, or “kinetic energy,” when the rocket is ignited. Kinetic energy is the energy that a body possesses due to its motion. Once a rocket is moving, it has energy of motion, or kinetic energy. The kinetic energy, KE, associated with motion depends on mass and velocity in the following way:

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1 mv 2. 2 The SI unit of energy is the “Joule,” J, m2 1J = 1kg × 1 2 s which, if you compare it to the Newton, is also equal to a Newton times a meter, or Nm. Having gained this energy during its acceleration, a body maintains its kinetic energy unless its speed changes. For example, should a rocketeer crash land a robotic lander on Mars, the lander’s energy of motion would be transferred to the ground, deforming it, and be converted to heat or thermal energy. KE =

5.4 Newton’s Third Law Newton’s second law dealt with the object on which the force was exerted. His third law explains what happens to the object exerting the force. This law is already familiar to you from chapter 1: For every action there is an opposite and equal reaction. The law is also know as the action/reaction law. And in rocketry, it is also called the rocket principle. In rocketry, we usually talk about the action being the rocket expelling gas out of the engine. In reaction, gas produces a thrusting force

Figure 5-5. Newton’s action/ reaction law has implications for people living and working in space. Here, astronaut Frank De Winne is attached to the T2 treadmill with bungee cords. If he was not attached to the treadmill, pushing off the “floor” with his feet would make him float to the “ceiling” of the ISS. (Wikipedia/NASA)

on the rocket. The thrust accelerates the rocket as described by Newton’s second law of motion. There is more to this law. Newton actually found that all forces in nature always come in pairs. A more explicit form of his third law reads: If object 1 and object 2 interact, the force F12 that object 1 exerts on object 2 is equal in amount but opposite in direction to the force F21 exerted by object 2 on object 1. Mathematically: F12 = −F21 The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the force on the first object equals the size of the force on the second object. This law also implies that a single isolated force, or a unidirectional force, cannot exist. The force F12 exerted by object 1 on object 2 is sometimes called the “action force,” and the force F21 is termed the “reaction force.” In reality, either force could be labeled the action or reaction force. This is because the action force is equal in amount to the reaction force and opposite in direction. In either case, the action and reaction forces act on different objects. Imagine that your car stopped and you are single-handedly trying to push it off the road with as much force as you can muster. According to Newton’s second law, the car is exerting an equal and opposite force back on you. How is this possible when you are not being accelerated backward by the car? The answer is that as you push the car, and the car pushes back on you, the friction between your shoes and the road also applies a force on you, which is just enough to keep you from accelerating backward away from 102

a simple Earth task, such as turning a nut with a wrench, can become quite difficult because the astronaut, and not the nut, may turn. To gain advantage over objects, the crew member must be braced through foot restraints.

the car. If you push on your car while the road is icy, the frictional force is much less. You do in fact accelerate away from the car; and it is much harder for you to gain any advantage over the car. A typical example given to visualize how Newton’s third law operates is with ice skaters. When two ice skaters push against one another, both of them start moving; they glide in opposite directions. The smooth ice provides very little frictional resistance against the ice skates being dragged across its surface. The low friction allows the skaters to glide away in opposite directions

5.4.1 Lift

Figure 5-6. Two ice skaters pushing against each other demonstrate Newton’s third law. When they push off, the force between them will accelerate them in opposite directions. (Wikipedia) with accelerations that are inversely proportional to their masses. This segues nicely into Outer Space examples. Newton’s action/reaction law also has interesting consequences for astronauts working in space. Pushing on an object causes the object and the crew member to float away in opposite directions. A simple action, like typing on a computer keyboard, will send the astronaut flying across the ISS. The rates at which the crew member and the object float away from each other is determined by their respective masses. For example, a massive spacecraft will move away much more slowly than the less massive astronaut pushing on it. In space

Lift is an aerodynamic force that acts perpendicular to an object’s direction of motion. It is a very important force in making airplanes fly, which have large wings to create a lift force. Lift, like drag, is a force that can only exists when a vehicle is flying through a medium, such as the Earth’s atmosphere. It also depends on the object’s velocity. A flat plate can generate lift, but not as much as a streamlined airfoil, and with somewhat higher drag. An airfoil is a streamlined shape that is capable of generating significantly more lift than drag. There are several ways to explain how an airfoil generates lift. Some are more complicated or more mathematically rigorous than others. For example, there are explanations based directly on Newton’s laws of motion and explanations based on Bernoulli’s principle. Either can be used to explain lift. An airfoil generates lift by exerting a downward force on the air as it flows past. According to Newton’s third law, the air must exert an equal and opposite, upward force on the airfoil, which is the lift. The air flow changes direction as it passes the airfoil following a path that is curved downward, and the overall result is that a reaction force is generated opposite to the directional change. In the case of an airplane wing, the wing exerts a downward force on the air and the air exerts an upward force on the wing. While rockets do not have wings, they are a shape moving through air. Any shape moving through air can produce some amount of lift force. Whenever the rocket is inclined to the flight path, a lift force is generated by the rocket’s body and its fins.

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be both attractive or repulsive depending on the electric charges of the particles involved. Gravitation on the other hand is always attractive. Newton’s law of gravity states that two bodies attract each other with a force that depends on their masses and separations as follows: The force of gravity is directly proportional to the product of the two masses and inversely proportional to the square of the distance between them.

Figure 5-7. Newton’s third law says that for every action there is an equal and opposite reaction. When an airfoil deflects air downwards, the air exerts an upward force on the airfoil. (NASA)

F=G

Lift forces are used differently on a rocket than on an airplane. On an airplane, lift is used to overcome the weight of the aircraft, but on a rocket, thrust is used in opposition to weight. On rockets, lift generated by the aft fins is useful to stabilize it, preventing it from tumbling about the flight path.

mM d2

In order to turn the proportionality in the relationship between force, masses, and distance into an equation, a constant is needed. This constant, G, is called the gravitational constant. It’s not to be confused with g, the gravitational acceleration, which you will find out about shortly. The Nm 2 value of the constant is G = 6.674×10−11 . kg2

5.5 Newton’s Law of Gravity Gravity is one of the four “fundamental forces of nature.” Gravity gives weight to physical objects and causes them to fall toward one another. It is attractive, and acts between all objects in the Universe, no matter their distance. Thus, gravity has an infinite range. In the previous sections, when you thought of a force as a push or pull, the implication was that there was a direct contact between the agent applying the force and the body that it was applied to. The force of gravity is different from such a “contact force.” It is considered an “action at a distance force.” No contact is needed between bodies which interact via the force of gravity. You encountered another such action at a distance force in chapter 3, the Coulomb or electrostatic force. The Coulomb force is a manifestation of another of the four fundamental forces of nature, namely, the electromagnetic force. There are similarities and differences between the Coulomb force and the force of gravity. For example, while the Coulomb force acts at a distance as well, it can

Here are some examples of how Newton’s law of gravity operates. Take two small masses at a given distance, and the attractive force is small. Compare with that a small mass and a large mass at the same distance. They will experience a larger attraction. Two large masses at that distance will exert an even larger attraction. Quantitatively, the force depends on the product of the two masses. For instance, if you double each of the two masses, keeping the distance the same, the force between the masses quadruples. The law also involves distance. It is another case of an inverse-square law, like the radiation law from chapter 3.2.1. The force rises and falls with the inverse square of the distance between the masses. If you move two masses further apart from one anther, the force between them weakens; if you move them closer together, the force strengthens. But this strength does not just change with distance, but with distance squared, which is a much stronger dependence. Quantitatively, if you cut the distance by half, the force is

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You can calculate the gravitational acceleration exerted on a body located on the surface of the Earth by combining the equation for weight with Newton’s law of gravity. When you do that, just imagine for simplicity all the mass of the Earth being compacted into a point at its center instead of also worrying about the extended mass distribution of the Earth. The mass of Earth is written as M⊕. Now you can use the radius of the

Figure 5-8. The force of gravity works to pull two bodies together, along the straight line between them. This figure shows two bodies, M1 and M2 being attracted to each other through the force of gravity acting along the line between their centers. The force with which M2 attracts M1 is denoted as F12, and the force by which M1 attracts M2 is denoted as F21. We know from Newton’s third law of motion that the two forces must be equal in amount, but opposite in direction. The amount of the force of gravity depends on the masses of the two bodies and on their distance. The force is larger for larger masses, and for a smaller separations between them. (Wikipedia)

Earth, r⊕ , as the distance between the mass of

Earth and the body with mass m on its surface. And use a special symbol for the gravitational acceleration by Earth, g⊕. This yields mg⊕ = G

5.5.1 Mass And Weight There is an important distinction in physics between the mass and the weight of an object. Mass is a fundamental property of objects connected to their inertia, while weight is due to the force of gravity on an object and therefore dependent on the context of the object. In United States customary units, the unit of weight is the pound. In the SI system, the unit of weight is the same as that of force: the Newton (N). Because weight is a force, it can be described by Newton’s force law

The weight is abbreviated W and the acceleration is the gravitational acceleration written with the symbol g. The definition of weight in mathematical form reads W = mg

2 r⊕

Divide both sides by m, and you are left with an expression for the gravitational acceleration by Earth

four times stronger. Or, if you double the distance, the force drops to one quarter, triple the distance, and the force is only one ninth, and so forth.

F = ma

m M⊕

g⊕ = G

M⊕ 2 r⊕

This equation tells you that the gravitational acceleration on the surface of the Earth is proportional to its mass divided by its radius squared. Because the Earth is not a perfect spheroid and there are mountains and valleys, plus it spins on its axis, the actual value of g⊕ varies with by a small amount with geographical location. The implication is that your weight varies slightly depending on where you are on Earth, while your mass remains the same. The conventional standard value of g⊕ is 9.80665 m/s2 (about 32.174 ft/s2). Hence, a mass of 1 kg on Earth weighs about 9.8 N. You can convert between Newton and pounds as follows: 1 lb = 4.45 N. Because the force of gravity causes a body on Earth to be accelerated by g⊕ it would fall toward the center of the Earth if it wasn’t for the sur-

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Figure 5-9. An object that moves because of gravity alone is said to be free falling. If the object falls through an atmosphere, there is an additional drag force acting on the object and the physics involved with the motion of the object is more complex. For an object in free fall in vacuum, you can easily predict the motion of the object. The net force is just the weight of the object. The acceleration of the object equals the gravitational acceleration. The mass, size, and shape of the object are not factors in describing the motion of the object. The remarkable observation that all free falling objects fall with the same acceleration was first proposed by Galileo Galilei, nearly 400 years ago. (Adapted from NASA) face of the Earth holding it up. This is similar to the case of holding a ball in your hand; the ball is not accelerated because there is a balance of forces. Gravity pulls the ball toward Earth, but the normal force between your hand and the ball compensates for it exactly. Similarly, when you stand on the surface of the Earth it pushes back on you exerting the normal force. It is this normal force that is measured when you step on a scale to appreciate your weight. When you rise above the surface of the Earth, such as by climbing a mountain or by flying in an airplane, the distance between you and the center of Earth increases, and your weight decreases. Typical low Earth orbits are just a few hundred kilometers/miles above the surface of the

Earth. Your separation from the center of the Earth increases by a very small amount that’s added to Earth’s radius. Your weight also decreases by a small amount from what it was on Earth’s surface. But you still have weight. There is still the force of gravity acting between you and Earth. You are not weightless! Consider the astronaut and the shuttle in Figure 5-9. Both are in free fall and are accelerated towards the Earth with the same acceleration. Because they are at some altitude above the Earth’s surface, that is, at a larger distance from the center of the Earth than if they were located on Earth’s surface, the acceleration is slightly less than the standard surface value. At a 200 mile altitude the acceleration is about 90% of the surface value. So the weight is 10% less. But

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both the astronaut and the shuttle do have weight; they are not weightless. Because the shuttle has a larger mass than the astronaut, it also has a larger weight. Because weight is due to the force of gravity between two objects, and forces occur in pairs, this means that while a body on the surface of Earth is attracted by the Earth, the Earth is also attracted by that body by the same amount of force. As long as that body has a mass which is much smaller than that of the Earth, like the astronaut or the shuttle in Figure 5-9, the Earth will accelerate by but a tiny, tiny amount. Gravitational acceleration depends on the mass-to-radius-squared ratio and is not the same on other astronomical bodies as it is on Earth. Imagine that for a smaller body, you can get closer to its center, but a smaller body has a smaller volume providing much less room to fit mass. By how much is the acceleration on other moons and planets different from what it is on Earth? You can insert the mass and the radius of another astronomical body into Newton’s universal law of gravity to calculate the answer. Some astronomical objects, such as the Moon, have smaller gravitational accelerations, because their smaller mass-radius values. The Moon, for instance, has only 1/80 the mass of Earth and less than 1/4 of Earth’s radius. It’s gravitational acceleration is only 1.62 m/s2. You find that your weight on the Moon is only 1/6 your weight on Earth. Or, take Mars as another example. Your weight on Mars is only about 1/3 your weight on Earth. The weight on the surface of another celestial body is important to know in order to calculate the thrust and hence, the propellant resources that your rocket engines need if, for instance, you want to launch your spaceship back to Earth.

5.5.2 Free Fall And Potential Energy Weight depends on mass, but gravitational acceleration does not. Near the surface of the Earth, an object in free fall (neglecting air resistance) accelerates at approximately 9.8 m/s², inde-

pendent of its mass. The gravitational acceleration is a constant acceleration, meaning the longer the body falls, the faster it will be moving. You can apply the equation for acceleration a=

Δv Δt

using gravitational acceleration and solve it for Δv. When you insert some time values you find that after a time interval of 1 s after being dropped from rest, a falling body will achieve a velocity of 9.8 m/s, after 2 s, the body has a velocity of 19.6 m/s and so forth going faster and faster. Two objects in free fall in vacuum dropped from the same height acquire the same speed and hit the ground at the same time. This is counterintuitive. Because for example, if you ever watched a leaf and an acorn fall from a tree you know the acorn hits the ground first. But that is only because the leaf is subject to larger drag and lift, or aerodynamic forces. Take away the air, and the two objects will hit the ground at the same time. This can be shown on Earth by dropping two objects of different masses and shapes inside of a vacuum tube. Free fall was spectacularly demonstrated on the Moon by astronaut David Scott on 2 August 1971. He simultaneously released a hammer and a feather from the same height above the Moon’s surface. The hammer and the feather both fell at the same rate and hit the ground at the same time. This demonstrated Galileo’s discovery that, in the absence of air resistance, all objects experience the same acceleration due to gravity. If you include air resistance, a body dropped from a great height will not pick up the same Δv you predict based on gravitational acceleration alone. The body will pick up a smaller speed instead, what’s called its “terminal velocity.” It is achieved when the drag of the object equals its weight. This is the reason why falling raindrops, which have the mass equivalent of a small bullet, don’t acquire enough momentum to harm anyone.

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Technically, an object is in free fall even when moving upwards or instantaneously at rest at the top of its motion. If gravity is the only influence acting, as is the case when the rocket engines have shut off, then the acceleration is always downward and has the same magnitude for all bodies. When an object is in free fall and its path is affected by gravity only, it is also said to be on a “ballistic trajectory.” A body that has gained some altitude or height, h, above the surface of the Earth, is said to possess a kind of energy termed “gravitational potential energy.” This energy arises from the difference between the energy of the object on the surface versus its energy at a height above the surface. For example, if you drop a glass of milk from a height of 1 inch above the floor, probably, nothing much is going to happen when the glass hits the ground. But if you drop it from standing, the glass will likely shatter when it hits the floor and the milk spills. By way of its position above the surface, an object possesses a stored up amount of potential energy, which is transformed as soon as the object falls. First, the potential energy is transformed into kinetic energy as it picks up speed. When the motion of the glass is stopped when it hits the ground, the kinetic energy is transformed into other forms of energy, such as sound energy. The gravitational potential energy, GPE, of an object depends on its weight, which is its mass times the gravitational acceleration of the planet it is on, and on the height of the object above the planet’s surface. This can be expressed mathematically as GPE = mgh. As an aside, chemical energy is also considered a form of potential energy. “Chemical potential energy,” CPE, is a form of potential energy stored in the structural arrangement of atoms or molecules. Chemical energy of a chemical substance can be transformed to other forms of energy by a chemical reaction.

Rockets that use combustion engines rely on the chemical potential energy that is stored in the propellant. When fuel is burned, or oxidized, the chemical energy of the propellant is converted to heat, creating the hot gas under pressure which is then exhausted to provide thrust.

5.5.3 True Weightlessness Weightlessness means the absence of weight. To be truly weightless, you would need to get away from Earth until the force of gravity between the two of you becomes zero. That is much further away from the Earth than Steven Hawking is on the cover image of this chapter; he is just in an airplane and not even in Outer Space! What he is experiencing is “apparent weightlessness.” Apparent weightlessness arises when bodies fall at the same rate as their immediate surroundings. There’ll be more discussion of apparent weightlessness in chapter 6. But can anything or anybody ever be truly weightless? The answer would have to be that you need to be at an infinite distance from Earth for that to happen. But you need to also consider that all objects in the Universe attract all other objects in the Universe with the force of gravity. You can’t get an infinite distance away from any gravitating body. Then the answer to “true weightlessness” can only be that it could occur when the forces on you are in balance. For instance, imagine that you are in a spaceship between the Earth and the Moon at the exact distance where the force of gravity between you and Earth is exactly the same in amount but opposite in direction to the force of gravity between you and the Moon. Then the forces would be balanced and there would not be a net force on you. This is the closest you might get to true weightlessness. The point where this balance of gravitational forces occurs is also called the “gravitational neutral point.” You can calculate it’s position by equating the gravitational force by Earth to the gravitational force by the Moon. It is an important point in flights to the Moon because if a spacecraft reaches the neutral point with any

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stronger than that of the Sun. This region around the planet is then referred to as the Sphere of Influence of that planet. Taking this a step further, to be retained by its planet, a moon or an artificial satellite must have an orbit that lies within the planet’s Hill Sphere. That moon would, in turn, have a Hill sphere of its own. Any object within that distance would tend to become a satellite of the moon, rather than of the planet itself. The size of the Hill Sphere depends on the ratio of the masses of the two bodies being considered, and the distance between them. When you use the Sun as one mass, the Earth as the other, and the Astronomical Unit as their separation, you find that the Sphere of Influence of the Earth has a radius of about 1 × 106 km. Compare that with the average Earth-Moon distance of about 378,000 km. The Moon is well within Earth’s Sphere of Influence, and is not at risk to be pulled away into an independent orbit around the Sun. Could an astronaut in low Earth orbit ever go in an orbit about his own spaceship? This, too, can be calculated by considering the mass of Earth, the mass of the spaceship, and their separation; and the answer is no. Astronauts could not orbit the ISS, for example, because the ISS’s Sphere of Influence is too small.

Figure 5-10. Konstantin Tsiolkovsky is considered to be one of the founding fathers of rocketry and a pioneer of astronautic theory. In 1903 he published the article “Investigation of Outer Space Rocket Devices,” in which for the first time it was proved that a rocket could perform spaceflight. The ideal rocket equation bears his name. (Wikipedia)

forward velocity toward the Moon, it can then fall to the Moon in free flight without expending additional propellant for thrust. If you include rotational or orbital motion in the calculation of the balance of forces, you can derive the location of the Lagrange points, which you first heard about in chapter 2.4.2.

5.6 The Ideal Rocket Equation

5.5.4 The Hill Sphere An astronomical body’s Hill sphere or “Sphere of Influence” is the region in which it dominates the attraction of satellites. It is an extension of the idea of the gravitational neutral point just discussed in the previous section, involving the behavior of one object in the presence of two others. The Sun is much more massive than any of the planets of our Solar System and its gravity dominates the Solar System. Only very close to the planets does the planetary gravity become

This is an equation that tells you how a rocket moves in Outer Space excluding aerodynamic effects and flying far away from any gravitating bodies. The ideal rocket equation was first derived by the Russian school teacher Konstantin Eduardovich Tsiolkovsky (1857-1935), and in his honor, it is also referred to as “Tsiolkovsky’s rocket equation.” Deriving the rocket equation requires the use of calculus. But you can get an appreciation for it by applying Newton’s second and third laws before and after just one burn. Start by inspecting Figure 5-11, which depicts the situation. The panel on the left shows a rocket at time t traveling to the left at velocity v. It

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Figure 5-11. Diagram for deriving the rocket equation (Wikipedia)

has an initial mass that is comprised of the body of the rocket, m, plus onboard propellant, Δm. The panel on the right shows the rocket a small time interval later, at t + Δt. The fuel has been burned and was ejected from the rocket with the exhaust velocity, ve. It left the rocket at a fixed speed relative to the rocket. You can now calculate the force, toward the right, that the rocket exerted on the accelerating fuel as it was burned and emitted. F = ma =

Δmve Δt

Δv = −ve

From here on we need calculus to determine the actual ideal rocket equation. The rocket does not simply throw out all of its exhaust out the back in one dump, it burns the fuel continuously. From the initial time of the burn to the final time of the burn, the mass of the rocket also decreases continuously from its initial mass with propellant, mi, to its final, or dry, mass, mf. By including this continuous change in mass, the rocket equation becomes:

By Newton’s third law, the fuel exerted an equal force on the rocket, toward the left, increasing the rocket’s velocity by Δv. −F = − m a = − m

Δm . m

Δv Δt

The minus sign comes from Newton’s third law. Together, you can express the action/reaction force pair as Δmve Δv =−m . Δt Δt You can multiply the equation with Δt, divide by m, and rearrange it so that you get a formula for Δv:

Δv = veln

mi ( mf )

where ln stands for the natural logarithm. This is the ideal rocket equation, neglecting all other influences on the rocket such as drag, lift, or weight. You can see from this equation that two factors influence just how large of a change in rocket velocity you can get. To obtain a large delta v, you need 1. a large exhaust velocity 2. a large initial-to-final mass ratio. You already knew about the need for large exhaust velocities. Take a closer look at what it takes to make the mass ratio large. You’d need the initial

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Figure 5-12. A graph of the rocket equation (Wikipedia)

mass to be much larger than the final mass. What does that mean? Your rocket is mostly made of propellant. There is very little mass left for the body of the rocket, and more importantly, the payload that you want to send into space. This is a true fact about rockets. The need for huge amounts of propellant in missions with a large delta v sparks research into propellant efficiency, since the cost of the propellant is a major factor in the cost of rocketry with chemical reaction engines. There are alternative ways to express the ideal rocket equation which then let you look at the propellant consumption. The rocket equation can be written in terms of the propellant mass ratio as mi mf

Δv

= e ve

the mass ratio increases for different ratios of Δv compared to ve. It illustrates how rapidly the mass ratio increases with respect to delta v. The rocket equation can also be written in terms of the amount of propellant mass: Δm = mi − mf = mf (e ve − 1). Δv

From this equation you can see that when Δv is Δv

very small compared to ve, e ve becomes a small number and little propellant mass is needed. When Δv is equal to ve, then using e 1 = 2.7128, you get that you need nearly twice as much as the rocket’s dry mass to be propellant. Finally, when Δv is large compared to ve, the growth is exponential and you need a lot of propellant mass relative to the dry mass. This is generally the case for rockets launching from Earth into space, as you’ll see in the next chapter.

where e is the e-function, the inverse function of the natural logarithm. Figure 5-12 shows you how 111

5 Test Your Understanding I. A NSWER IN A FEW SENTENCES . 1. How is acceleration related to velocity? 2. Can something have an acceleration in the opposite direction of its velocity? 3. What is inertia? 4. How does an accelerometer work? 5. How is the “Newton” defined? 6. What are the forces that act on a rocket in flight, and what are their directions? 7. Voyager 2 is traveling at high speed years after its rockets were last fired. Where is its velocity coming from? 8. Every acceleration is the result of a net force. Does every applied force produce an acceleration? 9. What does Newton’s 2nd law say about force? 10.What is meant by “delta v?” 11. What is momentum? 12.What is a burn? 13.What is impulse? 14.What is kinetic energy, and what does a body need to possess to have it? 15.Imagine you are an astronaut with a repair toolbox sent to work outside the ISS but you forgot to attach yourself to the ISS, and now you are drifting away from it in space. How could you try to get back to the ISS? 16.What does the gravitational force depend on? 17.What is the difference between mass and weight? 18.How is gravitational acceleration different on different celestial bodies in the Solar System? 19.What is true weightlessness and how could it be achieved? 20.What is meant by free fall? 21.What does it mean that something has potential energy? 22.How can you tell from its flight path when gravity is acting on a space probe? 23.If astronaut Julia has a mass of 60 kg on Earth, what would be her mass on the Moon?

24.If you dropped a hammer and feather from the same height above the Moon’s surface, where there are no aerodynamic forces, which would hit the ground first? 25.If you dropped a hammer and feather from the same height above the Earth’s surface, which would hit the ground first according to Newton’s laws, and which will actually hit the ground first? a) Why is there a difference? b) What do you think would happen if you dropped the feather quill first? 26.How can a rocketeer design an engine that delivers a large delta v? II. C ALCULATE THE A NSWER . 1. How fast is a rocket traveling 3s after firing its engines, which occurred at t=0, if it accelerated at a constant rate of 10 m/s2? (Neglect gravity, drag, and lift.) Convert your answer to mph. 2. Forces of 10N and 25N act on an objet in the same direction. a) What is the net force on the object? b) What is the net force on the object when the two forces act in opposite directions? 3. A rocket weighs 100,000 N. What are a) its mass in kg and b) its weight pounds? 4. Suppose a rocket matches acceleration with a satellite so both have 10 m/s2. It grabs the satellite doubling its mass, and it doubles the thrust of its engines. What is its new acceleration? 5. What is the force of gravity between Earth and a 70-kg astronaut a) standing on the surface of the Earth, b) in an airplane flying at 40,000 feet? Use 6,400 km, 5.97 ×1024 kg, for Earth’s radius and mass, and values of G, g from 5.5. 6. A rocket of dry mass of 20,000 kg is loaded with 100,000 kg of fuel. The propellant provides an exhaust velocity of 10,000 m/s. What delta v can this rocket achieve assuming no gravity, drag, or lift?

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The United Launch Alliance Delta IV Heavy rocket with NASAâ&#x20AC;&#x2122;s Orion spacecraft mounted atop, lifts off from Cape Canaveral Air Force Stationâ&#x20AC;&#x2122;s Space Launch Complex 37 at 7:05 am EST, Friday, 5 December 2014, in Florida. (NASA)

LIFT OFF! How does a rocket lift off from planet Earth? Does it go straight up? Does it fly at an angle? How much delta v is required to overcome gravity and atmospheric drag when a rocket delivers a spacecraft into an Earth orbit?

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6.1 Mission Profile Any space mission has a number of distinct, sequenced phases which make up its mission profile. The mission profile of NASA’s Orion flight test on 5 December 2014 is shown in Figures 6-1 and 6-2. The mission was an uncrewed, four-hour, two-orbit test of the Orion crew module featuring a high apogee on the second orbit and concluding with a highenergy reentry at about 20,000 miles per hour (32,000 km/h; 8,900 m/s). The Orion was launched on a Delta IV Heavy rocket from Cape Canaveral Air Force Station. In order to lift off, the rocket used thrust to overcome its weight. Rising through Earth’s atmosphere, the rocket became subject to the aerodynamic forces, drag and lift. It had to overcome the atmospheric drag by using thrust. Much of the rocket’s structure was jettisoned minutes after lift off, to reduce weight. This is a technique called “staging.” The rocket had been angled forward shortly after launch to place the Orion into an Earth orbit. Here, the rocket’s upper stage plus the Orion spacecraft completed a full orbit in free fall, or free flight. While in orbit, inertia carried the spacecraft forward, and the force of gravity in Earth’s Sphere of Influence bent the flight path into a low-eccentricity ellipse. The upper stage had sufficient propellant to initiate another burn which placed the Orion into a second, highly elliptical Earth orbit. Inertia would continue to carry it around and around, but only two orbits were part of this mission profile. Another application of thrust took the Orion out of the orbit, and caused it to fall down to Earth. Upon reentry into Figure 6-1. The mission profile of the Orion flight test in December Earth’s atmosphere, the spaceof 2014. Shown are the altitude versus mission elapse time (left) and craft heated up due to the fricthe altitude and shape of the Earth orbits (right). The numbers lational forces between its heat bels correspond to mission events detailed in Figure 6-2. (NASA) shield and air molecules. Descending through the atmosphere, it made use of lift and drag forces, using parachutes, to slow its descent to Earth. It finally came to rest when it splashed down in the Pacific Ocean, where it was recovered by a Navy amphibious transport dock warship. 114

During the mission, the net force on the Orion, its delta v, and its energy changed many times. The spacecraft was alternately in powered flight and in free flight. For example, launching from Earth required powered flight, but orbiting Earth occurred in free flight. Throughout the space mission, there were different net forces acting on the spacecraft. Hence, the spacecraft accelerated by different amounts at different times. Accelerations were due to changes in speed because of burns and changes in direction when the engines were gimbaled. There were changes in speed and direction because of gravitational forces. In addition to the thrust needed to launch it off the Earth, the Orion’s mission required the application of thrust twice more, for a change in orbital altitude on the second orbit, and for leaving that orbit to return to Earth. The “delta-v budget” is an accounting of the total delta v required for a space mission. The flight’s delta-v budget determined how much propellant was needed to carry out the mission. The energy of the spacecraft also varied throughout the mission. On the launchpad the Delta rocket had no velocity or altitude. It had maximum chemical potential energy but no gravitational potential energy. As soon as it lifted off, it started losing chemical potential energy, while gaining kinetic energy and gravitational potential energy. When the spacecraft reached apogee it slowed down. Recall this behavior is in accordance with Kepler’s second law, introduced in chapter 3. It’s kinetic energy was at a minimum, but its gravitational potential energy was at a maximum. As it swung back toward Earth, the craft’s kinetic energy increased. In descent back to Earth the rocket used the last of its chemical potential energy and lost gravitational potential energy and kinetic energy. Of the many phases that make up a space mission — the ascent, the travel through Outer Space, and the return to Earth and landing — the ascent requires the most net force to set the vehicle in motion. A spacecraft that is to travel to Outer Space must be able not only to accelerate its mass as described by the ideal rocket equation. It must first of all move away from Earth against the force of gravity. It must lift itself, its structure, its payload, and the huge amount of propellant it requires for generating its thrust against the attractive force of gravity. Recall the force of gravity is stronger for smaller separations. This is Figure 6-2. The mission profile of the Orion exactly the case when the rocket sits on a launch pad on flight test comprised eight events. (NASA) 115

Figure 6-3. A rocket reaching orbital velocity will enter into orbit around the Earth. For a circular orbit the flight path is the one labeled C. A higher speed will lead to an elliptical orbit such as trajectory D. A speed that is lower than the orbital velocity will cause the rocket to fall back to Earth, for example on trajectories A and B. When the escape velocity is attained, the rocket will be able to escape the Sphere of Influence of the Earth. One example is the parabolic trajectory E. (DFL)

Earth’s surface.

6.2 Rocket Velocities Two possibilities exist for a rocket to overcome the gravitational attraction between it and the Earth. It can get removed from the Sphere of Influence of the Earth by attaining escape velocity. Or it can transition into a free orbit around Earth. The same is true for a rocket that is in the Sphere of Influence of any other celestial body, be it the Sun, another planet, or a moon. They all have an escape velocity and an orbital velocity.

6.2.1 Escape Velocity Imaging that you throw a ball straight up in the air. You’d be putting it on a “ballistic” flight path since the only force acting on it after the throw is gravity. The ball starts out fast, but then slows down, stops at the highest point of its trajectory, and then falls straight back down to Earth, gaining speed. In reality it is not possible for even

the best baseball pitcher to throw a ball upward with enough speed so that it can escape from the Sphere of Influence of Earth. But we can ask, at what velocity (neglecting drag) would you in principle need to throw the ball up so that it never returns back to Earth? This velocity is called the “escape velocity.” It is the same velocity at which an object would strike Earth’s surface if it fell freely from an infinite distance to Earth. The escape velocity is a whopping 11.2 km/s, which is equal to about 25,100 mph. The simplest way of deriving the formula for escape velocity is to use conservation of energy. In this case the initial and final energy must be the same: Ei = Ef. Kinetic energy and gravitational potential energy are the only types of energy that you need to deal with in order to figure out the escape velocity. You already know the equations for them from chapter 5. The kinetic energy of the rocket is KE =

1 mv 2 2

and the gravitational potential energy is

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GPE = mgh.

vescape =

In the original equation for GPE the gravitation acceleration, g, was assumed constant. This is true near the surface of the Earth, at small altitudes, or heights, h. A different and more general equation for the GPE is used when you consider any distance, d, separating your spaceship from a massive astronomical body. This equation is GPE = −

Gm M . d

According to this equation, GPE is always a negative quantity for all distances. However, when the distance goes to infinity, GPE goes to zero. Hence it remains largest at great height, as before. When the distance is equal to the radius of the massive astronomical body, r, then GPE = −

2GM . r

Notice that the mass of the rocket drops out. The escape velocity is independent of the mass of the rocket! A spaceship and an air molecule have the same escape velocity. Since the escape velocity is independent of direction it is more correctly termed the escape speed. A rocket can escape straight up, or on an open orbit such as along trajectory E in Figure 6-3. On the surface of the Earth, the escape velocity, 11.2 km/s (about 21,500 mph), equals Mach 33, 33 times the speed of sound. Because of the at-

Gm M . r

Now have another look at the initial and final energies of your rocket, made up of KE and GPE 1 Gm M Gm M 1 mvi2 − = mvf2 − . 2 r 2 d For interpreting this equation, consider that the spacecraft blasts off at a high initial velocity from the surface of the planet at the moment of ignition, and then coasts without power until it reaches its final velocity of zero at infinite distance. Both terms on the right side of the equation become zero, yielding 1 Gm M mvi2 − = 0. 2 r The rocket’s initial velocity necessary to escape the planet is the escape velocity, vescape. Solving the

Figure 6-4. An illustration of the space canon from the 1872 edition of Jules Vernes’ science fiction novel “From the Earth to the Moon.” (Wikipedia)

above equation for velocity thus gives us the escape velocity as 117

falls back to Earth in free fall. mosphere it is not useful and Now imagine further, that you hardly possible to give an obdon’t just drop the ball, but that ject near the surface of the you throw it forward. The ball Earth the escape speed. It is will also fall back to the Earth too far in the hypersonic rebut it would hit the surface some gime for most practical prodistance away from the mounpulsion systems and would tain top from where you threw it. cause most objects to burn If you could throw a ball so fast up due to aerodynamic heatthat the Earth basically curves ing or be torn apart by ataway under it, then the ball mospheric drag. would never fall back down to Having a rocket Earth. Instead, its inertia would achieve escape velocity incarry the ball forward into a stantaneously upon lift off Figure 6-5.Velocity v and acceleration curved path around the Earth, a would be like firing the rocket out of a space canon, a in uniform circular motion at angu- stable, free-fall orbit. lar rate ω. The amount of velocity is Consider the simple case something that was enviconstant, but the velocity’s direction is of a spaceship that is in a circusioned by Jules Verne in his changing, being tangential to the orlar orbit around the Earth, such 1865 science fiction novel bit. The acceleration has constant magnitude, and always points toward as on trajectory C in Figure 6-3, “From the Earth to the the center of rotation. (Wikipedia) and try to figure out the orbital Moon.” Verne’s novel had velocity. At any moment in its cirgreat influence. The rocketry cular path, the object’s velocity changes. This is innovators Konstantin Tsiolkovsky, Robert Godbecause although it has a constant speed, its direcdard, and Hermann Oberth are all known to have tion changes. A changing velocity implies an acceltaken their inspiration from Verne’s book. Edwin eration. This is the centripetal (toward the center) Hubble, the American astronomer, was in his acceleration of gravity. youth fascinated by Verne’s novels, especially If the period for one rotation is P, the angu“From the Earth to the Moon” and “Twenty Thoular rate of rotation, also known as angular velocsand Leagues Under the Sea.” Like Verne, Hubble ity, ω is gave up the career path in law that his father intended for him, setting off instead to pursue his 3600 2π passion for science. The German’s tried to build a ω= = space canon during their World War II V-3 proP P ject, and there have been some other attempts where the units are degrees/s and can also be exsince. The space canon, however, has not turned pressed as radians/s. For motion in a circle of raout to be a viable launch alternative to a rocket dius r, the circumference of the circle is 2πr. launch. Hence the speed of the object traveling in the circle is 6.2.2 Orbital Velocity Now imagine that you are are standing on a 2πr mountain that is high enough to put you in Outer v = ωr = . P Space above the Kármán line. Such a mountain does not exist. Mount Everest is only about 8.8 The centripetal acceleration can be expressed as km (29,000 feet) tall. But imagine you are up there, and you let a ball drop from that height. It

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v2 , r

and because force is mass times acceleration, we find that the centripetal force is F=m

v2 . r

In an orbit, the centripetal force is supplied by the force of gravity, therefore m

v2 Gm M . = r r2

Solving this equation for v gives you the orbital velocity of the spacecraft, which is vorbit =

GM . r

The equation tells you that orbital velocity around the Earth, like escape velocity, is also independent of the mass of the vehicle, and that it is smaller, by a factor of the square root of 2, or 1.41, than escape velocity. The equation for orbital velocity can be applied to other bodies. The Sun, other planets, moons, all can be orbited at the orbital velocity. If a satellite is to remain in a circular orbit of radius r, it must have precisely the orbital velocity given by this equation. Notice that the closer a satellite is to the body that it orbits, the greater is its orbital speed. Satellites can orbit at a wide range of distances within Earth’s Sphere of Influence. And their orbits can have a full range of eccentricities. The above formula is for a circular orbit, only. The orbital velocity needed to maintain a stable, low Earth orbit, is about 7.8 km/s (about 17,500 mph). It reduces with increased orbital altitude. For instance, the orbital velocity of the Moon is only 1 km/s (or about 2,200 mph). The orbital velocity for a low Earth orbit, 17,500 mph, while it is much smaller than the escape velocity, 25,100 mph, is still an extremely high velocity. It is much

higher than the speed record achieved by the best supersonic aircraft, the Blackbird. The Lockheed SR-71 Blackbird holds the official Air Speed Record for a crewed airbreathing jet aircraft with a speed of 3,530 km/h (2,193 mph). It was capable of taking off and landing on conventional runways. Recall that airplanes use lift provided by their wings, in addition to thrust, for offsetting their weight. The speed record was set on 28 July 1976 by Eldon W. Joersz and George T. Morgan Jr. near Beale Air Force Base, California, US. The SR-71 pilot, Brian Shul, reported that he flew in excess of Mach 3.5, three and a half times the speed of sound, on 15 April 1986 over Libya in order to avoid a missile. Back to the analogy of throwing a ball straight up from the surface of Earth, or throwing it forward from a high altitude. There are a couple of differences between a real rocket launch and throwing a ball straight up for a steep ascent, or forward from great height, for a flat ascent. For one, if you want the rocket to go into orbit you still have to first hurl it up far enough. Also, in rocket flight, the velocity, be it escape velocity or orbital velocity, is not imparted instantaneously. Instead, there is a time during which the engine thrust builds up the vehicle’s velocity. This burn time is typically between 5 and 10 minutes long. And in reality, air drag is also a very important consideration for a rocket, impeding its ascent. Finally, in contrast to how an airplane flies, lift does not help a rocket offset its weight.

6.3 Launch Forces Take a look at the forces involved in accelerating a rocket vertically upward. When the rocket is sitting on the launch pad on Earth’s surface it is an object at rest relative to its immediate surroundings. You know from Newton’s first law, that the net force on the rocket must be zero. While on the launch pad, the weight, or force of gravity toward the center of Earth, is equal to and opposite in direction of the normal force. The normal force is provided by the launch pad which supports the rocket, preventing it from free falling to Earth’s

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center. The balance of forces is depicted in Figure 6-6. Expressed mathematically: F = N + (−W ) = N − W = 0. In order for the rocket to lift of, there needs to be a net force. This net force is supplied by the thrust. The amount of thrust must be greater than the amount of weight and thrust must be applied in the opposite direction of weight. Mathematically speaking:

At the instant it takes off, a rocket seems to hover over the launch pad as thrust overcomes weight. Then, as its velocity grows while propellant is exhausted, the rocket clears the launch tower and rises up. It must now fly through the densest parts of the atmosphere, where it encounters a considerable drag, D, from atmospheric friction. Drag resists thrust, and on its upward ascent, the rocket’s thrust must now also overcome drag. The force requirement is: F =T−W−D >0

F =T−W >0 or, the amount of thrust must be larger than the weight

or, the amount of thrust must be larger than weight and drag together for the rocket to lift off from Earth: T > W + D.

T>W. Rocketeers also sometimes say that the thrust-toweight ratio for lift off must be greater than one.

Figure 6-6. When the rocket sits on the launch pad, weight, acting downward, and normal force, acting upward, are balanced (A). In order for the rocket to lift off, upward thrust must be larger than weight (B). (NASA)

6.3.1 The g-force When the acceleration of a rocket or another vehicle is measured in terms of the gravitational acceleration, it is sometimes also termed the “g-force.” Despite the name, it is incorrect to consider g-force a force, as g-force is a type of acceleration. The g-force acceleration is the cause of an object’s acceleration in free fall. When a rocket takes a robotic spacecraft into orbit the g-force can be as high as the rocket is capable of delivering and which the equipment can withstand and still function. A crewed launch cannot accelerate at the highest g’s because the human body cannot endure very high g loads. The study of the effects of acceleration on humans began towards the end of World War I, when aircraft were becoming robust enough to perform violent aerial maneuvers without falling apart in mid-air. Pilots reported that those same maneuvers were giving them temporary vision problems, as well as making them feel heavier or lighter, depending on what their aircraft were doing at the time. Over the decades that followed, an understanding was developed of so-called g-forces and their effects on

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the body. When spaceflight began, a new element, that of weightlessness, was added to the research. The human body can endure any speed in space, or on Earth, but the time taken to reach that speed is governed, among other things, by the body’s ability to withstand the acceleration. We humans are accustomed to be accelerated by 1 g⊕

Figure 6-7. The record for peak experimental horizontal gforce tolerance is held by acceleration pioneer John Stapp. In a series of rocket sled experiments, Stapp was subjected to 15 g for 0.6 s and a peak of 22 g during a 19 March 1954 test. He would eventually survive a peak of more than 46 g, with more than 25 g for 1.1 s.

due to our weight on Earth. Hence, when an astronaut rides in a spaceship that accelerates at 1 g⊕, the experience is the same as standing or sitting on Earth. But when a rocket is accelerating at more than 1 g⊕, the astronaut will feel that his apparent weight has increased. He will be pressed into his seat. Positive, or “upward” g, drives blood downward to the feet of the astronaut; this causes problems with the eyes and brain in particular. Resistance to positive g-forces varies. A typical person can handle about 5 g⊕ before losing consciousness. Astronauts learn about their tolerance to gforces in a centrifuge. As positive vertical g-force is progressively increased the following symptoms may be experienced: • grey-out, where the vision loses hue, easily reversible on leveling out, • tunnel vision, where peripheral vision is progressively lost, • blackout, a loss of vision while consciousness is maintained, caused by a lack of blood to the head, • g-LOC, a loss of consciousness (“LOC” stands for “Loss Of Consciousness”), • death. If g-forces are not quickly reduced, death can occur. Resistance to “negative” or “downward” g, which drives blood to the head, is much lower. This limit is typically in the −2 to −3 g⊕ range. This condition is sometimes referred to as red out where vision is figuratively reddened due to the blood laden lower eyelid being pulled into the field of vision. Negative g is generally unpleasant and can cause damage. Blood vessels in the eyes or brain may swell or burst under the increased blood pressure, resulting in degraded sight or even blindness. Negative g-forces act on astro-

(Wikipedia)

nauts when their spaceship reenters Earth’s atmosphere and slows down for landing. Tests have shown that negative accelerations are best taken “eyeballs in.” Therefore, the astronauts are seated backwards, with their backs in the direction of descent and their eyes looking up. The maximum acceleration of the Space Shuttle during launch and reentry was an unpleasant 3 g⊕. The human body can tolerate violent accelerations for short periods, including the high-g acceleration necessary to reach Earth orbit. However prolonged periods of high-g acceleration during travel between planets would be very harmful to the body, and are therefore out of the question. Imagine traveling to Mars, accelerating all the way at 3 g⊕. You would weigh three times your normal weight for the duration of the trip and would barely be able to move. And what would the unrelenting acceleration be doing to your body? Heavy acceleration is hard on the body: the heart has to 121

do many times its proper work, tissues and capillaries break down. You could not count on being in good shape when you arrived!

Earth’s surface gravity. When you lift off vertically from the surface it becomes Δv = veln

6.4 The Rocket Equation For Launch The delta v budget for launch has several components. Luckily, different delta v’s can simply be added or subtracted to calculate the required delta v for launch. You already know that there is a component for thrust, weight, and drag. In addition, there are small losses due to steering the rocket, and a sizable gain or loss depending on whether the rocket is launched into the spin direction of Earth or opposite to it. So the delta v requirement for launch can be written as Δv = ΔvT − ΔvW − ΔvD − Δvsteer ± Δv⊕spin. In chapter 5 you learned about the ideal rocket equation which was derived for the case when a spacecraft is flying in Outer Space far away from any gravitating masses. The only net force considered was thrust. The ideal rocket equation alone gives you Δv = ΔvT = veln

mi . ( mf )

This equation is just for accelerating the mass of the rocket. It does not take into account its weight due to the force of gravity when the spaceship tries to lift off from Earth. You saw that the condition for lift off is that the rocket’s thrust must be greater than the rocket’s weight. In other words, the upward acceleration of the rocket must exceed one g. You know how gravitational acceleration relates to delta v: g⊕ =

Δv . Δt

This gives you an equation for ΔvW. You can now modify the rocket equation to include the effect of

mi − g⊕Δt ( mf )

where Δt is the ascent time in powered flight, or also the length of the burn. You have already seen, from the ideal rocket equation, that the only way to obtain a high delta v is to use a propellant with a high exhaust velocity and to take along more propellant with you than anything else. The second term in the above equations subtracts from the value of the first term. This means it is even harder to obtain a high delta v inside of a Sphere of Influence than in free space. If you want to get a high-enough delta v to lift off, you must try to make the second, negative term as small as possible. Gravitational acceleration, g⊕, is a near constant quantity; the small variation with altitude helps a little, but it isn’t something you can actively control. What you can try to do is lower the second term in the equation by reducing the ascent time, Δt. The faster you can move the rocket up, the larger your delta v. “Gravity losses” as a proportion of delta v are minimized if maximum thrust is applied for a short time. In terms of your propellant use, a rapid ascent is best. Lets put a number on this. Say, you do a 2 minute burn, then your ΔvW is approximately 1,200 m/s, or in the neighborhood of 1.2 km/s. Drag causes another negative term in the delta v budget, making it even harder for the rocket to thrust its way upward. Drag is a bit more complex to express with an analytic formula. It depends on the size of the rocket, the air density which is highest at sea level but drops with altitude, and it depends on the velocity of the rocket squared. To reduce drag, you can make the rocket small and streamlined. And a low speed at low altitude is preferred. But this preference of a low speed early in the flight conflicts with the rocket’s fight against gravity, for which a high-speed as-

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cent is best. A good value to use for ΔvD is about 1-1.5 km/s. Steering loss is the last major loss in our rocket equation for launch and is caused by the need to steer the launch vehicle. The steering loss is difficult to write down in an equation because it depends on the intended flight path. A rocket sent to orbit is often pitched away from vertical ascent, and may need to be steered onto a particular heading. Here we assume that Δvsteer will consume about 0.5 km/s of delta v. The last term in our delta v budget for a rocket launch has to do with the spin of Earth on its axis, Δv⊕spin. The rocket is not launching from a stationary platform, but from the moving surface of the Earth. The speed of the Earth’s surface at the Equator is highest, about 465 m/s (chapter 4.4.1). If you launch a rocket toward east from the Equator of Earth it takes all of this speed with it. Nearly 0.5 km/s is gained for which you don’t have to use fuel. But if you launch toward west, you must subtract that velocity. The Earth’s surface speed decreases with geographical latitude. To get the largest positive speed boost from the motion of the surface of the Earth, spacefaring na- Figure 6-8. This is a photo of the exhaust from the space tions seek to establish launch sites as close shuttle Discovery on its 27 May 1999 lift off from KSC. The exhaust helps to illustrate how the shuttle turned from vertito the Equator as possible. The US does cal to gain horizontal velocity for orbit. (NASA/MSFC) not have property on the Equator. So a site densely populated East Coast of the US, putting was chosen within the US that has a very southeveryone on its ground track in danger. A West erly latitude. The latitude of Kennedy Space CenCoast location would either send rockets over ter (KSC) in Florida is 28.5241° N. All spacecraft populated areas of the US or have to contend with launched to the east from KSC get an initial assist launching at azimuths which do not yield a speed of 0.4 km/s. boost. A second advantage of launching rockets Now you have all the tools to ask how much from Florida due east is that the rockets move out delta v from thrust you need to launch a rocket to over the Atlantic Ocean. This safeguards people. Earth orbit. You need to use thrust to get from 0 Nobody lives underneath the flight path if the to orbital velocity, 7.8 km/s, plus all of the velocity rocket blows up down range from the launch pad. losses incurred, minus the gain for launching eastImagine if the “launch azimuth” was not due east, wardly. This means the required ΔvT is but due north. Now the rocket flies over the 123

Figure 6-9. One method for changing a rocket’s flight direction is to gimbal the nozzle. A gimbaled nozzle is one that is able to sway while exhaust gases are passing through it. By tilting the engine nozzle in the proper direction, the rocket responds by changing course. Instead of going vertically upward, it can be tilted to move forward, and fly down range from the launch site. Once the rocket is pitched over, the force of gravity is no longer directly opposite of thrust, but at an angle to the thrust vector. (NASA) ΔvT = 7.8

km km km km km km + 1.2 +1 + 0.5 − 0.4 = 10.1 s s s s s s

implying that a launch vehicle needs the capability to provide a delta v from thrust of about 10 km/s for a mission from KSC into a low Earth orbit.

6.4.1 Gravity Turn At lift off the rocket’s velocity vector points straight up, but in an orbit, the velocity vector is sideways. The vectors for gaining altitude and gaining orbital velocity are at right angles to one another. But a rocket does not go up to orbital altitude, and then makes a sudden, 90o turn. Rather, it uses a gravity turn to go both up and forward. The rocket begins by flying straight up, gaining both vertical speed and altitude. During this

portion of the launch, gravity acts directly against the thrust of the rocket, as in Figure 6-6. Losses associated with this slowing can be minimized by executing the next phase of the launch, the pitchover maneuver, as soon as possible. The pitchover should also be carried out while the vertical velocity is small to avoid large aerodynamic stresses on the vehicle during the maneuver. The pitch-over maneuver consists of the rocket gimbaling its engine slightly to direct some of its thrust to one side. Now, thrust and weight make a small angle from being directly opposite on another, as in Figure 5-1. The pitch-over angle varies with the launch vehicle and is included in the rocket’s inertial guidance system. For some vehicles it is only a few degrees, while other vehicles use relatively large angles (a few tens of degrees). After the pitch over is complete, the engines are reset to point straight down the axis of the rocket again. This small steering maneuver is the only time during an ideal gravity turn ascent that thrust must be used for purposes of steering. The pitch-over maneuver turns the rocket slightly so that its flight path is no longer vertical; and it places the rocket on the correct heading for its ascent to orbit. After the pitch over, the rocket’s flight path is no longer completely vertical, and gravity acts to turn the flight path back towards the ground. If the rocket were not producing thrust, the flight path would be a simple ellipse like a ball thrown upward and forward, stopping at its highest point and then falling back to the ground. The rocket is producing thrust though, and rather than leveling off and then descending, by the time the rocket levels off, it has gained sufficient altitude and velocity to place it in a stable orbit.

6.5 Propulsion From chapter 5 you know that to obtain a large delta v from thrust, you need 1. a large exhaust velocity 2. a large initial-to-final mass ratio.

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Figure 6-10. Todayâ&#x20AC;&#x2122;s launch vehicles are combustion engines. For combustion to occur three things must be present: a fuel to be burned, a source of oxygen, and a source of heat. As a result of combustion, exhausts are created and heat is released. You can control or stop the combustion process by controlling the amount of the fuel available, the amount of oxygen available, or the source of heat. (NASA) Here we look at how you can get a large exhaust velocity.

6.5.1 Propulsion Types The word propellant does not mean simply fuel, as you know; it means both fuel and oxidizer. The fuel is the chemical that rockets burn but, for burning to take place, an oxidizer must be present. Jet engines draw oxygen into their engines from the surrounding air. Rockets do not have the luxury that jet planes have; they must carry oxygen with them into space, where there is no air. Current launch vehicles use chemical rockets for launch and operate with either solid or liquid propellants, or a combination of both. Solid propellants contain both the fuel and oxidizer combined together in the chemical itself. Usually the fuel is a mixture of hydrogen compounds and carbon and the oxidizer is made up of oxygen compounds. Liquid propellants, which are often gases that have been chilled until they turn into liquids, are kept in separate containers, one for the fuel and the other for the oxidizer. Then, when the engine fires, the fuel and oxidizer are mixed together in the engine. A solid-propellant rocket has the simplest form of engine. It has a nozzle, a case, insulation,

propellant, and an igniter. The case of the engine is usually a relatively thin metal that is lined with insulation to keep the propellant from burning through. The propellant itself is packed inside the insulation layer. Solid rockets can provide high thrust for relatively low cost. For this reason, solids have been used as initial stages in rockets, the classic example being the Space Shuttleâ&#x20AC;&#x2122;s Solid Rocket Boosters. Many solid-propellant rocket engines feature a hollow core that runs through the propellant. Rockets that do not have the hollow core must be ignited at the lower end of the propellants and burning proceeds gradually from one end of the rocket to the other. In all cases, only the surface of the propellant burns. However, to get higher thrust, the hollow core is used. This increases the surface of the propellants available for burning. The propellants burn from the inside out at a much higher rate, and the gases produced escape the engine at much higher speeds. This gives a greater thrust. Some propellant cores are starshaped to increase the burning surface even more. To fire solid propellants, many kinds of igniters can be used. Fire arrows were ignited by

Figure 6-11. A solid-propellant rocket (NASA)

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As with the inside of the rocket case, insulafuses, but sometimes these ignited too quickly and tion is needed to protect the nozzle from the hot burned the rocketeer. A far safer and more religases. The usual insulation is one that gradually able form of ignition used today is one that emerodes as the gas passes through. Small pieces of ploys electricity. An electric current, coming the insulation get very hot and break away from through wires from some distance away, heats up the nozzle. As they are blown away, heat is carried a special wire inside the rocket. The wire raises away with them. the temperature of the propellant it is in contact The other main kind of rocket engine is one with to the combustion point. that uses liquid propellants. This is a much more Other igniters are more advanced than the complicated engine, as is evidenced by the fact hot wire device. Some are encased in a chemical that solid rocket engines were used that ignites first, which then igfor at least seven hundred years benites the propellants. Still other fore the first successful liquid engine igniters, especially those for large was tested. Liquid propellants have rockets, are rocket engines themseparate storage tanks â&#x20AC;&#x201D; one for the selves. The small engine inside fuel and one for the oxidizer. They the hollow core blasts a stream of also have pumps, a combustion flames and hot gas down from chamber, and a nozzle. the top of the core and ignites the The fuel of a liquid-propellant rocket entire surface area of the propelis usually kerosene or liquid hydrolants in a fraction of a second. gen; the oxidizer is usually liquid oxyThe nozzle in a solidgen. They are combined inside a cavpropellant engine is an opening ity called the combustion chamber. at the back of the rocket that perHere the propellants burn and build mits the hot expanding gases to up high temperatures and pressures, escape. The narrow part of the and the expanding gas escapes nozzle is the throat. Just beyond through the nozzle at the lower end. the throat is the exit cone. The combustion of hydrogen and oxyThe purpose of the nozzle gen into water vapor is a reaction is to increase the acceleration of that looks like this the gases as they leave the rocket and thereby maximize the thrust. Figure 6-12. A liquid2H2 + O2 â&#x2020;&#x2019; 2H2O. It does this by cutting down the propellant rocket (NASA) opening through which the gases can escape. To see how this works, you can experiTo get the most power from the propellants, ment with a garden hose that has a spray nozzle they must be mixed as completely as possible. attachment. This kind of nozzle does not have an Small injectors on the roof of the chamber spray exit cone, but that does not matter in the experiand mix the propellants at the same time. Because ment. The important point about the nozzle is that the chamber operates under high pressures, the the size of the opening can be varied. propellants need to be forced inside. Powerful, Start with the opening at its widest point. lightweight turbine pumps between the propellant Watch how far the water squirts and feel the tanks and combustion chamber take care of this thrust produced by the departing water. Now rejob. duce the diameter of the opening, and again note With any rocket, and especially with liquidthe distance the water squirts and feel the thrust. propellant rockets, weight is an important factor. Rocket nozzles work the same way. In general, the heavier the rocket, the more thrust 126

6-13. Robert Hutchings Goddard (5 October 1882 – 10 August 1945) was an American engineer, professor, physicist, and inventor who is credited with creating and building the world’s first liquid-fueled rocket, which he successfully launched on 16 March 1926. In this picture, taken on the day of the successful launch, Goddard is seen bundled against the cold and holding the rocket’s launching frame. (NASA/ Wikipedia) is needed to get it off the ground. Liquid engines are much heavier than solid engines. Hydrogen becomes a liquid when it is chilled to about 20 K (-423 °F or -253 °C). Oxygen is a liquid at about 90 K (-297 °F, -183 °C). Despite criticism and early technical failures, the taming of liquid hydrogen proved to be one of NASA’s most significant technical accomplishments. In combination with an oxidizer such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency in relation to the amount of propellant consumed, of any known chemical rocket propellant. Because liquid oxygen and liquid hydrogen are both cryogenic — gases that can be liquefied only at extremely low temperatures — they pose enormous technical challenges. These

Figure 6-14. The Space Shuttle System had three components: the Orbiter which housed the crew; a large, rust-colored External Tank that held liquid propellant for the main engines; and two Solid Rocket Boosters which provided 80% of the Shuttle’s launch thrust during the first two minutes of flight. The lift-off weight of the Shuttle depended on its mission, but was around 4.5 million pounds. It reached altitudes of 115 to 400 miles. (NASA) liquids must be stored at extremely low temperatures and handled with extreme care. To keep them from evaporating or boiling off, rockets fueled with cryogenic liquids must be carefully insulated from all sources of heat, such as rocket engine exhaust and air friction during flight through the atmosphere. Once the vehicle reaches space, it must be protected from the radiant heat of the Sun. When cryogenic liquids absorb heat, they ex-

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pand rapidly; thus, venting is necessary to prevent the tank from exploding. Metals exposed to the extreme cold of cryogenic liquids become brittle. Moreover, liquid hydrogen can leak through minute pores in welded seams. Solving all these problems required an enormous amount of technical expertise in rocket propellants cultivated over a decade by researchers at the National Advisory Committee for Aeronautics (NACA) Lewis Flight Propulsion Laboratory in Cleveland, OH. Lack of liquid-hydrogen technology proved a serious handicap for the Soviets in the race of the two superpowers to the Moon. Controlling the thrust of an engine is very important to launching payloads, the rocket’s cargoes, into orbit. Too much thrust, or thrust at the wrong time, can cause a satellite to be placed in the wrong orbit or sent too far out into space to be useful. Too little thrust can cause the satellite to fall back to Earth. Liquid-propellant engines control the thrust by varying the amount of propellant that enters the combustion chamber. A computer in the rocket’s guidance system determines the amount of thrust that is needed and controls the propellant flow rate. On more complicated flights, such as going to the Moon or the planets, the engines must be started and stopped several times. Liquid engines do this by simply starting or stopping the flow of propellants into the combustion chamber. Solid-propellant rockets are not as easy to control as liquid rockets. Once started, the propellants burn until they are gone. They are very difficult to stop or slow down part way into the burn. Sometimes fire extinguishers are built into the engine to stop the rocket in flight. But using them is a tricky procedure and doesn’t always work. Some solid-fuel engines have hatches on their sides that can be cut loose by remote control to release the chamber pressure and terminate thrust. The burn rate of solid propellants is carefully planned in advance. Once ignited, it is difficult to change or stop the burn.

6.5.2 Exhaust Velocity And Specific Impulse Solid- and liquid-rocket propellants deliver exhaust velocities, or specific impulses, that depend on the chemicals they use. Different chemicals store different amounts of chemical energy to be released in the exothermic combustion reaction. Recall impulse was introduced in chapter 5.3, and exhaust velocity, in chapter 5.6. In addition to exhaust velocity, another performance parameter commonly used by rocketeers is “specific impulse.” One way of measuring the amount of impulse that can be obtained from a fixed amount of reaction mass is the specific impulse, the impulse per unit weight-on-Earth, Isp, Isp =

I . W

The unit for this value is seconds. The specific impulse can be thought of as the number of seconds that one pound of propellant will produce one pound of thrust. Specific impulse is a useful value to compare engines, much like miles per gallon is used for cars. A higher specific-impulse chemical is more propellant-efficient. Specific impulse is related to exhaust velocity as follows Isp =

ve . g⊕

You can easily rewrite the rocket equation and replace exhaust velocity with specific impulse times gravitational acceleration. Liquid propellants in general have higher specific impulses and hence, exhaust velocities, than solid propellants. Solid propellants are around 200 s or about 2 km/s, while liquid propellants are around 400 s or about 4 km/s. A typical, well-designed solid ammonium perchlorate composite propellant first-stage motor may have a specific impulse as high as 286 s, or an exhaust velocity of 2,803 m/s. On the other hand, a typical liquid propellant mix of liquid oxygen and liquid hydrogen has an exhaust velocity of 4,462 m/s and a specific impulse of 455 s.

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Looking at these real values for ve, from either solid or liquid propellants, now makes it clear that Δv for launch is always a few times ve. The mass ratio for launch is clearly in the exponential portion of Figure 5-12. Here are some real numbers for Space Shuttle propellant usage from NASA, given in gallons as well as in pounds. Also quoted are the associated propellant costs, in 2011 dollars. At lift off, the Shuttle carried 835,958 gallons of the principal liquid propellants – hydrogen, oxygen, hydrazine, monomethylhydrazine and nitrogen tetroxide – which in 2011 cost approximately $1,380,000. Their total weight was 1,607,185 lbs. The External Tank of the Shuttle carried about 384,071 gallons (227,800 lbs) of liquid hydrogen, but about 628,540 gallons (372,800 lbs) were used in all during Shuttle vehicle loading. Before final loading began, some liquid hydrogen was used to pre-chill fuel lines and the hydrogen compartment of the tank; additionally, during fueling, some boiled off as a gas. The hydrogen used was produced from natural gas by a steamreforming process in New Orleans, LA. It was shipped in 13,000-gallon mobile tankers. The price of hydrogen was 98 cents a gallon. The Shuttle used two types of liquid oxygen. The oxygen loaded into the External Tank, 141,750 gallons (1,350,000 lb), was produced at Mims, FL, by liquefying and separating air. The oxygen was trucked to KSC in 6,000-gallon tankers. As in the case of hydrogen, Shuttle servicing required more oxygen than the actual capacity of the oxygen compartment. About 250,000 gallons (2,580,000 lbs) were used in all. Small quantities were also used aboard the orbiter to provide its breathable atmosphere. The price of oxygen was 67 cents per gallon. The purer type of oxygen used in the Shuttle’s Power Reactant and Storage Distribution System required 327 gallons (2,340 lbs) per mission and was more expensive, $2.85 per gallon. About 800 gallons were used in total. Fuel cell oxygen was produced in Orlando, FL, by the same process as the propellant oxygen. Because it had to be of

higher purity, however, a more modern plant was required. The plant was used solely for this purpose during a production run, and the curtailment of other operations was among the reasons for the higher cost. Fuel cell oxygen was shipped in 4,000-gallon tankers. Some of the other chemicals used in smaller quantities were costlier; with price tags of up to around $500 per gallon.

6.6 Staging Here is a surprise: we cannot currently build a rocket that can carry a payload into orbit. Not with just using one single rocket stage, that is. No Earth-launched “single-stage-to-orbit” (SSTO) launch vehicles have ever been constructed. This is because a single rocket stage would simply be too heavy requiring not only a large amount of propellant but also a massive structure for the tanks, engine, etc. Many years of research have gone into developing technologies for achieving orbital speeds with a single rocket, or single stage. It is considered to be marginally possible to launch a SSTO spacecraft from Earth, given the right, lightweight but strong materials. SSTO has been achieved from the Moon by both the Apollo program’s Lunar Module and several robotic spacecraft of the Soviet Luna program. The lower lunar gravity and absence of any significant atmosphere make this much easier than STTO from Earth. To date, all orbital launches from Earth have been performed by multi-stage rockets. A multi-stage rocket is a rocket that uses two or more stages, each of which is its own rocket and contains its own tanks, engines and propellants. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Two-stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched. By jettisoning stages when they run out of propellant, the mass of the remaining rocket is decreased. This staging allows the thrust of the remaining stages to more easily accelerate the rocket to its final speed and height.

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In serial or tandem staging schemes, the first stage is at the bottom and is usually the largest, the second stage and subsequent upper stages are above it, usually decreasing in size. In parallel staging schemes several rocket boosters are used to assist with lift off. In the typical case, the firststage and booster rockets (also called the 0th stage) fire to propel the entire rocket upwards. When Figure 6-15. This is an example of serial staging. When the first stage burns the boosters run out of out, it is discarded and falls back to Earth. This reduces the mass of what refuel, they are detached mains and has to be accelerated to orbital speed. (NASA) from the rest of the rocket lightens itself. The thrust of future stages is rocket (usually with some kind of small explosive able to provide more acceleration than if the earcharge) and fall away. The first stage then burns to lier stages were still attached. When a stage drops completion and falls off. This leaves a smaller off, the rest of the rocket is still traveling near the rocket, with the second stage on the bottom, speed that the whole assembly reached at burnwhich then fires. Known in rocketry circles as stagout time. This means less total fuel to reach the ing, this process is repeated until the final stageâ&#x20AC;&#x2122;s final velocity and/or altitude. motor burns to completion. The main reason for multi-stage rockets and boosters is that once the propellant is exhausted, the space and structure which contained it and the motors themselves are useless and only add weight to the vehicle which slows down its future acceleration. What if you could drop some of that mass away and not accelerate it to orbit? That is the idea behind staging. By dropping the stages which are no longer useful, the Figure 6-16. This is an example of parallel staging. (NASA) 130

tural mass associated with the stage, mS, plus the payload mass,

A further advantage is that each stage can use a different type of rocket motor each tuned for its particular operating conditions. Thus the lower-stage motors are designed for use at atmospheric pressure, while the upper stages can use motors suited to near vacuum conditions. Lower stages tend to require more structure than upper as they need to bear their own weight plus that of the stages above them. Optimizing the structure of each stage decreases the weight of the total vehicle and provides another advantage. On the downside, staging requires the vehicle to lift motors which are not yet being used, as well as making the entire rocket more complex and harder to build. In addition, each staging event is a significant point of failure during a launch, with the possibility of separation failure, ignition failure, and stage collision. Nevertheless the savings are so great that every rocket ever used to deliver a payload into orbit has had staging of some sort. If you build a launch vehicle and it has multiple stages, n, the delta v’s of the individual stages can be added to calculate the mission delta v:

mi = mP + mS + mPL, Assume that you have a launch vehicle with two stages. You could then consider the second stage the payload of the first stage. And the second stage is the one which is carrying the original payload of the rocket. The initial and final masses of the first stage are then mi1 = mP1 + mS1 + mi2 mf 1 = mS1 + mi2 Now you eject mS1. The initial and final masses of the second stage are mi2 = mP2 + mS2 + mPL mf 2 = mS2 + mPL

Δv = Δv1 + Δv2 + Δv3 + . . . Δvn. If each stage had the same exhaust velocity and the same initial-to-final mass ratio, then the rocket equation could be written Δv = n × veln

mi . ( mf )

But in reality, the stages will be designed to be quite different. In a serial design, for instance, the first stage is usually the largest, and subsequent stages are smaller and smaller, fire later and later, and therefore have quite different characteristics. Let’s break the masses of the stages down. Your final mass is what you want to carry aloft, the payload mass, mPL. Each stage’s initial mass contains a propellant mass, mP, and the struc-

Now you eject mS2. You can see that there is a reduction in mass when you jettison mS1 and mS2 after the first and second stage burn out. Let’s look at an example, the mighty Saturn V, a three-stage serial launch vehicle used for the US’s Moon shot. Figure 6-17 shows the 281 ft tall launch vehicle, with its three stages plus the Instrument Unit. The latter was considered part of the launch vehicle and was a computer that controlled the operations of the rocket from just before lift off until the third stage was discarded. For the purpose of this exercise, count it as part of stage three. The payload was the 82 ft tall spacecraft. The spacecraft consisted of the Lunar Module, which took the crew to the surface of the Moon, the Service Module, for the propulsion and spacecraft support systems, and the Crew Module, which was the portion that returned to Earth. The weight of the spacecraft on Apollo 11 was 109,646 lb. The entire spacecraft was accelerated by the launch vehicle to achieve translunar injection velocity, which is a little higher than orbital velocity.

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Figure 6-17. The very large three-stage Saturn V launch vehicle launched a substantial spacecraft from the Earth to the Moon. (NASA) To this end, the third stage was fired twice, but weâ&#x20AC;&#x2122;ll gloss over that in our example calculation. First, add up all the fueled weights of the launch vehicle stages. Add to this the spacecraft weight, and you obtain the initial weights. For the first stage, this is 6,456,320 lb; for the second stage, you get 1,433,646 lb, and for the third stage, 264,829 lb. Next, calculate the final masses for each stage by reducing the initial masses by the propellant masses. The propellant weights can be found from the differences between the fueled and dry weights. A final piece of information you need are the specific impulses for the first stage, 263 s, giving an exhaust velocity of 2.58 km/s, and the second and third stages, which were the same, 421 s or 4.13 km/s. You can now calculate the delta v for each stage and estimate the total delta v from thrust after all stages are fired:

Figure 6-18. A photograph, taken from a military airplane, of the separation of the first stage from the Apollo 11 flight to the Moon of the Saturn V. Separation occurred at an altitude of about 38 miles, some 55 miles down range from KSC. (NASA)

km km km + 4.75 + 4.09 s s s km = 12.25 . s

Î&#x201D;vT = 3.41

The estimate shows that the Saturn V yielded quite a delta v, enough to place the Apollo space132

Figure 6-20. A Pegasus rocket is air-launched from the Stargazer carrier aircraft. (NASA)

Figure 6-19. This image shows a Pegasus rocket on the ground before its flight on a B-52. (NASA) craft into Earth orbit, even with gravity, drag, and steering losses.

6.7 Launch Alternatives Because the cost of launching a rocket to orbit is so high, there has been quite some research into developing alternative ideas. From space canons, to space towers, to space elevators, none of these theorized launch alternatives are in service. There is one operational launch alternative, which consists of the combination of an airplane and a rocket.

6.7.1 Air Launch To Orbit Air launch to orbit is the method of launching rockets at altitude from a conventional horizontal-takeoff aircraft, to carry satellites to low Earth orbit. It is a follow-on development of air launches of experimental aircraft that began in the late 1940s. This method, when employed for orbital payload insertion, presents significant advantages over conventional vertical rocket launches, particularly because of the reduced mass, thrust and cost of the rocket. The principal advantage of a rocket being launched by a highflying airplane is that it need not fly through the low, dense atmosphere, where drag requires a considerable amount of extra thrust and thus propellant. The carrier aircraft serves as a booster to launch payloads at reduced cost.

The winged aircraft makes use of lift in addition to thrust. Propellant needs are less because the carrier aircraft lifts the rocket using airbreathing engines that do not require on-board storage of oxidizer. This also reduces the overall size, and ultimately, payload launch costs. It is also possible to make use of higher-impulse fuels precluded from surface launches due to their toxicity, such as those containing beryllium or fluorine. Air launch to orbit offers the potential for aircraft-like operations such as launch on demand, and is also less subject to launchconstraining weather. This allows the aircraft to fly around weather conditions as well as fly to better launch points, and to launch a payload into any orbital inclination at any time. The Pegasus is an air-launched rocket developed by Orbital Sciences Corporation (Orbital). Capable of carrying small payloads of up to 443 kg (977Â lb) into low Earth orbit, Pegasus first flew in 1990. The vehicle consists of three stages, and is about 17 m (55 ft) tall. Pegasus is released from its carrier aircraft at approximately 12 km (40,000Â ft); and its first stage has wings and a tail to provide lift and attitude control while in the atmosphere. The first successful Pegasus launch occurred on 5 April 1990 with NASA test pilot and former astronaut Gordon Fullerton in command of the carrier aircraft. An altitude of 12 km is only about 4% of a low Earth orbital altitude, and the subsonic aircraft reaches only about 3% of orbital velocity, yet by delivering the launch vehicle to

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this speed and altitude, the reusable aircraft replaces a costly first-stage booster. Initially a NASA-owned B-52 Stratofortress served as the carrier aircraft. By 1994, Orbital had transitioned to their Stargazer L-1011, a converted airliner which was formerly owned by Air Canada. The name Stargazer is an homage to the television series Star Trek: The Next Generation. The character Jean-Luc Picard was captain of a ship named Stargazer prior to the events of the series, and his first officer William Riker once served aboard a ship named Pegasus. In a Pegasus launch, the carrier aircraft takes off from a runway with support and checkout facilities. Such locations have included KSC / Cape Canaveral Air Force Station, FL; Vandenberg Air Force Base and Dryden Flight Research Center, CA; Wallops Flight Facility, VA; Kwajalein Range in the Pacific Ocean, and the Canary Islands in the Atlantic. Upon reaching a predetermined staging time, location, and velocity vector the aircraft releases the Pegasus. After five seconds of free-fall, the first stage ignites and the vehicle pitches up. The 45o delta wing (of carbon composite construction and double-wedge airfoil) aids pitch-up and provides some lift. The tail fins provide steering for first-stage flight, as the stageâ&#x20AC;&#x2122;s motor does not have a thrust-vectoring nozzle. Approximately 1 minute and 17 seconds later, the stage burns out. The vehicle is at over 61 km (200,000 ft) in altitude and at hypersonic speed. The first stage falls away, taking the wing and tail surfaces, and the second stage ignites. The second stage burns for approximately 1 minute and 18 seconds. Attitude control is by thrust vectoring the engine in two dimensions, pitch and yaw; roll control is provided by nitrogen thrusters on the third stage. Midway through second-stage flight, the launcher has reached a near-vacuum altitude. The fairing splits and falls away, uncovering the payload and third stage. Upon burnout of the secondstage motor, the stack coasts until reaching a suitable point in its trajectory, depending on mission. Then the second stage is discarded, and the third stage ignites. It too has a thrust-vectoring nozzle,

assisted by the nitrogen thrusters for roll. After approximately 64 seconds, the third stage burns out.

6.8 Landing It might seem strange that landing is part of the lift off chapter. But if you think about it, landing is in principle just the opposite of launching. When landing, the very large delta v of the spacecraft now needs to be brought back down to zero, preferably to a gentle stop. A â&#x20AC;&#x153;landerâ&#x20AC;? is a spacecraft which descends toward and comes to rest on the surface of an astronomical body. There are two fundamentally different landing scenarios. The first one is for landing on a body that has no atmosphere, such as the Moon or Mercury, while the second one is for bodies with atmospheres, and applies to Earth, Mars, Titan, to name a few.

6.8.1 Landing On Bodies Without Atmospheres For an airless body, the landing vehicle must provide the full delta v necessary to land safely on the surface. A powered descent with a gravity turn is the best option. The Apollo Lunar Module used a slightly modified gravity turn to land from lunar orbit. This was essentially a launch in reverse except that a landing spacecraft is lightest at the surface while a spacecraft being launched is heaviest at the surface. The vehicle begins by orienting for a retrograde burn to reduce its orbital velocity, lowering its point of periapsis to near the surface of the body to be landed on. After the deorbit burn is complete the vehicle can either coast until it is nearer to its landing site or continue firing its engine. If it is not already properly oriented, the vehicle lines up its engines to fire directly opposite its current surface velocity vector, which at this point is only slightly vertical. The vehicle then fires its landing engine to slow down for landing. As the vehicle loses horizontal velocity the gravity of the body to be landed on will begin pulling the trajectory closer and closer to a vertical descent. In an ideal maneuver on a perfectly spherical body the

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vehicle could reach zero horizontal velocity, zero vertical velocity, and zero altitude all at the same moment, landing safely on the surface. However due to rocks and uneven surface terrain the vehicle usually picks up a few degrees of angle of attack near the end of the maneuver to zero its horizontal velocity just above the surface. This process is the mirror image of the pitch over maneuver used in the launch procedure and allows the vehicle to hover straight down, landing gently on the surface.

6.8.2 Landing On Bodies That Have Atmospheres

Figure 6-21. Top: Artist’s drawing of the Apollo 11 Command and Service Module Columbia docked with the Lunar Module Eagle, in lunar orbit. Middle: The Eagle was jettisoned from the Columbia for descent to the Moon. This is a photo of the Columbia taken from the Eagle. Bottom: The Eagle in landing configuration as photographed from the Columbia. Inside the module were Commander Neil A. Armstrong and Lunar Module Pilot Buzz Aldrin. The lander fired rockets to slow itself down. Starting sideways after detaching from the Command and Service Module, it pitched upward during descent until the landing pods pointed down toward the surface. The long rod-like protrusions under the landing pods are lunar surface sensing probes. Upon contact with the lunar surface, the probes sent a signal to the crew to shut down the descent engine. (NASA)

When a spacecraft is targeted to land on a body which has an atmosphere, it must first enter into the atmosphere at high speeds. In the case of a spacecraft that is returning to Earth, this phase is called “reentry.” The altitude at which the atmosphere causes drag relevant for entry is called the “entry interface.” For Earth, the entry interface occurs near the Kármán Line while Venus’ is at 250 km and Mars’ is at about 80 km. Spacecraft which land on a planet with an atmosphere depend on drag rather than thrust to slow the craft. A side effect of drag is aerodynamic heating. Entry can be uncontrolled, such as due to natural decay of a satellite’s orbit or due to a failure, or it can be purposeful and controlled following a predesigned descent trajectory and soft landing. If the spacecraft arrives at the planet from deep space, it will approach the planet with a velocity that is at least equal to the escape velocity characteristic of the planet. Several kinds of approach orbits are of interest. A direct hit on the planet would involve an entry path similar to that of a ballistic rocket, but with a higher velocity. A more gradual entry can be accomplished by either approaching the planet tangentially or by maneuvering into a satellite orbit before descending. Shifting into a satellite orbit can be accomplished either with thrusters and using up what propellant may be remaining, or by the “aerobraking” procedure.

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Figure 6-22. The entry corridor into Earth’s atmosphere has a very narrow range of allowable angles. This artist’s drawing shows the reentry corridor for Apollo. The entry angle could be no more than 7 0. 7 and no less than 5 0. 3. If a spacecraft flies above the corridor, it may skip out of the atmosphere. If it flies below the corridor, it may burn up. (NASA)

a shallow angle, you can make it bounce off the water’s surface. You know from experience that, if the rock is not flat enough or its angle of impact is too steep, it’ll make only a noisy splash rather than a quiet and graceful skip. This situation is not unlike what happens when a spacecraft enters an atmosphere. It flies at a very high speed and the atmosphere presents a dense, fluid medium, which, at orbital velocities, is not all that different from a lake’s surface. Mission planners have to make a vehicle hit the atmosphere at the precise angle and speed for a safe landing. If it enters too steeply or too fast, you risk making a big “splash,” which would mean the spacecraft burns and breaks up. If the entry is too shallow, the vehicle may literally skip off the atmosphere and back into Outer Space. These two extremes dictate safe entry angles into an atmosphere that define a very narrow “entry corridor.” This is the flight path that the vehicle must be on for a safe landing on the surface. The entry angles leave little margin for error, so engineers make great efforts to ensure spacecraft are carefully guided along the proper trajectory to survive the extremes of atmospheric entry.

Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit by flying the vehicle through the atmosphere at the low point of the orbit. Aerobraking requires less or no propellant compared to the use of thrust. Although the theory of aerobraking is well developed, utilizing the technique is difficult because a very detailed knowledge of the character of the target planet’s atmosphere is needed in order to plan the maneuver correctly. Entering into an atmosphere from Outer Space at high speeds is quite challenging. If you’ve ever skipped a pebble on a pond, you know that when you Figure 6-23. Atmospheric entry trajectory illustrating the basic phases of flight in throw a flat stone a skip reentry. In this case, a single skip is used to extend the landing range of the across water at just vehicle, for example to reach a landing site on the far side of the Earth. (Wikipedia) the right speed and 136

reduce sustained g loading over extended time periods. A lifting entry is one in which a lift force is generated. Drag is also present throughout the entry, and very important for slowing the vehicle. In a lifting entry the resulting flight path can be adjusted to change both vertical motion and flight direction while the velocity is slowing. As you know, all kinds of shapes can generate a lift force, and so, when you pitch a space capsule to a certain angle of attack, you can get a lift force on it. Creating even a Figure 6-24. There are special concerns for entry, descent and small amount of lift can help reduce landing in the thin atmosphere of Mars. The Curiosity Mars the g-forces of the reentry and deRover had seven minutes from the top of the Martian atmosscent, as well as reducing the peak phere to touchdown on the ground, using a parachute, thrusters and a skycrane. (NASA) reentry heat. Such so-called “semiballistic” reentries were used for the There are three major strategies available Gemini and Apollo capsules. for aerodynamic entry, descent and landing: balThe primary design parameter for lifting listic entry, lifting entry, and skip entry. entry is the “Lift-to-Drag Ratio.” Low values of In a ballistic entry, the craft falls through L/D produce moderate g loads, moderate heatthe atmosphere under the influence of weight ing levels, and low maneuverability. High values pointing down, and drag pointing up. The drag of L/D produce very low g loads, but entries are force on the body of the vehicle slows it down enough so that parachutes can be deployed and create an even larger drag to decelerate it further for a soft touchdown. A ballistic reentry vehicle is subject to high g- and heat loads. The landing point of a ballistic entry has low accuracy because of a lack of steering, requiring careful preplanning of the entry, and a large landing area. Safety on Earth is why NASA let such spacecraft splashdown in the ocean and the Russians landed their’s on the underpopulated Kazakh steppe. The first manned ballistic reentry was the flight of John Glenn’s Friendship 7 Mercury Figure 6-25. An artist’s rendition of the Orion capsule in February 1962. Glenn was subjected Crew Module’s fireball as it reenters into Earth’s atmosphere. A shock wave forms to a substantial deceleration of over 8 g for about around the heat shield and directs the heat 30 s. This was the impetus for the development away from the vehicle. The heat shield ablates, of lifting entry for crewed spaceflight in order to and the pieces that fly away also take heat away from the vehicle. (NASA) 137

Figure 6-26. Two T-38 chase planes follow Space Shuttle Columbia as it lands at Northrop Strip in White Sands, New Mexico, ending its third mission, STS-3. (Wikipedia /NASA) of very long duration and have continuous heating. An example is Space Shuttle reentry at a L/D value of around unity with a total entry time of about 25 minutes. Although the peak temperatures of a lifting entry are below the peak temperature of a ballistic entry, the total heat load that must be absorbed over the duration of the entry is higher. The Space Shuttle was a winged NASA craft that used lift to land on a runway like a glider. Yet another reentry option that combines features of both ballistic and gliding trajectories is the skip entry trajectory. Skip entry is an entry technique involving one or more successive dips and skips off the atmosphere to achieve greater entry range or to slow the spacecraft before final entry, which helps to dissipate the huge amount of heat that is usually generated on faster descents. The technique was used by the Russian Zond series of circumlunar spacecraft. As a spacecraft reenters the Earth’s atmosphere, it is traveling at hypersonic speed, by about 25 times the speed of sound. Strong shock waves are generated on the lower surface of the spacecraft. The intense heat which is created by the compression of the atmosphere causes air molecules to break apart producing an electrically charged ionized plasma around the vehicle. A “thermal protection system” is the barrier that protects a spacecraft during the searing heat

of atmospheric entry. Reentry into the Earth’s atmosphere creates heat loads from 3,500 °F (1,926 °C) up to as high as 6,000 °F (3,315 °C). Several approaches for the thermal protection of spacecraft are in use. Examples are ablative “heat shields,” passive cooling and active cooling of spacecraft surfaces. The Orion Crew Module uses an ablative heat shield which is similar to the one that the Apollo Crew Module used, but with much more modern and better ablative material. In a basic sense, “ablative” material is designed to slowly burn away in a controlled manner, so that heat can be carried away from the spacecraft by the gases generated by the ablative process while the remaining solid material insulates the craft from superheated gases. There is an entire branch of spaceflight research involving the search for new fireproofing materials to achieve the best ablative performance; this function is critical to protect the spacecraft occupants and payload from otherwise excessive heat loading. Spacecraft reentering the Earth’s atmosphere suffer from a “communications blackout” with ground control on Earth. The communications blackouts are caused by the envelope of ionized air around the craft. The ionized air interferes with radio signals. For the Mercury, Gemini, and Apollo spacecraft, such communications blackouts lasted for several minutes. Until the creation of the Tracking and Data Relay Satellite System (TDRSS), the Space Shuttle endured a 30 minute blackout. The TDRSS allowed the Shuttle to communicate by relay with a Tracking and Data Relay Satellite during reentry, through a hole in the ionized air envelope at the tail end of the craft, created by the Shuttle’s shape. Communications blackouts are not solely confined to reentry into Earth’s atmosphere. They apply to entry into any atmosphere where such ionization occurs around a craft. The Mars Pathfinder endured a communications blackout as it entered Mars’ atmosphere, for example. The Huygens probe experienced a communications blackout as it entered the atmosphere of Titan.

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Figure 6-27. NASAâ&#x20AC;&#x2122;s Orion spacecraft awaits the US Navyâ&#x20AC;&#x2122;s USS Anchorage for a ride home. Orion launched into space in December 2014 on a two-orbit, 4.5-hour test flight, and safely splashed down in the Pacific Ocean, where a combined team from NASA, the Navy and Orion prime contractor Lockheed Martin retrieved it for return to shore on board the Anchorage. (US Navy/NASA)

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6 Test Your Understanding I. A NSWER IN A FEW SENTENCES . 1. What is a mission profile? 2. What are typical phases of a space mission? Describe them in the order they occur. 3. What is meant by the escape velocity, and what does escape velocity depend on? 4. What is meant by orbital velocity, and what does orbital velocity depend on? 5. Why is the velocity of a satellite in orbit always changing? Include a drawing. 6. What path would an object take, following its inertia, if you could remove the centripetal force from its motion? Include a drawing. 7. What are the forces acting on a rocket as it sits on the launch pad? Include a drawing. 8. What is Loss of Consciousness and how does it occur? 9. What is meant by a delta-v budget? 10.What are the components to the delta-v budget that must be included when launching a rocket from Earth? Give the equation and describe each term. 11. What might be the consequences of launching a rocket from KSC with a launch azimuth of 00? 12.What is the pitch-over maneuver? 13.What are the pros and cons of different propellant types? List and describe at least two. 14.What is the purpose of a nozzle? 15.What chemical forms the exhaust in the most common liquid-propellant rocket? 16.What are some of the challenges posed by cryogenic propellants? Name and describe at least three. 17.How can you demonstrate that specific impulse has the unit of second? 18.What does single-stage-to-orbit mean, and has it ever been achieved by a spacecraft? 19.What is meant by staging? 20.What is serial staging? Draw the phases of a launch of a three-stage rocket that uses serial staging.

21.Why are alternative to using a rocket for orbital launches so sought after? 22.What is the difference between landing on a body that doesn’t have an atmosphere versus one that does have one? 23.What is the aerobraking maneuver? 24.What is an entry corridor? 25.What were the forces acting on the Space Shuttle when it landed like a glider? Include a drawing which shows the force vectors. 26.A heat shield is made of ablative material. What does that mean? II. C ALCULATE THE A NSWER . 1. The International Space Station orbits the Earth at an altitude of 249 miles. a) What is the orbital speed of the ISS? b) How long does it take the ISS to make one Earth orbit? 2. The Orion Crew Module was in an elliptical second orbit which took it to an altitude of 3,600 miles at apogee and 115 miles at perigee. Calculate the velocity of the Orion at both perigee and apogee. 3. Can this rocket launched from Kourou on the Equator achieve escape speed? It’s engines supply it with a total thrust of 14.5 km/s. Gravity loss is 2 km/s, drag loss is 1 km/s, and steering loss is 0.5 km/s. 4. A serial, two-stage rocket has the following masses: first stage propellant mass 100,000 kg, first stage structural mass 8,000 kg, second stage propellant mass 40,000 kg, second stage structural mass 4,000 kg, and payload mass 1,000 kg. The specific impulses of the first and second stages are 280 s and 455 s respectively. Calculate the launch vehicle’s total delta v from thrust.

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The flight path of the Rosetta spacecraft from launch to its encounter with the comet 67P/Churyumov-Gerasimenko. (ESA/NASA)

SPACECRAFT TRAJECTORIES After a rocket is launched, it can follow a flight path into an Earth orbit or it can be sent onward to a planet. What are some flight paths that space probes follow to travel throughout our Solar System? How can a flight path be changed? And how does this change affect propellant usage?

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7.1 Trajectories The terms trajectory and orbit both refer to the flight path of a body in space. Trajectory is commonly used in connection with flight paths having clearly identified initial and end points. Orbit is commonly used in connection with natural bodies (planets, moons, etc.). And it is associated with flight paths of a repetitive character, like the orbit of the Moon around the Earth. In discussions of spaceflight, both terms are used depending on the nature of the flight path. Thus we speak of trajectories from the Earth to the Moon, and of satellite orbits around the Earth. The type of path that a spacecraft takes depends on the Sphere of Influence it is in and its velocity. One of Newton’s remarkable achievements was his deep understanding of the physics of motion and gravity which enabled him to explain why Kepler’s laws work the way they do. Kepler derived his laws by describing how the planets orbit the Sun. Newton used his laws to demonstrate how orbits result from inertia and the force of gravity. You can use Newton’s laws to show that, in the idealized gravitational field of a point mass or a spherically-symmetrical extended mass, the trajectory of a moving object is a conic section, usually an ellipse or a hyperbola. Here is one example that illustrates how Newton’s laws explain Kepler’s laws. You know from the previous chapter that the distance-velocitytime relationship for a spacecraft on a circular orbit can be expressed as v=

2πr P

and that the centripetal force of gravity of the massive body being orbited results in the spacecraft having an orbital velocity of v= Figure 7-1. Geometry of a circular orbit (Wikipedia)

GM . r

Equating the two equations and squaring yields

GM 4π 2r 2 . = r P2 Now solve this equation for P 2 and you get P2 =

4π 2 3 r GM

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This is exactly Kepler’s third law (compare chapter 3.7), only for a circular orbit. So instead of saying, “The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit” it reads, “The square of the orbital period of any object on a circular orbit is proportional to the cube of the orbital radius.” The equation works for both natural and artificial bodies on circular orbits. You can also solve the equation to get an expression for the orbital period of a circular orbit

Porbit =

4π 2r 3 . GM

You can do a similar kind of derivation for an elliptical orbit only the math is harder. Newton was able to derive all of Kepler’s laws using his laws of motion and gravity, and he invented calculus along the way to do so. This is the foundation of understanding flight paths. In reality, objects are not spherical, have nonspherical mass distributions, plus additional gravitational forces of other massive bodies may also have to be taken into account. Most systems that involve multiple gravitational attractions present one primary body which is dominant in its effects (for example, a star, in the case of the star and its planets, or a planet, in the case of the planet and its satellites). Then the gravitational effects of the other planets or satellites can be treated as perturbations of the hypothetical, unperturbed motion of the planet or satellite you’re interested in. The trajectories can be calculated numerically using a computer.

7.2 Sub-orbital Trajectories Sub-orbital trajectories are ones in which the spacecraft reaches space, but it does not complete one full orbital revolution. If the primary is Earth, then these are flights where the apogee of a spacecraft reaches space, i.e., 100 km or more, but its perigee is too low, less than the radius of the Earth. Thus the trajectory intersects the Earth’s

Figure 7-2. Example mission profile of a suborbital spaceflight (Wikipedia) surface before the spacecraft can complete an orbit. The first American human spaceflight in 1961 was a suborbital flight, by Mercury astronaut Alan Shepard. Sub-orbital trajectories combine powered and free flight. To minimize the required delta v, the high-altitude part of the flight is made with the rockets off. This is technically called free fall even for the upward part of the trajectory. The maximum speed on such an arc is attained at the lowest altitude of the free-fall trajectory, both at the start and at the end of it. An astronaut experiences apparent weightlessness both on the upward and the downward portion of the free-fall portion of the trajectory. The delta v requirements for sub-orbital spaceflight are much lower than for orbital spaceflight. If the goal of the mission is simply to reach space, horizontal motion is not needed. In this case the lowest required delta v is about 1.4 km/s, for a sub-orbital flight with a maximum speed of about 1 km/s. For sub-orbital spaceflights covering a horizontal distance the maximum speed and required delta v are in between those of a vertical flight and orbital velocity. The V-2 rocket, the first rocket that just reached space and had a range of about 330 km, required a maximum speed of 1.6 km/s (3,580 mph). Intercontinental ballistic missiles achieve maximum speeds of about 7 km/ s. Because sub-orbital craft fly at lower speeds than orbital ones, they generate less heat upon reentry. The major use of suborbital vehicles today is for scientific experiments, but there may soon be sub-orbital flights serving the space tourism in-

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most of the duration of the mission. When orbiting the Earth, their spacecraft and their bodies are both continually free falling around the Earth. Orbiting spacecraft can sometimes be seen from Earth as points of light tracking across the night sky. For instance, observers at certain geographic latitudes can see the ISS moving against the background of the fixed stars at night (see chapter 4).

7.3.1 Orbital Eccentricity Figure 7-3. Mission profile of ESA’s IXV experimental spaceplane, which was launched on a Vega rocket from Kourou, French Guiana, 11 February 2015, on a suborbital arc to test its reentry system. The IXV was released from the Vega rocket at an altitude of 333 km, then climbed in free fall to the targeted altitude of 412 km, before stalling out and starting its downward free fall for reentry. (ESA) dustry. Private companies such as Virgin Galactic, XCOR, Armadillo Aerospace, Airbus, Blue Origin and Masten Space Systems are taking an interest in sub-orbital spaceflight, due in part to ventures like the Ansari XPRIZE. The Ansari XPRIZE was a space competition in which the XPRIZE Foundation offered a US$10,000,000 prize for the first non-government organization to launch a reusable manned spacecraft into space twice within two weeks. It was modeled after early 20th-century aviation prizes, and aimed to spur development of low-cost spaceflight. More on this in Chapter 9.

Artificial satellites in orbit around a primary behave just like planets in orbit around the Sun: they follow Kepler’s laws (see chapter 3.7). Their orbits are ellipses, with the primary body in the focus of the ellipse. The shape of the orbit is given by orbital eccentricity. Ellipses have eccentricities from zero, where zero is a circular orbit, to less than one. Eccentricity is one of six orbital parameters which together, fully describe the properties of an orbit.

7.3.2 Orbital Directions The orbit of a spacecraft around a primary like the Earth is usually in the same direction as the spin of Earth on its axis. As you know, there is a delta v boost that can be obtained when launching from Earth due east, with Earth’s spin. Orbits that are in the same spin direction as their central celestial body are called direct or “prograde” or-

7.3 Orbital Trajectories Orbital trajectories are flight paths on which spacecraft remain in space for at least one orbit. The main proven technique for reaching orbit involves launching a rocket nearly vertically for a few kilometers while performing a gravity turn, and then progressively flattening the trajectory out and accelerating on a horizontal trajectory to the orbital speed. Orbital speed is slower for higher orbits, but attaining them requires higher delta v. Astronauts participating in orbital space missions experience apparent weightlessness for

Figure 7-4. Orbital inclination is the angle, i, between the plane of an orbit and the equator. (NASA)

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it has a high enough horizontal velocity. However, an Earth-orbiting satellite in orbit of less than roughly 200 km above Earth is considered unsta7.3.3 Orbital Inclinations ble due to atmospheric drag. For a satellite to be Spacecraft always orbit about the center of in a stable orbit (i.e. sustainable for more than a the primary body. The plane in which they orbit few months), 350 km is a more standard altitude intersects the body’s center. The inclination, i, is for low Earth orbit. Even then the satellite may octhe angular distance of the orbital plane from the casionally have to fire small thrusters to maintain plane of reference, usuits orbit. This is also really the primary’s equaferred to as “station keeptor, and is normally ing.” The exact behavior of stated in degrees. Here objects in orbits depends are the possible scenaron their altitude, shape, ios: and details of space • an inclination weather which can affect of 0° means the satelthe height of the upper atlite orbits the primosphere. mary in its equatoThere are three main rial plane, in the bands of orbits around the same direction as Earth: low Earth orbit the primary rotates; (LEO), medium Earth oran inclination • bit (MEO) and high orbit greater than 0° and (HEO). Low Earth orbits less than 90° is an have altitudes up to 2,000 inclined, prograde km (1,240 miles). Medium orbit; Earth orbits range in alti• an inclination Figure 7-5. A comparison of spacecraft in differtude from 2,000 km to just greater than 90° and ent orbital bands. The Hubble Space Telescope and the ISS are in LEOs. Several navigation satel- below geosynchronous orless than 180° is an bit at 35,786 kilometers lites, such as the GPS satellites, occupy MEO orinclined, retrograde bits. (Wikipedia) (22,236 mi). High Earth orbit; orbits have altitudes of • an inclination of exactly 90° is a polar or35,786 km (22,240 miles) or more. bit, in which the spacecraft passes over the north and south poles of the planet; and 7.3.5 Orbital Synchronicity: Geo• an inclination of exactly 180° is a retrosynchronous And Geostationary grade equatorial orbit. Orbits bits. A “retrograde” orbit has a direction opposite of the rotation of its primary.

7.3.4 Orbital Altitudes Satellites can orbit at a range of distances from their primary, and this is referred to as the orbital altitude. By definition the highest orbit around Earth is determined by Earth’s SoI. Eventually, as a satellite moves further away, the Sun’s gravity will take over. Theoretically, a satellite could orbit Earth at a very low altitude as long as

An orbit whose period is a multiple of the rotational period of the body being orbited and in the same direction of rotation as that body is on a synchronous orbit. In practice, only 1:1 ratios (synchronous) and 1:2 ratios (semi-synchronous) are common. GPS satellites, for instance, are on semisynchronous orbits which take them around Earth once every 12 hours, or twice a day.

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A satellite in synchronous orbit around Earth is said to be in “geosynchronous” orbit. The orbital period for a geosynchronous orbit is 24 hours. The satellite circles the Earth at great height once a day. A special geosynchronous orbit that is both circular and lies in the equatorial plane of Earth is called a “geostationary orbit.” A satellite on a geostationary orbit hovers above the Earth’s Equator. It appears to be suspended motionless, like a star, on the celestial equator.

7.3.6 Ground Tracks The ground track of a satellite is the time series of its subsatellite points (see chapter 4.3.1). The ground track of a satellite can take a number of different forms, depending on the values of the orbital elements − the parameters which define the size, shape, and orientation of the satellite’s orbit. Satellites have roughly sinusoidal ground tracks. Since all satellite orbital planes must pass through the center of the Earth, a satellite’s ground track must pass over the Equator. For inclinations other than 0o the ground track must cross the Equator twice for each orbital revolution. The inclination of an orbit determines the highest north and south latitude of the ground track. The geographic latitudes covered by the ground track range from −i to i. A satellite with an inclination of exactly 90° is said to be in a polar orbit, meaning it passes over the Earth’s North and South poles. If the Earth did not rotate, a satellite’s ground track would simply repeat itself until some force changed the orbit. However, the Earth rotates from the west towards the east once a day. By the time a satellite returns to the same place in its orbit after one revolution, the Earth has spun eastward by some amount, and the ground track looks like it has moved westward on the Earth’s surface. The orientation of the satellite’s orbital plane did not change, the Earth has just rotated under it. The satellite’s orbital period determines the amount the Earth rotates eastward and hence its ground track’s westward regression.

Figure 7-6. The ground track of a satellite depends on its orbital parameters. This satellite is on a polar orbit. As the satellite orbits the Earth, different locations on the surface of Earth are directly underneath the satellite, the subsatellite point. As the Earth spins, and depending on the field of view of the satellite’s instruments or antennas, a range of locations around the ground track can be within the satellite’s field of view. This determines which surface areas are visible to the satellite for surveillance and communication. (USGS) Earth rotates through 360o in 24 hours. This equates to an angular rotation speed of about 15o per hour. The result is that each successive track will be offset 15o to the west for each hour of the satellite’s period. A period of 90 minutes, as is typical in LEO, results in an offset of 22 o. 5 (15o/hr x 1.5 hr = 22 o. 5 ) for each successive orbit. A satellite in a LEO will have its ground track shifted by that amount to the west on subsequent orbits as the Earth rotates beneath it. As the altitude of an orbit increases, the period becomes longer and the satellite’s speed is lower. The result is that it takes the satellite longer to survey the globe. A satellite in an eccentric orbit moves faster near perigee and slower near apogee; and it is possible for a satellite to track eastward during part of its orbit and westward during an-

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Figure 7-9. The orbit of the ISS is inclined 51 o. 6 to the equator. This is because the Russian components of the station are launched from the Baikonur Cosmodrome in Kazakhstan. The black line marks Earth’s Equator. The yellow line is the ground track of the ISS. The red lines mark the geographic latitudes in between which the ISS is visible from the ground. (NASA) Figure 7-7. If the Earth did not rotate, the ground track of a satellite in a circular orbit would be the projection of a great circle onto the Earth’s surface. On a flat map it is a sine wave that repeats exactly from one orbit to the next. (US Air Force)

A satellite in a polar orbit can see all of the Earth’s surface over time, due to a combination of its orbital inclination, period, and Earth’s rotation underneath it. These orbits are preferred for meteorological and defense satellites. Polar orbits are also used for the exploration of other Solar System objects. They are especially useful for mapping surfaces.

Figure 7-8. The effect of the Earth’s rotation on the ground track (US Air Force) other part. This phenomenon allows for ground tracks which cross over themselves. A special case of the geosynchronous orbit, the geostationary orbit, has an eccentricity of zero, and an inclination of zero. The ground track in this case consists of a single point on the Earth’s Equator, above which the satellite sits at all times. Due to their high altitudes, geostationary satellites can see almost the entire hemisphere of Earth below them. Most commercial communications satellites and broadcast satellites operate in geostationary orbits.

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Figure 7-10. The geostationary orbit (top) and its ground track (bottom), which is a single subsatellite point on the Earth’s Equator (US Air Force)

7.3.7 Launch Azimuth And Launch Window

tion, launching due east, it also determines the maximum inclination by launching due west. The maximum inclination is 180o minus the latitude. The launch sites of different countries vary in geographic latitude. For example, the US launches from KSC with a latitude of 28 o. 5 , or from Vandenberg Air Force Base, 34 o. 7. The latitude of the Russian Baikonur Cosmodrome is 45 o. 9. The Europeans launch from Kourou, at 5 o. 2, and the Russians have also launched Soyuz rockets from this spaceport. As you know, all orbits pass over the Equator, so an equatorial launch site not only gives the largest delta v boost from Earthâ&#x20AC;&#x2122;s spin, but also access to the full range of orbital inclinations by varying the launch azimuth. Recall from chapter 5 that the allowable launch azimuth from any launch site

Launches from Earth have but a brief phase of powered flight, after which gravity takes over and dictates the flight path in free flight. Because of this, the geographic latitude of the launch site determines the orbits you can reach. You need the smallest delta v and expend the least amount of propellant if you launch directly into your desired orbit. The reason is that for launching to a certain orbit from the surface of the Earth, the orbital plane must intersect the launch site. When the orbit passes over your launch site, you can launch into it either going prograde, or retrograde. A location that is naturally above your launch site is the inclination that equals the latitude of your launch site. You can see this as follows. The angle between the Earthâ&#x20AC;&#x2122;s Equator and a line drawn from the center of the Earth to the surface where the launch site is determines the geographic latitude of the launch site (cf. Figure 4-4). If you extend the line through the launch site into space, you get the inclination of the orbital plane of a satellite launched directly from that site (see Figure 7-4 and section 7.3.3). For launches due east, with a 90o azimuth (or due west, with a 270o azimuth) from a launch site, the resulting orbital inclination equals the latitude of the launch site. For example, as shown in Fig. 7-11, a launch azimuth of 900 places a rocket launched from Kennedy Space Cen- Figure 7-11. The range of allowable launch azimuths and the resulting orbital inclinations for US space launches ter, which has a geographic latitude of (NASA) 28 o. 5, into an orbit that has an inclination of 28 o. 5. might be restricted due to safety considerations. If you want to launch into a different orbital Azimuths that take the rocket over populated arinclination, you have to ask yourself if the orbit eas right after launch are not allowed. will ever pass over your launch site. If the inclinaIf the orbital inclination equals the latitude tion of the orbit is larger than the latitude of your of the launch site, then, owing to the Earthâ&#x20AC;&#x2122;s rotalaunch site, the orbit can pass overhead and you tion, the ground track passes over the launch site can launch into it by varying your launch azimuth. just once a day. This particular time defines the But if the inclination of the orbit is less than your launch window, which is the time of day when the geographic latitude, you cannot reach it. Just like launch needs to occur. If the orbital inclination is the launch site determines the minimum inclinagreater than the latitude of the launch site, the or148

bit passes over the launch site twice a day, producing two launch windows per day.

7.3.8 Orbital Maneuvers: Changing Orbital Altitude Once a spacecraft is on an orbital trajectory, it is in free flight, and keeps coasting on that trajectory due to gravity and inertia. If you want to change the orbit, you need to perform an orbital maneuver. To maneuver requires delta v, in other words, you must fire thrusters and have enough propellant for your desired delta v. One aspect of an orbit that you might wish to change is orbital altitude. You can either fire the thrusters in the direction of your orbit, to increase velocity which also raises the apogee, or you can turn the spacecraft and fire the rockets opposite your direction of travel, to slow down and lower the apogee. To raise the apogee of an Earth orbit, thrust is applied along the direction of flight at a point in the orbit that is 180° away from the desired new apogee. Reducing a point in the orbit is done in a similar fashion. Thrusting to reduce speed will reduce the altitude of the spacecraft 180° away from the point of the burn.

7.3.9 Changing Orbital Inclination An orbital inclination change requires a burn, to occur in one of the two points where the initial and desired orbits intersect. In general, inclination changes take a very large amount of delta v to perform, and most mission planners try to avoid them whenever possible to conserve fuel. Instead, getting a satellite into the right orbital plane is typically achieved by launching a spacecraft directly into the desired inclination, or as close to it as possible so as to minimize any inclination change required over the duration of the spacecraft life. The simplest way to perform a plane change is to perform a burn around one of the two crossing points of the initial and final planes. The engine must be fired perpendicularly to the initial

Figure 7-12. An engine burn in the direction of travel (prograde) at perigee raises the orbit’s apogee. (NASA) direction of travel. The delta v required is the vector change in velocity between the two planes at that point. If both orbits are circular and have the same radius, the delta v for the inclination change is proportional to the orbit’s velocity and the inclination change between the two orbits. The equation reads Δv = 2v × sin

Δi . (2)

In the case of elliptical orbits, maximum efficiency of inclination changes is achieved at apogee, where orbital velocity is the lowest. In some cases, it can require less total delta v to raise the satellite into a higher orbit, change the orbit plane at the higher apogee, and then lower the satellite to its original altitude.

7.3.10 Orbital Rendezvous A rendezvous is a maneuver during which two spacecraft approach at a very close distance. Rendezvous requires a precise match of the positions and orbital velocities of the two spacecraft, allowing them to remain at a constant distance. Sometimes it is desired to dock them, which is bringing the spacecraft into physical contact and creating a link between them.

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done quite carefully to avoid damaging the two The standard technique for rendezvous and spacecraft. docking is to intercept a passive vehicle, the “tarAn example of an orbital rendezvous is reget,” with an active vehicle, the “chaser.” This techsupplying the ISS. The European Space Agency nique has been used successfully for the Gemini, (ESA) has used its Automated Transfer Vehicle Apollo, Apollo/Soyuz, Salyut, Skylab, Mir, ISS, (ATV), an expendable cargo spacecraft, to furnish and Tiāngōng programs. There are several importhe ISS with propellant, water, air, payloads, and tant requirements when trying to rendezvous with experiments. ATVs also re-boosted the station an orbiting spacecraft: into a higher orbit. The ISS orbits at an altitude of 1. The chaser and the target must be in 350 km with an inclination of 51 o. 6 . The ATV is the same orbital plane. 2. The chaser and the target must be at the same orbital altitude. 3. The chaser’s orbit must be adjusted to “synchronize” with the target’s orbit, meaning that the spacecraft must not only be in the same orbit, but that both vehicles reach the rendezvous point at very close to the same time. You could satisfy the first two conditions by choosing your chaser’s launch window when the target’s orbit is over your launch site, and you can choose a launch azimuth and the correct delta v to carry your chaser into the orbit of your target. Now, both spacecraft are in the same orbit, but they might not necessarily be in Figure 7-13. In this example rendezvous, the Space each other’s vicinity within the orbit. Their orShuttle is trying to reach a satellite in order to service bital positions are said to be separated by the it. The Shuttle and the satellite are in the same orbit, phase angle, ϕ. but the Shuttle is ahead of the satellite. When the When two spacecraft are in the same orphase angle, ϕ, between the two vehicles is just right, the Shuttle can burn to enter a phasing orbit. This will bit, they have the same orbital velocity. Hence the chaser can never catch up with the target. If make it go higher and slower. When the two orbits intersect again, the satellite is in the right position for you burn the rockets of the chaser to make it go the Shuttle to come near it. When the two vehicles are faster, it will also go into a higher orbit. at the rendezvous point, the Shuttle must retrofire to You can do a burn and this creates a new exit the phasing orbit and return into the original orbit. (Wikipedia) orbit called the “phasing orbit” which can be larger or smaller than the original orbit resulting launched into a 300-km orbit when the orbit of in a different period than the original orbit. When the ISS crosses over the European launch site in the phasing orbit intersects with the original orbit Kourou. This puts the ATV into an orbit which is and the chaser is nearer to the target, another in the same plane as the ISS. However, the ATV burn, equal and opposite to the first, is applied to and the ISS are in different places in their orbits, return the chaser to the original orbit. Now the and, having different radii, they orbit the Earth at chaser and the target are close to one another, and different speeds. The ATV needs to transfer to the more delicate proximity operations begin. As you ISS’s orbit, and needs to get to the rendezvous can imagine, the final approach and docking are point at the time when the ISS also gets there. The ATV uses GPS and a star tracker when it carefully 150

Figure 7-14. The ATV “Jules Verne” is docked to the bottom of the ISS. The photo was taken from the departing Space Shuttle “Discovery.” (NASA)

approaches the ISS. At a distance of 249 m, the ATV computers calculate small burns for the final approach and docking maneuver. The actual docking is fully automatic. If there are any last-minute problems, a pre-programmed sequence of anticollision maneuvers, fully independent of the main navigation system, can be activated by the flight engineers aboard the station. With the ATV docked, the station crew enters the cargo section and removes the payload. The ATV’s liquid tanks are connected to the station’s plumbing and discharge their contents. The station crew manually releases air components directly into the ISS’s atmosphere. For up to six months, the ATV, mostly in dormant mode, remains attached to the ISS with the hatch remaining open. The crew then steadily fills the cargo section with the station’s waste. Each day, the ISS loses about 100 m in altitude due to residual air resistance. So in intervals of 10 to 45 days, the ATV’s thrusters are used to boost the station’s altitude. Once its mission is accomplished, the ATV, filled with up to 6.5 tons of waste, separates. Its thrusters move the spacecraft out of orbit (deorbit) and place it on a steep flight path to perform a controlled destructive re-entry high above the Pacific Ocean.

7.4 Deep Space Trajectories Deep space trajectories refer to flight paths through our Solar System. They require a delta v that exceeds escape velocity from Earth. To have enough propellant for a journey away from Earth, mission planners choose trajectories which minimize propellant use in favor of gravity, so that the

spacecraft coasts most of the way from Earth to the other celestial body in free flight. One way to minimize delta v is to chose how to depart from Earth orbit. The Earth revolves around the Sun at an average speed of 30 km/s (18.5 mi/s, or 66,600 mph). Just like launching east (or west) with (or against) Earth’s spin adds (or subtracts) velocity, departing into or against the direction of Earths’s orbital path around the Sun can be used to add or subtract velocity. Whether you want to add or subtract velocity depends on whether your jour-

Figure 7-15. A rocket makes use of Earth’s rotational and orbital motions to gain delta v for a deep-space mission away from the Sun. (NASA) ney is outbound, away from the Sun, or inbound, toward the Sun, from planet Earth. You might think that the best time to fly to another planet is when it is close to Earth. For example, when an outer planet is at opposition, or an inner planet is at inferior conjunction (cf. Figure 3-21). But that would not take into account the finite travel time, and minimizing delta v. When flying across the Solar System, mission planners do not take the shortest, or direct, trajectory from one place to another. The delta v and consequently, the amount of propellant needed depend largely on what route you choose. Trajectories that by their nature need a minimum delta v are therefore of great interest. With current technology, only those trajectories are practical and travel is only possible within certain time windows. Outside these windows the planets are essentially inaccessible from Earth

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The spacecraft lifts off the launch pad, rises above Earth’s atmosphere, and uses its rocket to accelerate in the direction of Earth’s revolution around the Sun to the extent that the delta v added at perihelion will cause its new orbit to have an aphelion equal to Mars’ orbit. The acceleration is tangential to the existing orbit. After this brief acceleration away from Earth, the spacecraft has achieved its transfer orbit, and it simply coasts the rest of the way. Getting to the planet Mars,

Figure 7-16. A Hohmann transfer from a lower to a higher circular orbit around the Earth requires two engine burns, one to insert the vehicle into the elliptical transfer orbit, shown in red, and one to insert it into the desired final orbit. (Wikipedia) with current technology. This constrains flights and prevents rescue in an emergency.

7.4.1 The Hohmann Trajectory The Hohmann trajectory is an elliptical orbit used to transfer between two circular orbits of different radii in the same plane using the minimum amount of delta v. Conceptually, the Hohmann transfer is very similar to the orbital rendezvous discussed in the previous section. In transferring from Earth to a planet, the space probe is the chaser and the planet becomes the target. The maneuver to perform the Hohmann transfer uses two engine burns, one to move a spacecraft onto an elliptical transfer orbit and a second to move the craft off the transfer orbit. This maneuver was named after Walter Hohmann (18 March 1880 - 11 March 1945), the German architect who became interested in spaceflight and who published a description of it in his 1925 book “Die Erreichbarkeit der Himmelskörper” (“The Accessibility of Celestial Bodies”). To launch a spacecraft to an outer planet such as Mars, using the least propellant possible, first consider that the spacecraft is already in an orbit around the Sun as it sits on the launch pad on Earth. Its existing solar orbit must be adjusted to cause it to take the spacecraft to Mars. In other words, the spacecraft’s perihelion will be Earth’s orbit, and the aphelion will intercept the orbit of Mars.

Figure 7-17. The geometry of the Hohmann trajectory (yellow) from a lower orbit (green) to a higher orbit (red). The central body is marked O. For interplanetary transfers, the central body is the Sun. The radius of the lower orbit is R , and of the higher orbit, is R′. The transfer is half of an ellipse, the solid portion of orbit 2. In the case of a transfer from Earth to Mars, Earth is on orbit 1, Mars is on orbit 3, and the spacecraft is placed on an independent orbit, 2, around the Sun. Δv shows the tangential burn to leave the lower orbit, and Δv′ shows the second burn when the spacecraft arrives at the higher orbit. (Wikipedia)

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2018 – May 2018, and rather than just to its or2020: Jul 2020 – Sep bit, requires that the 2020, and so forth. There spacecraft be inserted are several missions in into the transfer trajecprogress and planned to tory at the correct time so make use of these opporit will arrive at the martunities. tian orbit when Mars will The Hohmann trajecbe there. This is similar tory is also used to transto the task of performing fer to the inner planets of an orbital rendezvous. the Solar System. To You have to lead the aim achieve this, the spacepoint by just the right craft uses its rocket to acamount, given by the Figure 7-18. Positions of Earth and Mars at the celerate opposite the diphase angle, to hit the tar- time of launch and arrival of the two Mars Exrection of Earth’s revoluget. Whenever the phase ploration Rovers MER-A “Spirit” and MER-B “Opportunity.” The mission began in 2003. tion around the Sun, angle is just right, there (NASA) thereby decreasing its veis an “opportunity” for a locity while at aphelion to the extent that its new transfer. orbit will have a perihelion equal to the distance Once the spacecraft gets to Mars, if you do of, say, Venus’ orbit. Once again, the acceleration nothing, it will be carried along its elliptical trajecis tangential to the existing orbit. Of course the tory on a return voyage toward the Sun and Earth. spacecraft will continue going in the same direcTo be captured into a martian orbit, the spacecraft tion as Earth orbits the Sun, but a little slower must burn its rockets again. now. To get to Venus, rather than just to its orbit, The Hohmann transfer is propellant effiagain requires that the spacecraft be inserted into cient, but it is slow. Thus, a trip to Mars takes its interplanetary trajectory at the correct time so about 8.5 months, and you can only depart to it will arrive at the Venusian orbit when Venus is Mars every 26 months, when the planets have the proper alignment. Engineering interplanetary jour- there. Venus launch opportunities occur about every 19 months. The flight time to Venus via a neys is very complicated, so the exploration of Hohmann trajectory is about 5 months. Mars has experienced a high failure rate. Roughly two-thirds of all spacecraft destined for Mars 7.4.2 Gravity Assist Trajectory failed before completing their missions. However, A gravity assist maneuver, or gravitational missions have also met with unexpected levels of slingshot, is used when a spacecraft swings by an success, such as the twin Mars Exploration Rovers astronomical body to alter its flight path. Gravity operating for years beyond their original mission assistance can be used to accelerate a spacecraft, specifications. The Indian Space Research Organithat is, to increase or decrease its speed and/or rezation (ISRO) launched its Mars Orbiter Mission direct its path. In interplanetary spaceflight, grav(MOM) on 5 November 2013. It was successfully ity assists are sometimes used multiple times, ininserted into Mars orbit on 24 September 2014. cluding when flying by the Earth, to change a India’s ISRO is the fourth space agency to reach spacecraft’s flight path relative to the Sun. Mars, after the Soviet space program, NASA and The gravity assist can be understood from ESA. India became the first country to successNewton’s third law and the conservation of energy fully get a spacecraft into the martian orbit on its and momentum. The spaceship gains speed, and maiden attempt. Opportunities to launch to Mars the planet loses some, during the interaction. The occur in 2016: Jan 2016 – Apr 2016, 2018: Apr 153

Figure 7-20. A gravitational slingshot that includes the motion of Jupiter around the Sun. When both, the gravitational interaction and the orbital velocity of Jupiter are taken into account, you can see that the spacecraft changes direction and speed. (Wikipedia)

Figure 7-19. A gravitational slingshot assumingJupiter itself is not moving around the Sun. The gravitational interaction bends the spacecraft’s trajectory, but its outgoing speed is the same as the incoming speed. (Wikipedia)

speed, the spacecraft flies against the movement of the planet. The main practical limit to the use of a gravity assist maneuver is that planets and other large masses are seldom in the right places to enable a voyage to a particular destination. For example the Voyager missions which started in the late 1970s were made possible by the “Grand Tour” alignment of Jupiter, Saturn, Uranus and Neptune. A similar alignment will not occur again until the middle of the 22nd century. That is an extreme case, but even for less ambitious missions there are years when the planets are scattered in unsuitable parts of their orbits.

kinetic energy and momentum gained by the spaceship are equal in magnitude to that lost by the planet. The effects on the planet are so tiny, because planets are so much more massive than spacecraft, that they can be ignored. Gravitational slingshots that accelerate space probes are extremely important to make flights to distant planets possible with chemical rockets. To increase speed, the spacecraft flies with the movement of the planet; to decrease

7.4.3 Free Return Trajectory

Figure 7-21. A diagram of the slingshot trajectories that enabled NASA’s twin Voyager spacecraft to tour the four gas giant planets (NASA)

The free return trajectory is a special case of a gravity assist trajectory: it uses gravity to completely turn the path of a spacecraft around. In a nutshell, the free return trajectory idea is that you aim the spacecraft at the destination, and burn engines only once, when you depart. After that you just coast. When you get to the destination, you approach the body from the front. Gravity will turn and slingshot your vehicle around in such a way that it is sent on a journey back to Earth with-

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Figure 7-22. A conceptual sketch of the figureeight shaped circumlunar free return trajectory (NASA)

Figure 7-23. An illustration of the geometry of the Earth and the Moon for the Apollo lunar free return trajectory. (Wikipedia.) out the use of any propellant burns or maneuvers. Although in theory it is possible to execute a perfect free return trajectory, in practice small correction burns are often necessary during the flight. The free return trajectory was used in the human exploration of the Moon. The ill-fated Apollo 13 used the Lunar Module to maneuver from its planned lunar orbit insertion to a free return trajectory. This brought the Crew Module and the astronauts back to Earth. A free return trajectory ranks highly on the list of possible initial trajectories for human missions to Mars.

7.4.4 Constant Thrust Trajectory What if you could apply thrust for more than just a brief impulse, and be in powered flight most of the way? If you had a high-thrust rocket, and enough propellant, your flight time would be much shorter than a Hohmann transfer and you could take a more direct route. It has been suggested that some type of nuclear propulsion could do the job. But that engine does not yet exist. What has been feasible, however, is to constantly apply low amounts of thrust. One such proven low-thrust method is deploying a solar sail. A solar sail carries no reaction mass, but in-

Figure 7-24. Trajectory of NASA’s Dawn space probe on its way to dwarf planet Ceres. Dawn was thrusting with its ion drive on portions of the trajectory colored in light blue, but was coasting where the trajectory is colored green. Dawn also used a gravity assist. (NASA) stead, uses the radiation pressure of sunlight to propel itself. There are also low-thrust reaction engines. An entire category of engines uses electromagnetic propulsion which accelerates charged particles. They are capable of producing a low thrust over a long period of time using little propellant, and are flight proven. While these low-thrust propulsion systems cannot be used for lift-off from Earth, they work quite well once in space (cf. also, chapters 9.6.1 & 9.6.2). They depart from Earth orbit spiraling outward under low thrust. An example of an electric propulsion system is the ion drive. NASA recently used the ion drive to send “Dawn” to Vesta and Ceres. Figure 7-24 shows how Dawn spiraled around the Sun several times in an ever growing orbit. Using a spiraling, low-thrust trajectory takes longer than a Hohmann transfer. However, the high specific impulse of the ion drive means that less propellant is required compared with chemical propulsion. Finally, continuous thrust over a very long time can build up a larger velocity than traditional chemical rockets.

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7 Test Your Understanding I. A NSWER IN A FEW SENTENCES . 1. What is the difference between sub-orbital and orbital spaceflight? 2. What kind of orbit is a spacecraft on if its orbital inclination is 00, 900? 3. Space starts at 100 km above Earth. Would it be feasible to orbit Earth at that altitude? 4. A satellite has an inclination of 200. Will it ever fly through the zenith of a Pittsburgh observer? 5. Can an observer in Pittsburgh ever see the ISS in the night sky? 6. What would be the ground track of a satellite in a LEO with a 00 inclination? Include a drawing. 7. How many times does one revolution of a MEO with a 900 inclination cross over the equator? 8. What is the farthest southern latitude where a satellite in a 300 orbit can fly through the zenith of an observer on Earth? 9. In what orbit is a spacecraft launched from KSC with a launch azimuth of 1800 to an altitude of 500 km? 10.What orbit is a satellite in that was launched from a launch site on the Equator with a 00 launch azimuth? 11. Can you launch to the ISS from KSC? 12.Can you launch into a geostationary Earth orbit from KSC? 13.What are the advantages of launching a rocket from Earth’s Equator? 14.How and when would you perform an engine burn to lower the apogee of a probe in an elliptical Earth orbit? Make a sketch of the orbits around Earth. 15.How and when would you perform and engine burn to change the orbital inclination of your vehicle? Make a sketch of the orbits around Earth. 16.If the phase angle between the ATV and the ISS is 1800 while they are in the same orbit, what is their relative position? Draw a sketch.

17.What happens when you depart Earth in a direction that is opposite its orbital motion around the Sun? 18.What is a Hohmann transfer? 19.What is meant by an “opportunity?” 20.If you wanted to fly to Mercury by doing a swing by at Venus to get a gravity assist, what would determine when such an opportunity might exist? 21.What happens to a spacecraft that flies into the SoI of a planet in the same direction that the planet orbits the Sun? 22.What happens to a spacecraft that approaches the SoI of a planet from the opposite direction that the planet orbits the Sun? 23.What is the shape of a free return trajectory? II. C ALCULATE THE A NSWER . 1. What is the orbital period of the ISS which orbits at an altitude of 350 km? 2. Calculate the orbital altitude of the Hubble Space Telescope given that it takes the telescope 1 h and 37 min to orbit the Earth. 3. Calculate the radius of a satellite that is in geostationary orbit. 4. A satellite orbits with a velocity of 8 km/s in an orbit that is inclined by 100 to Earth’s Equator. Calculate the delta v required to change its orbital inclination by 100, 600, 800. What do you conclude from this exercise? 5. Calculate how long it takes a vehicle to fly from Earth to Mars along a Hohmann trajectory. Also given: Earth orbits the Sun at a distance of 1 AU in 1 year. Mars orbits the Sun at a distance of 1.52 AU in 1.88 years. You may assume that both orbits are circular and coplanar (that means they have the same inclination). Hints: Refer to Figure 7-17 and make a sketch. Then use Kepler’s 3rd law.

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Selfie of the Mars Curiosity Rover (NASA)

SPACE EXPLORATION How did we find out all the information that we know about planets, stars, and galaxies? What kind of techniques have scientists come up with that leverage spaceflight to explore the nature of celestial objects?

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8.1 Direct and Remote Sensing Space exploration is the ongoing discovery and exploration of objects and phenomena in Outer Space using scientific equipment in Outer Space. While the study of Outer Space has been carried out remotely for centuries by astronomers using telescopes, the direct exploration of the Solar System, which only became possible with the advent of spaceflight, is the domain of specialists like solar physicists and planetary scientists. Remote sensing is the acquisition of information about an object without making physical contact with it. It is the study of objects from a distance. Astronomy is the perfect example of a science which employs remote sensing. Astronomers study the Universe mainly by collecting and analyzing electromagnetic radiation that travels from distant celestial sources to Earth. There are many information carriers that bring knowledge about Outer Space to you right here on Earth, enabling remote study. These are electromagnetic radiation, cosmic ray particles, neutrino particles, and meteorites which are the leftover cores from shooting stars. You can collect these waves and particles on Earth, and try to figure out where they came from and what their nature tells you about their sources of origin. This contrasts with direct sensing, where a sensor is in direct contact with the object it is exploring. A thermometer, for example, can sense the temperature of a substance it is in contact with. Direct sensing is Figure 8-1. The Deep Space Network, or DSN, is an international network of communication facilities that supports inter- used in the laboratory sciences planetary spacecraft missions, as well as radio and radar as- on Earth. Examples of laboratronomy observations for the exploration of the Solar System tory sciences are physics, chemand the Universe. It is best known for its large dish radio anistry, and biology. Here you can tennas. The network also supports selected Earth-orbiting misdirectly examine and experisions. The DSN is part of NASAâ&#x20AC;&#x2122;s Jet Propulsion Laboratory. A photo of one of the antennas, located in California, is shown in ment with the objects and natural phenomena that you are tryFigure 1-16. (NASA) ing to understand. Ever since the advent of spaceflight, direct sensing has been extended to the exploration of objects in our Solar System. In a few rare cases, humans have been able to do the exploring and experimenting in situ. Astronauts have visited the surface of the Moon, studying it, and even returning lunar soil samples to Earth. Several scientists have conducted experiments in material and life sciences onboard the International Space Station.

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More commonly, direct sensing experiments have been carried out by robots. Robotic space probes have flown by or orbited the Sun, planets, moons, and other bodies. In some cases, space probes have entered the atmospheres or landed on bodies of our Solar System. While you can bring a space probe in direct contact with, say, a soil sample on Mars, the information must still be relayed back to Earth remotely by using telemetry. Although the sensors are in direct contact with the phenomena being studied, the information still needs to be sent back to us on Earth. Nevertheless, we consider scientific payloads that bring sensors in direct contact with the phenomena or objects under Figure 8-2. NASA researchers analyzed the light reflected by the planstudy in the category of direct ets and plotted the results on a “color-color” diagram. By plotting the sensing. ratios of red to green light on the x-axis as well as blue to green on the y-axis, the planets cluster into “color families.” On the diagram, Earth There are many differis easily distinguishable from the other major planets. The gas giants ent kinds of scientific instruJupiter and Saturn occupy one corner of the chart, Uranus and Nepments. Remote and direct senstune a different one, and the rocky inner planets Mars, Venus, and Mering experiments can often pro- cury cluster in the third one. But Earth is the true loner in color space. vide the same information, Its uniqueness traces to two factors. One is the scattering of blue light by the atmosphere. The second is because Earth does not absorb a lot of but in different ways. For exinfrared light. That’s because our atmosphere is low in infraredample, you can measure the absorbing gases like methane and ammonia, compared to the gas giant temperature of a substance by planets Jupiter and Saturn. (NASA/GSFC) inserting a thermometer in it ject and then collect the re(direct) or by recording the peak 8.1.1 Passive And Acflected radiation that comes strength of the radiation that tive Remote Sensing back to you. Hence the informathe object emits (remote). The Remote sensing can be tion content of electromagnetic two methods use our undersplit into passive and active reradiation received from remote standing of two different physimote sensing. In passive remote objects depends on the nature cal processes, the expansion of a sensing you simply collect and of the illuminating radiation material at different temperaanalyze what radiation you get source and interactions that the tures versus the black-body spec- from a source. In active remote radiation has before reaching trum, to masure an object’s temsensing, you actively send out you. perature. radiation to illuminate the ob159

you see that there is much information that can be decoded from passively reflected light. Active remote sensing experiments involve emitting a beam of electromagnetic radiation to an object, and then recording the signal as is returns. These types of experiments are combined with the velocity-distance-time relationship to range the distances of objects from us. For example, the Apollo astronauts left a small mirror on the Moon (cf. Figure 4-25), and now the Lunar Laser Ranging Experiment gives us the varying distance to the Moon throughout its orbit with an accuracy of a few millimeters. We can use a radar detector on an orbiting spacecraft to make a topographical map of the surface terrain, say, the mountains and valleys on cloud-covered Venus.

Figure 8-3. In passive remote sensing, the Sun illuminates an object (1), the space probe collects radiation from the interaction of the object with the sunlight (2), and then sends the information to a ground station (3) for analysis by space scientists. In active remote sensing, the space probe itself provides the illumination of the object, collects the radiation after it has interacted with the object under study, then relays the information gathered to a ground station. (Wikipedia)

8.2 Observatory space probes & Space Telescopes

Here are a few examples of passive remote sensing. Reflected sunlight is the most common source of radiation measured by passive sensors. Figure 3-4 shows the radiation emitted by the Sun, and how it peaks in the visible range. An object that absorbs and reflects radiation from the Sun, such as the surface of the Moon, interacts with that radiation, altering it. We see dark areas on the Moon where the surface absorbs solar radiation, and bright areas where it reflects more than it absorbs. Similarly, the colors you see in nature are the result of an object’s emission, absorption, reflection, and transmission properties. The colors of sunlight that the planets reflect can tell us much about them. For example, Mars is red because its soil contains iron oxide, also known as rust. And the famous tint of our planet, the “blue marble?” It’s because the atmosphere scatters blue light rays more strongly than red ones. Therefore the atmosphere looks blue from above and below. So

An observatory spacecraft does not travel to a destination to explore it. Instead, it occupies an Earth orbit, or a solar orbit, from where it can observe distant targets free of the obscuring and blurring effects of Earth’s atmosphere. There have been several observatories in space remotely exploring space phenomena in all ranges of the electromagnetic spectrum: radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma ray radiation. Space observatories have also collected particles in space. An example is the Alpha Magnetic Spectrometer. Because visible radiation is the most accessible type of radiation for us, information in this chapter is centered on gathering and detecting visible light with space telescopes.

8.2.1 Purpose Of A Space Telescope A telescope generally has two purposes: to collect light from and to resolve fine detail in the object being studied. In chapter 3, you learned how sunlight spreads out over a larger and larger area as it moves away from the Sun, making it look fainter when seen from farther away. This was the first inverse-square law you encountered. Similarly, given two stars of same intrinsic bright-

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Figure 8-4. When light strikes the concave primary mirror of the Hubble Space Telescope, it is reflected to the convex secondary mirror, then back through a hole in the center of the primary mirror. There, the light comes to the focal point and passes to Hubble’s science instruments. By ground-based standards, the primary mirror of the Hubble, measuring 2.4 m (7.9 ft) in diameter, is on the small side. The length of the telescope structure is 13.2 m (43.5 ft). It was launched in 1990 on the Space Shuttle. (NASA/STScI) ness, the one that is farther away is going to appear fainter to you. The lens in a human eye is about 0.01 m in diameter, and that limits the amount of light it can gather from a light source,

as well as the amount of detail, or sharpness, it can achieve. Now, imagine collecting that spread out light back with a lens that is bigger than the eye. The bigger you make the lens, the more of the light you can collect, the brighter the source will appear from a distance. The collecting area of a telescope is also called its aperture. The aperture can be a lens, or a mirror. Most modern telescopes use mirrors to collect light. Since the area of a circle grows with the square of its diameter, D, you can say about the light collecting power, LCP, of a telescope’s aperture, that it is proportional to diameter squared: LCP ∝ D 2. The power to resolve fine detail, let’s call it R, also grows with the size of the telescope’s aperture. Resolving more detail will give you a sharper image than resolving less detail. In the case of resolving power, there is a simple linear dependance with aperture diameter, such that R ∝ D.

Figure 8-5. A diagram of the electromagnetic spectrum with the Earth’s atmospheric transmittance (or opacity) and the types of telescopes used to investigate parts of the spectrum (NASA/Wikipedia) 161

Figure 8-6. Remember Orion and the Orion Nebula from chapter 3? Here is a multi-wavelength view of the Orion nebula from the ground and using space telescopes. (ESA / AOES Medialab (overall composition); Kosmas Gazeas (visible light, large image); STScI-DSS (visible light, small image); ESA, LFI & HFI Consortia (microwave and (sub)millimetre); AAAS / Science, ESA XMM-Newton and NASA Spitzer data (mid-infrared and X-rays); NASA, ESA, M Robberto (Space Telescope Science Institute / ESA) and the Hubble Space Telescope Orion Treasury Project Team (visible and near-infrared)) Bigger is better for telescopes. This is easier to implement on the ground than it is in space. Telescopeâ&#x20AC;&#x2122;s lenses and mirrors have traditionally been made of glass. This produces very large, very heavy objects. Taking them to Outer Space is expensive needing a carrier rocket that can accommodate the required mass. Also, rockets are limited by the sizes of their payload fairings; there is a limit to the size of any monolithic object that will fit in the nose cone of a rocket. Although not the first space telescope, Hubble is one of the largest and most versatile, and is well known as both a vital research tool and a public relations boon for astronomy. A picture of the Hubble as it orbits Earth is shown in Figure 1-10. The diameter of its aperture, the primary mirror, is 2.4 m (7.9 ft). The telescope weighs 11,110 kg (24,500 lb). In comparison, the largest telescope mirror on Earth (in 2015) is the Gran Telescopio Canarias, with an aperture measuring 10.4 m

(34.1 ft) in diameter. It is not a monolithic mirror, but made out of twelve individual mirror segments.

8.2.2 Why Put Telescopes In Space? There are three advantages of having telescopes in space. First, the atmosphere of Earth moves. There are winds at different levels in the atmosphere. This causes a blurring of images from distant sources. When a telescope is above the Earth, there is no atmospheric blurring. Hence it can produce images as sharp as its resolving power will allow. Second, as you first read in chapter 2, the atmosphere of Earth blocks most types of electromagnetic radiation, allowing only some to reach the surface of the Earth, as illustrated in Figure 85. So, if you want to obtain the full range of information from a remote source, you need to send a

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Figure 8-7. The spectrum of a tube filled with glowing carbon gas is shown in two ways. The top shows it as an image, the bottom show it as a strength-wavelength graph. Notice there is a bit of continuum, in addition to the bright lines. The continuum came from ambient light in the room, not from the glowing gas tube. (NASA/GSFC) telescope into space that can collect all of the different types of radiation. Third, when you have a telescope on the ground, you can only see what’s above your horizon. In chapter 3 you learned that an observer on the northern hemisphere can only see objects in the northern hemisphere of the sky. The southern hemisphere of the sky is occulted by the Earth. Also, you know that you have to wait an entire year to see all of the constellations in your sky, because every day, about half of the sky is bathed in sunlight and we can only see the stars at night. Not so in space. The sky is dark even with the Sun in it, because there are no atmospheric particles to scatter sunlight. A telescope in low Earth obit like the Hubble circles the Earth every 97 minutes. Hubble’s low orbit (about 560 km or 350 mi) means targets are blocked by Earth for about half of an orbit’s elapsed time. Hubble’s view is also blocked by the Moon and the Sun. Still, over time, Hubble can access all regions of the celestial sphere.

8.2.3 Images And Spectra Space telescopes record electromagnetic radiation in a variety of ways that enable scientist to deduce information about the source being studied. You are very familiar with images; scientists are interested in images as well. An image is a representation of the strength of the radiation from an object as a function of position. The image is a

two-dimensional object where the x and y axis are positions, and the value plotted is the strength of the object’s radiation at those positions. Images give information of the positions, brightnesses, or shapes of objects, and can be put together to make maps. A color image uses colors to represent different wavelengths of radiation; the strengths of each show how bright they are relative to one another. The relative strengths of colors can yield information about physical processes involved. In order to be able to produce a color image, you have to have the means of selecting or filtering out parts of the wavelengths of the electromagnetic spectrum, and you need to be able to line up the images in different wavelengths to combine them. A familiar way of producing color images to everyone who has a digital camera is color photography. In color photography, electronic sensors record color information at the time of exposure. This is usually done by analyzing the spectrum of colors into three channels of information, one dominated by red, another by green and the third by blue, in imitation of the way the normal human eye senses color. The recorded information is then used to reproduce the original colors by mixing various proportions of red, green and blue light. You can alter the amount of color in your image in post-processing, and even produce false-color images. Scientific cameras work in a similar way, only scientists record the information that the camera gathers in a quantitative way. There are many other ways to analyze radiation. One other technique that is very frequently used is spectroscopy. A spectrum is a plot on which the strength of radiation is shown as a function of wavelength. Once again, you need to have the ability to select out different wavelengths from the entire range of electromagnetic radiation. In this technique, you want to spread out or disperse the wavelengths as much as you can, so you can glean information about the continuous or discrete nature of the radiation. You can show a spectrum in the form of an image, or in the form of a graph. Example repre-

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8.2.4 Kirchhoff’s Laws Gustav Robert Kirchhoff (12 March 1824 – 17 October 1887) was a German physicist who contributed to the fundamental understanding of spectroscopy, and the emission of black-body radiation by heated objects. Kirchhoff’s three laws of spectroscopy are:

1. A solid object or opaque gas produces light with a continuous spectrum. Kirchhoff coined the term black-body radiation. 2. A transparent gas produces light with spectral lines at discrete wavelengths (i.e. specific colors) which depend on the energy levels of the atoms in the gas. This is called an emission spectrum. 3. A solid object or opaque gas surrounded by a cooler, transparent gas produces light with an almost continuous spectrum which has gaps at discrete wavelengths depending on the energy levels of the atoms in the gas. This is called an absorption spectrum.

Figure 8-8. The three types of spectra. Here they are shown as images. Another representation is a graph. The spectral lines are from hydrogen gas. They are part of the Balmer series; the red line is the Balmer α line, n=3 to n=2, with a wavelength of 656 nm. (Wikipedia) sentations of spectra are shown in Figure 8-7. Another example, of a graph of the Sun’s radiation spectrum as seen from Outer Space and from Earth, is shown in Figure 3-4. When you record images in different wavelengths, or when you record spectra, you obtain complementary information on the source that is radiating, and all of the interactions that its light has experienced before reaching you.

Figure 8-9. The Bohr model of the hydrogen atom resembles our Solar System. Only instead of the Sun, there is the proton, and instead of the orbits of the planets, there are the orbits of the electron. The only orbits that are allowed are orbits of specific energies. When an electron transitions between two orbits, the energy difference corresponds to a specific wavelength of radiation. The Balmer series produces lines in the visible, the Lyman series, in the ultraviolet and the Paschen series, in the infrared. (Wikipedia) 164

Figure 8-10. (Top) When the right amount of energy is transferred to an atom, either by absorbing a photon of just the right wavelength, or by collision with another atom, it’s electron can be raised to a higher level. The atom becomes excited. (Bottom) When the electron transitions back down from the excited state, it emits a photon with a wavelength that corresponds to the energy difference between the two levels involved. (NASA/GSFC) This is how information can be obtained from the colors of objects. The peak wavelength in the black-body spectrum can tell you the temperature of the emitter. The spectral lines you record in the emission or absorption spectrum of an object tell you about the atoms and ions which are present in the gas. This also tells you about the

temperature of the gas and, additionally, about its chemical composition. Let’s look at how an emission line is formed in the simplest, and universally most abundant chemical element, hydrogen. A hydrogen atom consists of an electron orbiting its nucleus. The electromagnetic force between the electron and the nuclear proton leads to a set of quantum states for the electron, each with its own energy. These states are visualized by the Bohr model of the hydrogen atom as being distinct, “allowed” orbits around the nucleus. Each energy state, or orbit, is designated by an integer, n, as shown in Figure 89. Spectral emission occurs when an electron transitions, or jumps, from a higher energy state to a lower energy state. The energy of the emitted photon corresponds to the energy difference between the two states. Because the energy of each state is fixed, the energy difference between them is fixed, and the transition will always produce a photon with the same energy. Because energy is proportional to frequency (and inversely proportional to wavelength), the transition will produce radiation of a certain frequency (or wavelength). This is the unique “fingerprint” of radiation of hydrogen. You see this signature in emission, when you look at a transparent hydrogen cloud, or you may find it in absorption, when you look at a transparent hydrogen cloud in front of a hotter opaque object. The spectral lines are grouped into series depending on the lowest state that the electron transitions to. The brightest line in the visible range is the hydrogen Balmer α line, between the n=3 and the n=2 level of the atom. It is at 656 nm, which corresponds to red wavelengths. Therefore, images of nebulas or of galaxies with regions of hydrogen gas show these with red colors (cf. Figure 2-22).

Figure 8-11. The emission spectrum of iron shows iron’s unique spectral “fingerprint.” (Wikipedia) 165

Figure 8-12. The Doppler effect for a moving radiation source (Wikipedia) An atom can become excited in two different ways. It can be excited by collisions with other particles. Recall the northern lights from chapter 2, where we talked about collisional excitation of atoms in Earth’s atmosphere by charged particles from space spiraling into Earth’s magnetic field. Collisions between its atoms happen more frequently when the gas is dense and hot. Alternatively, an atom can become excited by absorbing a photon of just the right frequency, or wavelength of radiation, as corresponds to the difference in the energy levels that are involved.

8.2.5 The Doppler Effect Emission and absorption line spectra are also extremely useful in telling us about the rela-

Figure 8-13. The three spectra above show how spectral lines shift when the source is moving relative to the observer. The top spectrum is produced by a source that is stationary with respect to the observer. In the middle spectrum, the absorption lines are shifted to the right (toward the red), and are thus redshifted and produced by a source that is moving away from the observer. In the bottom spectrum, the absorption lines are shifted to the left of where they lie in the stationary spectrum. In this case the source is moving toward the observer. (NASA/GSFC)

tive motion of the source and the observer. You already learned of the redshift of the galaxies in chapter 2. Here is more detail on how this works. When the source of the waves is moving toward the observer, each successive wave peak is emitted from a position closer to the observer than the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrival of successive wave crests at the observer is reduced, causing an increase in the frequency, or a decrease in wavelength. While they are traveling, the distance between successive wave peaks is reduced, so the waves “bunch together.” This results in a blueshift of the radiation. Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency/increasing the wavelength. The distance between successive wave peaks is then increased, so the waves “spread out.” This results in a redshift of the radiation. Scientists have been able to quantify the amount of blueshift or redshift, to measure the relative line-of-sight speeds of the source and the

Figure 8-14. Unlike the Hubble Space Telescope, which orbits Earth, Kepler has been placed in an “Earth trailing” orbit around the Sun. (Wikipedia/NASA) 166

observer. Thus, the amount by which the wavelength of a certain spectral line is shifted compared with where it appears at rest can be translated into the speed of the motion in m/s.

8.2.6 Sample Mission: Kepler Even though it seems likely that the Galaxy is teeming with exoplanets, finding them isn’t easy. Planets are millions of times dimmer than the stars they orbit and incredibly distant. Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars. The spacecraft, named after the German Renaissance astronomer Johannes Kepler (of Kepler’s three laws of planetary motion described in chapter 3), was launched on 7 March 2009. It has helped humanity make great strides in finding exoplanets. By 6 January 2015, 1,000 confirmed exoplanets had been discovered with the Kepler Space Telescope. At the time of launch, Kepler, containing a primary mirror measuring 1.4 m (4.6 ft), was the largest mirror on any telescope outside Earth orbit. Kepler’s camera was also the largest camera

Figure 8-15. If a planet passes directly between a star and an observer’s line of sight, it blocks out a tiny portion of the star’s light, thus reducing its apparent brightness. Sensitive instruments can detect this periodic dip in brightness. (NASA/JPL) yet launched into space. Kepler used the transit method to detect extrasolar planet candidates. When a planet passes in front of a star as viewed from Earth, the event is called a “transit.” On Earth, we can observe an occasional Venus or Mercury transit. These events are seen as a small black dot creeping across the Sun—Venus or Mercury

Figure 8-16. The Kepler Space Telescope is used to measure tiny changes in brightness (flux) as a planetary candidate transits in front of its stars. Note that you can measure the tiny dip in flux, but you cannot see the planet in front of the star. The top panel is just an illustration of the star-planet configuration that was derived from the flux versus time data. (NASA/Kepler) 167

Figure 8-17. The unseen planet causes a motion in the star about the common center of mass. The more massive the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing periodic blue- and redshifts in the spectrum of the star. (NASA/JPL)

Figure 8-18. The sizes of the candidate exoplanets discovered with the Kepler Space Telescope as of 23 July 2015 (NASA Ames/W. Stenzel)

blocks sunlight as the planet moves between the Sun and us. Kepler finds planets by looking for tiny dips in the brightness of a star when a planet crosses in front of it—we say the planet transits the star. For a planet to transit, as seen from our Solar System, the orbit must be lined up edgewise to us. Transits by Earth-like planets produce a very small change in a star’s brightness of about 1/ 10,000 (100 parts per million, ppm), lasting for 1 to 16 hours. This change must be periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method. The measurement must be space-based to obtain the photometric precision needed to reliably see an Earth-like transit and to avoid interruptions caused by day-night cycles, seasonal cycles and atmospheric perturbations. Once a candidate planet it found, extensive follow-up observations, including spectroscopic observations from the ground, are used to weed out false positives due to confusion with other phenomena, such as binary star systems. When confirmed, the planet’s orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the

star using Kepler’s Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet’s characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.

8.3 Flyby space probes In a flyby, a space probe follows past a planet or other Solar System body, but does not go in an orbit around that body. The first lunar flyby was by Luna 1, a Soviet space probe, in 1959. Luna 1 became the first man-made object to reach the escape velocity of the Earth. The first successful interplanetary flyby was the 1962 Mariner 2 flyby of Venus (closest approach 34,773 kilometers).

8.3.1 Purpose Of A Flyby Mission Flyby missions are used to conduct the initial reconnaissance of extraterrestrial bodies in the Solar System. They follow a continuous trajectory, never to be captured into a planetary orbit. They must have the capability of using their instruments to observe targets they pass. As they draw closer to the body to be investigated, that body covers a larger part of their camera’s field of view, and the radiation is also blueshifted. Ideally, the

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ment will not occur again until the middle of the 22nd century. That was an incredible opportunity to visit the outer planets at a reasonable cost. During a flyby mission, a space probe is coming close to a planet. This means that the planet will subtend a larger angle to the camera on the space probe than it would to the same camera on Earth. Getting images up close means you can see more detail in the atmosphere or on the surface of the planet. A flyby also enables direct sensing science. For example, the particle detector data on Mariner 2 showed the absence of a detectable magnetosphere. It detected no radiation belt at Venus similar to that of Earth. A disadvantage of a flyby mission is that once the spacecraft has flown past the object, it can’t return for further investigation.

optical instruments can pan and compensate for the target’s apparent motion in the instruments’ field of view, and for the Doppler shift. Such probes must also be able to survive long periods of interplanetary cruise. They must downlink data to Earth, storing data onboard during the periods when their antennas are off Earth. The prime example of the flyby spacecraft category is the pair of Voyager spacecraft, which conducted encounters in the Jupiter, Saturn, Uranus, and Neptune systems, during a time when the positions of these planets favored such a mission design to visit all of the outer planets. More recently, the New Horizon spacecraft flew by the dwarf planet Pluto and is now continuing into the Kuiper Belt.

8.3.2 Why Do A Flyby? A flyby is the first step in closer exploration. Mariner 2 (Mariner-Venus 1962), a US space probe to Venus, was the first ever robotic space probe to conduct a successful planetary encounter. It was a flyby mission. As a the first step in exploration, a flyby makes sense and is cheaper than sending an orbiter, or a lander. The flyby mission returns the first close-up images of the body, as well as measurements about its environment. Examples include measurements of the magnetic field, radiation belts, and properties of moons and planetary rings. These are important to know in detail before investing in a more expensive mission. There are also practical reasons. The Voyager missions were enabled by several opportunities for gravitational slingshots. The Voyager missions which started in the late 1970s were made possible by the “Grand Tour” alignment of Jupiter, Saturn, Uranus and Neptune. The concept of the Grand Tour began in 1964, when Gary Flandro of the Jet Propulsion Laboratory noted that an alignment of Jupiter, Saturn, Uranus, and Neptune that would occur in the late 1970s would enable a single spacecraft to visit all of the outer planets by using gravity assists. The particular alignment occurs once every 175 years. A similar align-

8.3.3 The Flyby Anomaly The flyby anomaly is an unexpected energy increase during Earth-flybys of spacecraft. There is a significant unaccounted velocity increase of up to 13 mm/s during flybys. The flyby anomaly is an unsolved problem. Gravitational assists are valuable techniques for Solar System exploration. Because the success of flyby maneuvers depends on the geometry of the trajectory, the position and velocity of a spacecraft is continually tracked during its encounter with a planet by the Deep Space Network. The flyby anomaly was first noticed during a careful inspection of DSN Doppler data shortly after the Earth-flyby of the Galileo spacecraft on 8 December 1990. While the Doppler residuals (observed minus computed data) were expected to remain flat, the analysis revealed an unexpected 66 mHz shift, which corresponds to a velocity increase of 3.92 mm/s at perigee. An investigation of this effect has not yielded a satisfactory explanation. No anomaly was detected after the second Earth-flyby of the Galileo spacecraft in December 1992, because any possible velocity increase was masked by atmospheric drag of the lower altitude of 303 km.

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Figure 8-19. Timeline of the New Horizons mission (NASA/JHU)

On 23 January 1998 the Near Earth Asteroid Rendezvous (NEAR) spacecraft experienced an anomalous velocity increase of 13.46 mm/s after its Earth encounter. Cassini–Huygens gained ~0.11 mm/s in August 1999 and Rosetta 1.82 mm/ s after its Earth-flyby in March 2005. An analysis of the MESSENGER spacecraft (studying Mercury) did not reveal any significant unexpected velocity increase. This may be because MESSENGER both approached and departed Earth symmetrically about the equator. This suggests that the anomaly may be related to Earth’s rotation. In November 2009, ESA’s Rosetta spacecraft was tracked closely during flyby in order to precisely measure its velocity, in an effort to gather further data about the anomaly, but no significant anomaly was found. A wide range of explanations has been suggested. They range from proposing a dark matter halo around Earth, to certain problems that only

occur with the data of the DNS stations. So far, no satisfactory explanation has been given.

8.3.4 Sample Mission: New Horizons New Horizons was a flyby mission of Pluto. Pluto is so faint in our sky that we cannot see it with the unaided eye. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus’s orbit was being disturbed by yet another planet besides Neptune. Pluto was discovered in 1930 by Clyde Tombaugh, and was originally considered the ninth planet from the Sun. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 AU (4.4–7.3 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Nep170

tune. Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. When New Horizons was launched in 2006, Pluto was still considered a planet. No humanbuilt spacecraft had ever come close to Pluto. On 14 July 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. It is now headed for the Kuiper Belt. A small amount of Clyde Tombaugh’s ashes is onboard the spacecraft. On 19 January 2006, New Horizons was launched from Cape Canaveral directly into an Earth-and-solar escape trajectory with a speed of about 16.26 kilometers per second (58,536 km/h; 36,373 mph). After a brief encounter with asteroid 132524 APL, New Horizons proceeded to Jupiter, making its closest approach on 28 February 2007, at a distance of 2.3 million kilometers (1.4 million miles). The Jupiter flyby provided a gravity assist that increased New Horizons‍’ speed by 4 km/s (14,000 km/h; 9,000 mph). The encounter was also used as a general test of New Horizons‍’ scientific capabilities, returning data about the Jupiter’s atmosphere, moons, and magnetosphere.

Most of the post-Jupiter voyage was spent in hibernation mode to preserve on-board systems, except for brief annual checkouts. On 6 December 2014, New Horizons was brought back online for the Pluto encounter, and instrument check-out began. On 15 January 2015, the New Horizons spacecraft began its approach phase to Pluto. On 14 July 2015 11:49 UTC (07:49 EDT), it flew 12,500 km (7,800 mi) above the surface of Pluto, making it the first spacecraft to explore the dwarf planet. At Pluto’s distance, transmitting data back to Earth takes nearly 4.5 h. New Horizons transmits at a low bit rate, so data from the encounter were stored onboard. New Horizons required approximately 16 months after it left the vicinity of Pluto to transmit the full dataset back to Earth. New Horizons carries seven instruments: three optical instruments, two plasma instruments, a dust sensor and a radio science receiver/ radiometer. The instruments were used to investigate the global geology, surface composition, surface temperature, atmospheric pressure, atmospheric temperature and escape rate of Pluto and its moons. Some of the interesting findings from New Horizons are discussed below.

Figure 8-20. New Horizons’ trajectory (NASA/JHU) 171

Figure 8-21. Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this enhanced color global view of Pluto. (The lower right edge of Pluto in this view currently lacks high-resolution color coverage.) The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away, show features as small as 1.4 miles (2.2 kilometers). (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) Pluto is 1,473 miles (2,370 kilometers) in diameter. This is just 18.5% of Earth’s diameter. Charon, Pluto’s largest Moon, has a diameter of 750.6 miles (1208 km), which is 9.5% that of Earth’s. Pluto’s moon Nix is about 26 miles (about 42 kilometers) across and its moon Hydra is roughly 34 miles (roughly 55 kilometers) across. New Horizons discovered the “Sputnik

Planum,” an icy plain no more than 100 million years old. The probe also found an 11,000 ft high mountain range near Pluto’s equator. Pluto’s surface sports a remarkable range of subtle colors, enhanced in Figure 8-21 to a rainbow of pale blues, yellows, oranges, and deep reds. Many landforms have their own distinct colors, telling a complex geological and climatological story that scientists 172

change. Similarly, after the initial reconnaissance by flyby, the next step in exploring an extraterrestrial body is a mission that goes into orbit around it. An orbiter space probe stays in the vicinity of the object that it orbits for a longer duration than during a flyby, allowing it to make studies of changes over time, such as, for instance, seasonal changes.

8.4.2 Why Do An Orbiter? Figure 8-22. Just 15 minutes after its closest approach to Pluto on 14 July 2015, NASA’s New Horizons spacecraft looked back toward the Sun and captured a near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon, as well as some atmospheric haze. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) have only just begun to decode. Spectroscopy of the surface in reflection, and of the atmosphere in transmission, has been used to glean these new results. There is water ice on the surface. A curious aspect of the detection is that the areas showing the most obvious water ice spectral signatures correspond to areas that are bright red in color images. The chemicals that give Pluto it’s red color are collectively termed “tholins.” Pluto’s nitrogen atmosphere reaches as far as 1,000 miles (1,600 kilometers) above the surface. It appears blue in color. The dwarf planet has a plasma tail. Ionized gas was found to be tens of thousands of miles behind the dwarf planet, due to the planet’s atmosphere being stripped away by the solar wind.

8.4 Orbiter space probes An orbiter space probe is a spacecraft that enters in an orbit around the body that it is exploring. Missions include observation satellites and the International Space Station orbiting the Earth.

8.4.1 Purpose Of An Orbiter Space Probe Earth-observing space probes allow scientists to study phenomena that affect the Earth as a whole, such as global weather patterns or climate

Observations of a body from an orbiting spacecraft have two main advantages. First, when you place the spacecraft in a polar orbit (cf. Figure 7-6), it can map the entire surface of the object. Second, an orbiting spacecraft may make repeated observations of the same area, thereby recording any changes that may have taken place on the body over time. Some planets have no solid surfaces. In this case, the orbiter can provide information about the cloud layers, their rotation, and changes in

Figure 8-23. Active remote sensing via radar from orbit using NASA’s Magellan data was used to generate this image of mountains on cloudenshrouded Venus. A volcano named Sapas Mons dominates this computer-generated view of the surface of Venus, which measures 248 miles across 0.9 mile high. The simulated hues are based on color images recorded by the Soviet Venera 13 and 14 spacecraft. The image was produced by the Solar System Visualization project and the Magellan Science team at the JPL Multimission Image Processing Laboratory. (NASA/ JPL)

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storm patterns with time. Remember from chapter 3 how cloud-covered Venus is? Venus has a very dense atmosphere that you cannot look through in the visible range. But a radar altimeter can be used to map its surface. Detailed maps of the terrain, including how the terrain changes with latitude, longitude, and altitude, are important pieces of knowledge about a rocky object, and a prerequisite to further exploration by a lander. Planetary cartography, or cartography of extraterrestrial objects, is the Figure 8-24. Trajectory of the MESSENGER spacecraft (Wikipedia/ cartography of solid objects outside of the Earth. Planetary NASA) maps can show any spatially 8.4.3 Sample Mission: MESSENmapped characteristic (such as topography, geolGER ogy, and geophysical properties) for extraterresMESSENGER, short for MErcury Surface, trial surfaces. Space ENvironment, GEochemistry, and Ranging, and a reference to the Roman mythological mes-

Figure 8-25. MESSENGERâ&#x20AC;&#x2122;s first (29 March 2011) and last (30 April 2015) images from Mercuryâ&#x20AC;&#x2122;s orbit (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington) 174

senger, Mercury, was a NASA robotic spacecraft which orbited the planet Mercury between 2011 and 2015. The spacecraft was launched in August 2004 to study Mercury’s chemical composition, geology, and magnetic field. The instruments carried by MESSENGER were used on a complex series of flybys – the spacecraft flew by Earth once, Venus twice, and Mercury itself three times, allowing it to decelerate relative to Mercury using minimal fuel. MESSENGER became the second mission after Mariner 10’s 1975 flyby to reach Mercury during its first flyby of the planet in January 2008. MESSENGER entered orbit around Figure 8-26. The view on the right, from 2013, is similar to Mercury on 18 March 2011, becoming the an earlier one, from 2011, but now the coverage is more complete. The globe on the left was created from the MDIS monofirst spacecraft to do so. It successfully chrome surface morphology base map campaign. The globe completed its primary mission in 2012. on the right was produced from the MDIS color base map campaign. Each map is composed of thousands of images, Following two mission extensions, the and the color view was created by using 3 of the 8 color filMESSENGER spacecraft used the last of its maneuvering propellant and deorbited ters acquired. (1000, 750, and 430 nm wavelengths are displayed in red, green, and blue, respectively.) (NASA/Johns as planned, impacting the surface of Mer- Hopkins University Applied Physics Laboratory/Carnegie cury on 30 April 2015. Institution of Washington) Traveling to Mercury requires an the cost of prolonging the trip by many years and extremely large delta v, because Mercury’s orbit is to a total distance of 4.9 billion miles. To further deep in the Sun’s gravity well. On a direct course minimize the amount of necessary propellant, the from Earth to Mercury, a spacecraft is constantly spacecraft orbital insertion targeted a highly ellipaccelerated as it falls toward the Sun, and will artical orbit around Mercury. rive at Mercury with a velocity too high to achieve Among the discoveries made by MESSENorbit without excessive use of fuel. For planets GER are the unexpectedly high concentrations of with an atmosphere, such as Venus and Mars, magnesium and calcium found on Mercury’s nightspacecraft can minimize their fuel consumption side, and the fact that Mercury’s magnetic field is upon arrival by using friction with the atmosphere offset far to the north of the planet’s center. In Febto enter orbit (aerocapture), or can briefly fire ruary 2013, NASA published the most detailed their rocket engines to enter into orbit followed by and accurate 3D map of Mercury to date, assema reduction of the orbit by aerobraking. However, bled from thousands of images taken by MESSENthe tenuous atmosphere of Mercury is far too thin GER. As its orbit began to decay in early 2015, for these maneuvers. Instead, MESSENGER extenMESSENGER was able to take highly detailed sively used gravity assist maneuvers at Earth, Veclose-up photographs of ice-filled craters and nus, and Mercury to reduce the speed relative to other landforms at Mercury’s north pole. The Mercury, then used its large rocket engine to enter probe discovered carbon-containing organic cominto an elliptical orbit around the planet. The pounds and water ice inside permanently shadmulti-flyby process greatly reduced the amount of owed craters near the north pole. MESSENGER propellant necessary to slow the spacecraft, but at 175

sent.

Figure 8-27. Dr. Carl Sagan (9 November 1934 – 20 December 1996) poses with a model of the Viking lander in Death Valley, CA. Sagan examined possible landing sites for Viking along with Mike Carr and Hal Masursky. (NASA) also discovered large amounts of water present in Mercury’s exosphere, which was an unexpected finding. MESSENGER provided visual evidence of past volcanic activity on the surface of Mercury, as well as evidence for a liquid iron planetary core.

8.5 Lander space probe A lander is a spacecraft which descends toward and comes to rest on the surface of an astronomical body. The first interplanetary surface mission to return at least limited surface data from another planet was the 1970 landing of Venera 7 on Venus which returned data to Earth for 23 minutes.

There are several other missions that we can do when approaching a celestial body up close. Sometimes, we just want to fly through the atmosphere; that is an atmospheric probe. Lander means the soft landing after the probe stays active while impact probe (as a rule, preceding the lander) just achieves the surface by hard landing with crush. When a high velocity impact is planned not for just achieving the surface but for study of consequences of impact, the spacecraft is called an impactor. After achieving a soft landing on the surface of a body, another mission design we can go for is a rover. A rover allows us to study the surface of the body in several nearby locations, not just at the landing site.

8.5.2 Why Do A Lander? Sending a lander is the closest we can come to doing laboratory research on a celestial body itself. The prime example are the Mars Viking landers, which visited the red planet in the mid 1970s. Viking 1 and Viking 2 both successfully arrived in 1976, deploying a lander each to the surface while the orbiter remained working above.

8.5.1 Purpose Of A Lander Space Probe Lander spacecraft are designed to reach the surface of a planet and survive long enough to telemeter data back to Earth. The purpose of a lander space probe is to perform detailed work on the surface, exploring surface features up close and analyzing the chemical composition of the Figure 8-28. The location of the science instruments on the Mars surface and the atmosphere, if preCuriosity Rover (NASA/JPL) 176

Figure 8-29. This is the first laser spectrum from the Chemistry and Camera (ChemCam) instrument on NASA’s Curiosity rover, sent back from Mars on 19 August 2012. The plot shows emission lines from different elements present in the target, a rock near the rover’s landing site dubbed "Coronation" (see inset). (NASA/JPL-Caltech/LANL/CNES/IRAP)

The principal reason for the Viking mission was to look for evidence of life. The two Viking landers each carried four types of biological experiments to the surface of Mars. The landers used a robotic arm to put soil samples into sealed test containers on the craft. The two landers were identical, so the same tests were carried out at two places on Mars’ surface, Viking 1 near the equator and Viking 2 further north. After the landers dug soil samples from the frozen surface, they looked for signs for biological activity. Though the initial results were thought promising, Viking found no conclusive signs of life.

8.5.3 Sample Mission: Mars Science Laboratory & Curiosity The Mars Science Laboratory mission is part of NASA’s Mars Exploration Program, a longterm effort of robotic exploration of the red planet. Mars Science Laboratory is a space probe launched on 6 November 2011, which successfully landed Curiosity, a Mars rover (shown on the front image of this chapter), in Gale Crater on 6 August 2012. The overall objectives include investigating Mars’ habitability, studying its climate and

geology, and collecting data for a crewed mission to Mars. Curiosity comprised 23 percent of the mass of the 3,893 kg (8,583 lb) Mars Science Laboratory spacecraft, which had the sole mission of delivering the rover safely across space from Earth to a soft landing on the surface of Mars. The remaining mass of the spacecraft was discarded in the process of carrying out this task. The task of setting Curiosity down softly on the martian surface required parachutes, thrusters, and a skycrane (cf. Figure 6-24). Curiosity was designed to assess whether Mars ever had an environment capable of supporting small life forms called microbes. In other words, its mission is to determine the planet’s “habitability.” Curiosity is equipped with 17 cameras: HazCams (8), NavCams (4), MastCams (2), MAHLI (1), MARDI (1), and ChemCam (1). The general sample analysis strategy begins with high-resolution cameras to look for features of interest. If a particular surface is of interest, Curiosity can vaporize a small portion of it with an infrared laser and examine the resulting spectral signature to query the rock’s elemental composition. If that signature is intriguing, the rover will

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use its long arm to swing over a microscope and an X-ray spectrometer to take a closer look. If the specimen warrants further analysis, Curiosity can drill into the boulder and deliver a powdered sample to either the SAM or the CheMin analytical laboratories inside the rover. Curiosityâ&#x20AC;&#x2122;s mission has eight main scientific goals: Biological 1. Determine the nature and inventory of organic carbon compounds 2. Investigate the chemical building blocks of life (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur) 3. Identify features that may represent the effects of biological processes (biosignatures and biomolecules) Geological and geochemical 4. Investigate the chemical, isotopic, and mineralogical composition of the martian surface and near-surface geological materials 5. Interpret the processes that have formed and modified rocks and soils Planetary process 6. Assess long-timescale (i.e., 4-billionyear) martian atmospheric evolution processes

7. Determine present state, distribution, and cycling of water and carbon dioxide Surface radiation 8. Characterize the broad spectrum of surface radiation, including galactic and cosmic radiation, solar proton events and secondary neutrons. As part of its exploration, it also measured the radiation exposure in the interior of the spacecraft as it traveled to Mars, and it is continuing radiation measurements as it explores the surface of Mars. These data are important for a future crewed mission. Curiosityâ&#x20AC;&#x2122;s discoveries as of spring 2015 are summarized in Figure 8-31.

8.6 Other modes of exploration In this chapter, atmospheric probes and impactors received but a short mention. The communication satellites supporting space exploration by relaying signals from space probes were not described at all. There are yet more modes of exploration, so here is a brief account of those. As far as laboratory research is concerned, another option is to conduct a sample return mission. Via sample return missions, material is brought back to Earth for detailed analysis by sci-

Figure 8-30. This composite image looking toward the higher regions of Mount Sharp was taken with the Mastcam on 9 September 2015, by NASAâ&#x20AC;&#x2122;s Curiosity rover. (NASA/JPL-Caltech/MSSS) 178

Climate Orbiter and Mars Polar Lander). The manned US Apollo 11 mission in July 1969 achieved the first successful sample return from another Solar System body. The material was collected by humans. Scientists and engineers have used the International Space Station to conduct experiments on low Earth orbit, and to prepare for long-duration, interplanetary space missions with humans. The question of whether we should send robots, or whether we should send humans to explore space, has been debated for a long time. Humanity has certainly been able to learn a great deal about deep space and our Solar System using robotic space probes alone. Since signal travel times become larger and larger the further you are from Earth, there is an interest in enabling robotic space probes to conduct more and more of their projects on their own, by developing artificial intelligence. However, the spirit of exploration is strong in many humans, and favors the human exploration of space. Pro and cons have been discussed by scientists, engineers, and the public for quite some time.

Figure 8-31. Curiosityâ&#x20AC;&#x2122;s discoveries (NASA/JPL) entists in Earth-bound laboratories. There have been some sample return missions, both human and robotic. Comet Wild 2 and asteroid 25143 Itokawa were visited by robotic spacecraft which returned samples to Earth. NASA is considering launching an international sample return mission of to Mars in the near future, depending on its budget. Previous attempts to launch this type of sample return mission have been scrubbed due to technical difficulty, budget constraints, and other factors such as recent mission failures (e.g., Mars

Figure 8-32. On 13 December 1972, scientistastronaut Harrison H. Schmitt is photographed standing next to a huge, split lunar boulder during the third Apollo 17 extravehicular activity at the Taurus-Littrow landing site. The Lunar Roving Vehicle, which transported Schmitt and Eugene A. Cernan to this extravehicular station from their Lunar Module, is seen in the background. (NASA) 179

II. R ESEARCH THE A NSWER .

8 Test Your Understanding I. A NSWER IN A FEW SENTENCES . 1. What is direct sensing? 2. What is remote sensing? 3. Describe active remote sensing. Include a drawing. 4. What is the purpose of a telescope? 5. What are the advantages of placing a telescope in Earth orbit? 6. Sketch the strength-wavelength graphs for a continuous spectrum, an emission-line spectrum, and an absorption-line spectrum. 7. What are Kirchhoff’s Laws? 8. What is the Bohr model of the atom? 9. When an electron in the hydrogen atom jumps from level 3 to level 2, it emits radiation at which wavelength? Sketch the atom, showing the transition, and discuss why this transition is important in observing astronomical objects. 10.Why does the emission spectrum of iron show many more lines than that of hydrogen? 11. Describe how you can measure that an object is moving away from us. Include a drawing. 12.How can scientists detect the presence of a planet orbiting another star? Describe both the transit and the spectroscopic method. 13.What is the purpose of a flyby mission? 14.Why is it better for an object to subtend a large angle to a camera? Include a sketch drawing. 15.Does Pluto have an atmosphere? 16.What is the purpose of an orbiter space probe? 17.How can scientists generate images of the surface of Venus when it is always enshrouded in clouds? 18.What makes sending an orbiter to Mercury so difficult? 19.What is the purpose of a lander space probe? 20.What is the advantage of a rover over a simple lander? 21.How was the Curiosity Rover able to chemically analyze the “Coronation” rock on Mars? 22.What is the purpose of a sample return mission?

1. The next-generation space telescope is the James Webb Space Telescope. Research this telescope on the internet, then write a short essay, 10-20 sentences long, which includes the following terms: carrier rocket, diameter of the primary mirror, folded segmented mirror, orbital location of the telescope, Sun, operating temperature, deployable sunshield. 2. When scientists analyze the spectrum of the Sun, what kind of a spectrum do they find? Research the Sun’s spectrum on the internet, then write a short essay, 10-20 sentences long, which includes the following terms: photosphere, atmosphere, opaque gas, transparent gas, continuous spectrum, absorption-line spectrum, Fraunhofer lines. Include a sketch. 3. How does a spacecraft magnetometer work? Research magnetometers on the internet, then write a short essay, 10-20 sentences long, which includes the following terms: scientific instrument, magnetic field, Explorer 1, Gauss, survey magnetometers, scalar magnetometers, vector magnetometers, fluxgate magnetometers, Luna 10. 4. What kind of scientific instrument is the Alpha Magnetic Spectrometer? Research the AMS on the internet, then write a short essay, 10-20 sentences long, which includes the following terms: particle experiment, cosmic rays, positrons, orbital location, particles rejected, antimatter, dark matter, space radiation environment. 5. Should robots or humans explore space? Research this issue on the internet, then write four lists, each containing three items. The four lists are: pro robot, con robot, pro human, con human. Conclude by stating, in a few sentences, which mode of exploration you personally favor, and why.

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The satellite business in low and geostationary Earth orbits is growing. (NASA/JPL)

SPACE COMMERCIALIZATION How can a private business make money in Outer Space? What kind of opportunities exist for the commercialization of Outer Space? Is there an economic boom in Outer Space? And where is commercial spaceflight headed in the near future?

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9.1 What is Space Commercialization? The commercialization of space is the use of equipment sent into or through Outer Space to provide goods or services of commercial value, either by a country, or by a private enterprise. In recent years, private companies have invested increasing amounts of capital in developing new technologies to access space or operate in space. There are many types of commercial uses of space. You may be thinking of far-off schemes like asteroid mining. Thatâ&#x20AC;&#x2122;s not where space commercialization is at, yet. The largest and most profitable market so far has been that for commercial satellites for communications and television. But there are also successful attempts to build private space launch systems. Indeed, today the four major sectors of the space industry are: satellite manufacturing, satellite services, supportive ground equipment manufacturing, and the launch industry. The satellite manufacturing sector is composed of satellite and their subsystems manufacturers. It is nearly impossible to imagine modern life without satellites. Satellite services enable phone calls, television programs, weather forecasts, airplane navigation, disaster management, agricultural planning, energy resource discovery, and online maps, to name but a few applications. The ground equipment sector is composed of manufacturing items like GPS devices, network gateways, control staFigure 9-1. Public-private partnerships are important to tions, direct broadcast satellite stimulate the space economy. (NASA) dishes, and other specialized equipment. The launch sector is composed of launch services and vehicle manufacturing. According to a 2014 NASA report, the global satellite industry is the largest commercial space market, with $190 billion in 2012 revenues, of which $113.5 billion was in satellite services (see Figure 9-2).

9.1.1 First Commercial Communications Satellite In chapter 1, you learned about the first commercial satellite, the Telstar 1 communications satellite, paid for by the American phone company AT&T. Belonging to AT&T, Telstar 1 was part of a multi-national agreement among AT&T, Bell Telephone Laboratories, NASA, the British General Post Office and the French National Post, Telegraph & Telecom Office 182

to develop experimental satellite communications over the Atlantic Ocean. In 1962, Telstar 1 ushered in a new age of the commercial use of space technology. It successfully relayed through space the first television pictures, telephone calls, and fax images, and provided the first live transatlantic television feed. Telstar 1 also became the first satellite used to synchronize time between two continents, bringing the United Kingdom and the United States to within 1 microsecond of each other (previous efforts were only accurate to 2,000 microseconds). On the occasion of the 50th birthday of its launch, Wired magazine called Telstar 1 “the little satellite that created the modern world.” As of 2015, there are 40 countries which are operating at least one commercial communications satellite or currently have one on order.

project in cislunar space is the stimulation of several organizations to pursue a lunar lander and rover/hopper to capture a $30 million Google Lunar XPRIZE.

9.1.2 US Space Commercialization Process Space commercialization has been referred to as the “space revolution,” linking it with other economic game changers such as the industrial revolution. The US is presently witnessing the convergence of several powerful ecoFigure 9-2. Space communications is the major market highlighted nomic forces that foster space in a 2014 report on public-private partnerships in space. (NASA) commercialization. These include The national space policy of the US seeks to the need to restore American capability to reach develop more public-private partnerships in low Earth orbit for the servicing of the Internaspace. A public-private partnership is a governtional Space Station, the rise of a hospitality/ ment service or private business venture which is tourism/entertainment industry interested in funded and operated through a partnership of govspace, the development of expansive remote sensernment and one or more private sector compaing and other applications in Earth orbit, and the nies. Telstar 1 is a perfect example of the model of possibilities envisioned for opening commercial public-private partnerships, which is crucial for space activities in the cislunar region. A specific the success of the commercial space industry. The 183

Telstar 1 satellite was the first commercial payload, built by a team at Bell Telephone Laboratories. Launched by NASA aboard a Delta rocket from Cape Canaveral, Telstar 1 was the first privately sponsored space launch. On 28 June 2010, President Obama issued a national space policy directive providing comprehensive guidance for all government activities in space, including the commercial, civil, and national security space sectors. The new policy leans farther forward in support of US business interests than any previous space policy. The principles section of the policy states, “The United States is committed to encouraging and facilitating the growth of a US commercial space sector that supports US needs, is globally competitive, and advances US leadership in the generation of new markets and innovation-driven entrepreneurship.” The Office of Space Commercialization is the principal unit for space commerce policy activities within the Department of Commerce. Its mission is to foster the conditions for the economic growth and technological advancement of the US commercial space industry. The Commercial Spaceflight Federation is a private spaceflight industry advocacy group for spaceflight. A 2014 NASA report on public-private partnerships lists 8 areas of space capability development that show positive indicators of privatesector interest and investment, new business formation, and alignment with NASA’s goals, thus making them strong candidates for economic stimulation with increased NASA partnerships: space communications, satellite servicing, interplanetary small satellites, robotic mining, microgravity research for biomedical applications, liquid rocket engines for launch vehicles, wireless power, and Earth observation data visualization. This chapter will touch on a few of these areas, with an emphasis on private space launch enterprises. As you read through this chapter, bear in mind that it was written near the end of 2015. Space commercialization is a very rapidly developing topic. New companies may enter the market while others may cease to do business in space. To

Figure 9-3. This is the third rocket in the first attempts at private spaceflight, the Conestoga1620. It is seen on the launch pad at Wallops Island. The launch vehicle would fail about a minute after lift off due to a problem with its guidance system. (Wikipedia/ NASA)

a large extent, the success of the launch vehicle manufacturing industry rest on the success rate of private space launches. Another important enabling factor without which the commercial spaceflight industry cannot succeed is the continued underwriting by space insurance companies.

9.2 Private Spaceflight Private spaceflight is flight beyond the Kármán line that is conducted and paid for by an entity other than a government agency. As you know, Outer Space is not very far away. Yet, launching vehicles to transport payloads or humans into Outer Space was, for the most part of the 20th century, something only nation states could do. Spaceflight was thus the monopoly prov-

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Figure 9-4. White Knight One, carrying the SpaceShipOne, taxis on the runway of the Mojave Spaceport for an air-launched, suborbital spaceflight. (Wikipedia/D. Ramey Logan) ince of a small group of national governments. Both the US and Soviet space programs were operated using mainly military pilots as astronauts. During this period, no commercial space launches were available to private operators, and no private organization was able to offer space launches. Eventually, private organizations were able to both purchase and offer space launches, thus beginning the period of private spaceflight. With appropriate international and national space policies and deregulations in place, entrepreneurs began designing and deploying competitive space systems to the national-monopoly governmental systems of the early decades of the space age. Successes to date include flying suborbital spaceplanes, launching orbital rockets, and flying two inflatable orbital test habitats (Genesis I and II). Planned private spaceflights beyond Earth orbit include personal spaceflights around the Moon.

9.2.1 First Private Space Launch: Conestoga 1 The first privately-funded rocket to achieve spaceflight was the Conestoga 1, which was launched by Space Services Inc. of America on 9 September 1982. It was a suborbital flight to

309 km (192 mi) altitude. The Conestoga rocket consisted of surplus Minuteman ICBM stages with additional strap-on boosters, as required for larger payloads. It was the world’s first privately funded commercial rocket, but was launched only three times, and successfully only once, before the program was shut down. One rocket exploded on the launch pad, and the other failed less than a minute after lift-off due to a problem with its guidance system. This served to illustrate the technical difficulties which private spaceflight would encounter.

9.2.2 First Private Launch To Orbit: Orbital Sciences Corporation On 5 April 1990, the US company Orbital Sciences Corporation became the first company to launch a privately developed rocket into orbit. It was a Pegasus rocket. As discussed in Chapter 6.7, the Pegasus is a small, air-launched rocket. Capable of carrying payloads of up to 443 kg (977 lb) into LEO, Pegasus remains active as of 2015. A merger of Orbital Sciences Corporation and the defense and aerospace divisions of Alliant Techsystems (ATK) was announced on 29 April 2014. Orbital ATK since secured a NASA contract to develop the four-segment Space Shuttle Solid

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Rocket Booster into the five-segment boosters for the Space Launch System. They also produce the Cygnus spacecraft, which delivers cargo to the International Space Station. Continuing the story of setbacks to private spaceflight, Orbital suffered the loss of an Antares rocket and its payload. On 28 October 2014, CRS Flight 3, an attempted flight of the Cygnus which would have been its fourth to the ISS and the fifth of an Antares launch vehicle, resulted in the Antares rocket exploding seconds after lift off.

9.2.3 First Private Crewed Space Launch: SpaceShipOne SpaceShipOne was a suborbital airlaunched spaceplane. Its mother ship was named White Knight. SpaceShipOne was developed by Mojave Aerospace Ventures (a joint venture between Paul Allen and Scaled Composites, Burt Rutan’s aviation company, in their Tier One program), without government funding. On 21 June 2004, SpaceShipOne made the first privately funded human spaceflight. On 4 October 2014, it won the $10 million Ansari XPRIZE, by reaching 100 kilometers in altitude twice in a two-week period with the equivalent of three people on board and with no more than ten percent of the non-fuel weight of the spacecraft replaced between flights. Development costs were estimated to be $25 million, funded completely by Paul Allen. SpaceShipOne’s first official spaceflight, known as flight 15P, was piloted by Mike Melvill. A few days before that flight, the Mojave Air and Space Port was the first commercial spaceport licensed in the United States. A few hours after that flight, Melvill became the first licensed US commercial astronaut. SpaceShipOne’s spaceflights were watched by large crowds at Mojave Spaceport.

9.3 Case Study: Virgin Galactic With the success of SpaceShipOne/White Knight flights, a successor project started in 2004. The successor ships were named SpaceShipTwo

and White Knight Two. In 2005, Burt Rutan and Richard Branson founded The Spaceship Company, to build commercial spaceships and launch aircraft for space travel. The company was jointly owned by Scaled Composites and Virgin Group until 2012 when Virgin Galactic became the sole owner. Virgin Galactic intended to go into spaceflight tourism. It sold several hundred tickets for spaceflights on SpaceShipTwo to private citizens. SpaceShipTwo was twice as large, measuring 18 m (60 ft) in length. Whereas SpaceShipOne could carry a single pilot and two passengers, SpaceShipTwo had a crew of two and room for six passengers. SpaceShipTwo’s planned trajectory would achieve a suborbital journey with a short period of weightlessness. Carried to about 16 kilometers, or 52,000 ft, underneath the carrier aircraft, the vehicle would continue to over 100 km after separation. The time from lift off of the White Knight Two carrying SpaceShipTwo until the touchdown of the spacecraft after the suborbital flight would be about 2.5 hours. The suborbital flight itself would only be a small fraction of that time, with weightlessness lasting approximately 6 minutes. Passengers would be able to release themselves from their seats during these 6 minutes and float around the cabin. By August 2013, 640 customers had signed up for a flight, initially at a ticket price of $200,000 per person, but raised to $250,000 in May 2013. Tickets are available from more than 140 “space agents” worldwide. Passengers who reportedly submitted deposits include Stephen Hawking, Tom Hanks, Ashton Kutcher, Katy Perry, Brad Pitt, and Angelina Jolie. In September 2014, Richard Branson described the intended date for the first commercial flight as February or March of 2015; by the time of this announcement, a new plastic-based fuel had yet to be ignited inflight. By 2014, the three test flights of the SpaceShipTwo have only reached an altitude of around 71,000 ft, approximately 13 miles; in order to receive a Federal Aviation Administration license to

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Figure 9-5. SpaceShipTwo underneath White Knight Two were photographed by Jeff Froust during a flyover for the runway dedication of Spaceport America, NM, in October 2010. (Wikipedia) carry passengers, the craft needs to complete test missions at full speed and 62-mile height. On 31 October 2014, the fourth rocket powered test flight of one of the company’s SpaceShipTwo craft, VSS Enterprise, ended in disaster, as it broke apart in midair, with the debris falling into the Mojave desert in California, shortly after being released from the mothership. The flight was the first test of SpaceShipTwo with new plastic-based fuel, replacing the original—a rubber-based solid fuel that had not met expectations. The 39-year-old co-pilot Michael Alsbury was killed and 43-year-old pilot Peter Siebold was seriously injured. On 28 July 2015, Richard Branson released a statement that the National Transportation Safety Board had finished its investigation into the accident, and found that the feather, a unique system used for the spaceship’s safe reentry from space, was manually and prematurely unlocked by the co-pilot. The spaceplane almost immediately broke apart. Branson stated that engineers already designed a mechanism to prevent

the feather from being unlocked at the wrong time. A second SpaceShipTwo has been under construction and is expected to fly in 2016. While Virgin Galactic has not given up its goal to transport humans into Outer Space, it has recently repositioned its business to take advantage of the growing satellite launch market. LauncherOne is an orbital launch vehicle that was publicly announced by Virgin Galactic in July 2012. The LauncherOne configuration is proposed to be an expendable, two-stage, liquid-fueled rocket, air-launched from a White Knight Two. This would make it a similar configuration to that used by Orbital Sciences. LauncherOne is being designed to launch “smallsat” payloads of 200 kilograms (440 lb) into Earth orbit, with launches projected to begin in 2016. Several commercial customers are said to have already contracted for launches, including GeoOptics, Skybox Imaging, Spaceflight Services, and Planetary Resources. Virgin Galactic has also been awarded a transportation contract from NASA. And it has signed up an-

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Figure 9-7. SpaceX CRS-5 was a cargo resupply mission to the International Space Station, conducted by SpaceX for NASA, and was launched on 10 January 2015 and ended on 11 February 2015. It was the seventh flight for SpaceX’s uncrewed Dragon cargo spacecraft and the fifth SpaceX operational mission contracted to NASA under an ISS resupply services contract. (SpaceX)

Figure 9-6. A Falcon 9 rocket is being rolled out to the launch pad for a May 2015 launch of the Thales missions. (SpaceX) other high-profile customer, OneWeb. The OneWeb satellite constellation is a proposed constellation of approximately 700 satellites expected to provide global internet broadband service to individual consumers as early as 2019.

9.4 Case Study: SpaceX

September 2008 by Falcon 1 from the Omelek Island, Marshall Islands; and its first launch from a US spaceport was the Falcon 9’s Flight 1 on 4 June 2010 from Cape Canaveral. SpaceX’s Dragon Spacecraft successfully docked with the International Space Station on 22 May 2012, and has since been delivering supplies. A crew version of

Space Exploration Technologies Corporation (SpaceX) is an American aerospace manufacturer and space transport services company. It was founded in 2002 by former PayPal entrepreneur and Tesla Motors CEO Elon Musk. His vision was to build a simple and relatively inexpensive reusable rocket that would go into space multiple times, similar to the turn around time capabilities that commercial airliners currently exhibit. Musk’s stated end goal has been the colonization of Mars. SpaceX became the second company to launch a rocket into orbit using a rocket developed with private—not government—funds. Its first successful launch was performed on 28 Figure 9-8. Dragon maneuvers in the proximity of the ISS. (NASA) 188

Figure 9-9. A comparison of the Falcon Heavy with other launch vehicles (SpaceX)

Dragon is in development. SpaceX was also the first private company to send a satellite into geosynchronous Earth orbit, on 3 December 2013. This is an impressive list of achievements, which is coupled with a rising financial success of SpaceX. Musk initially went a similar route as Space Services Inc., in that he tried to purchase ICBMs, in Russia. When the deal did not work out, Musk decided to develop his own rocket. The Falcon 1 was completely developed and built by SpaceX. SpaceX developed their own engine, the Merlin 1, for the rocket. SpaceX operated and launched the Falcon 1 themselves. It is the first privately funded, liquid-propellant rocket that was launched from the ground to reach orbit. SpaceX used what’s called a vertically integrated business model, which has been critical in reducing launch cost. Since the founding of SpaceX in 2002, the company has developed three families of rocket engines — Merlin and Kestrel for launch vehicle propulsion, and the Draco RCS control thrusters. SpaceX is currently developing two further rocket engines: SuperDraco and Raptor. SpaceX has flown, or is developing, several orbital launch vehicles: the Falcon 1, Falcon 9, and Falcon Heavy. As of May 2015, the Falcon 9 v1.1 was in active use and the Falcon Heavy was under development. The Falcon 1 was a small rocket capable of placing several hundred kilograms into low Earth

orbit. It functioned as an early testbed for developing concepts and components for the larger Falcon 9. Falcon 1 made five flights in 2006–2009. On 28 September 2008, the Falcon 1 succeeded in reaching orbit on its fourth attempt, becoming the first privately funded, liquid-fueled rocket to do so. The Falcon 1 carried its first successful commercial payload, RazakSAT, into orbit on 13 July 2009, on its fifth launch. SpaceX has now retired the Falcon 1 and transferred Falcon 1 class payloads to be secondary payloads on the Falcon 9, which proved to be more efficient. The Falcon 9 is a medium-lift vehicle capable of delivering up to 10,450 kg (23,000 lb) to orbit, and is intended to compete with the Delta IV and the Atlas V rockets, as well as other launch

Figure 9-10. The Dragon crew capsule is sitting on a test stand for testing conducted in May 2015. (SpaceX)

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Figure 9-11. A look through the open hatch of the Dragon V2 reveals the layout and interior of the seven-crew capacity spacecraft. The Dragon V2 is designed to carry people into Earth orbit. (SpaceX)

providers around the world. It is a two-stage rocket, which has nine Merlin engines in its first stage, and one in its second stage. The Falcon 9 rocket successfully reached orbit on its first attempt in June 2010. The second flight for the Falcon 9 vehicle was the COTS Demo Flight 1 on 8 December 2010, the first launch under the NASA Commercial Orbital Transportation Services (COTS) contract, and was similarly successful. Its third flight, COTS Demo Flight 2, launched on 22 May 2012, and was the first commercial spacecraft to reach and dock with the International Space Station. Further launches have continued with many successes and one launch failure. On 28 June 2015, the Falcon 9 disintegrated beginning at 2 minutes and 19 seconds after launch, resulting in a total loss. SpaceX was expected to resume flights by the end of 2015.

The Falcon Heavy began development as a heavy-lift configuration using a cluster of three Falcon 9 first stage cores with a total 27 Merlin engines. SpaceX is aiming for the first demonstration flight of the Falcon Heavy in 2016. When it finally flies, the Falcon Heavy will be the most powerful rocket in operation. It is surpassed only by the retired US Saturn V and Russian Energia rockets. SpaceXâ&#x20AC;&#x2122;s low launch prices (less than $2,500 per pound to orbit for Falcon 9 v1.1 and $1,000 for Falcon Heavy), especially for communications satellites flying to geostationary orbit, have resulted in market pressure on its competitors to lower their own prices. The communications satellites launch market had been dominated in the years preceding 2013 by companies using rockets that had been developed with government funding, Europeâ&#x20AC;&#x2122;s Arianespace (flying the Ariane 5

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Figure 9-12. Location of the SpaceX Orbital Launch Site (Wikipedia) rocket) and the Russian-American International Launch Services (flying Russia’s Proton vehicle). SpaceX’s capabilities and prices are affecting the global market considerably. Arianespace requested in early 2014 that European governments provide additional subsidies to face the competition from SpaceX. In 2014, Falcon 9 mission pricing to geostationary Earth orbit was approximately US$15 million less than a launch on a Chinese Long March 3B. However, the Chinese Government and the Great Wall Industry company—which markets the Long March for commsat missions—made a policy decision to maintain commsat launch prices at approximately US$70 million. SpaceX’s capabilities and lower launch prices have also begun to affect the launch market for US military payloads, where for nearly a decade the large US launch provider United Launch Alliance (ULA) had faced no competition for military launches. In October 2014 ULA announced a

major restructuring of processes and workforce in order to decrease launch costs by half, in part as a result of competition from SpaceX. In May 2015, ULA stated that it would go out of business unless it won commercial and civil satellite launch orders to offset an expected slump in US military launches. On 16 September 2014, NASA announced that both SpaceX and Boeing have received contracts to provide crewed launch services to the ISS. For completing the same contract requirements, Boeing can receive up to 4.2 billion dollars, while SpaceX can receive up to 2.6 billion dollars. Both Boeing and SpaceX were awarded for the same set of requirements: completing development and certification of their crew vehicle, then flying up to six operational flights to the ISS following a certification flight. Both companies were guaranteed at least two of the operational flights. As of 2015, the SpaceX Dragon V2.0 was expected to launch on an uncrewed test flight to the ISS in December 2016, followed by a crewed test flight in April 2017. SpaceX is currently building the first commercial orbital launch facility. The spaceport will be located at Boca Chica Village near Brownsville, TX. SpaceX started construction on the new launch facility in September 2014, with the first launches from the facility expected to take place no earlier than 2016. When completed, it will become SpaceX’s fourth active launch facility, following three launch locations that are leased from the US government. In 2011, Elon Musk received the Heinlein Prize. The Heinlein Prize for Advances in Space Commercialization was founded in 1988 to reward individuals who make practical contributions to the commercialization of space. It honors the memory of Robert A. Heinlein, one of the most popular science fiction writers of the 20th century. The Heinlein Prize offers a cash award of $500,000 to one or more individuals for practical accomplishments in the field of commercial space activities and is awarded by the International Aeronautical Congress in Bremen, Germany. 191

services in Asia. The launch, on 24 December 1997, was provided by International Launch Services, an American-Russian joint venture with exclusive rights to the worldwide sale of commercial Proton rocket launch services from the Baikonur Cosmodrome in Kazakhstan. However, a failure of the fourth stage left it stranded in a highly inclined and elliptical orbit, although still fully functional. It was declared a total loss by its insurers. The satellite was transferred to Hughes Global Services Inc., with an agreement to share any profits with the insurers. Hughes subsequently used an Apollo-style free return trajectory that required only a few days to complete to salvage the satellite, with a trajectory designed and subsequently patented by Hughes Chief Technologist Jerry Salvatore. Using on-board propellant and lunar gravity, the orbit’s Figure 9-13. A detail of the sometimes rugged lunar apogee was gradually increased with several masurface. Rima Ariadaeus on the Moon is thought to neuvers at perigee until it flew by the Moon at a be a graben. The lack of erosion on the Moon makes its structure with two parallel faults and the sunken distance of 6,200 km from its surface in May block in between particularly obvious. (Wikipedia/ 1998. Another lunar flyby was performed later NASA) that month at a distance of 34,300 km to further improve the orbital inclination. Hughes claimed to 9.5 Private Spaceflight to the have flown the first commercial mission to the Moon Moon. Now that private space probes and rockets The first commercial lunar flyby mission by have reached low Earth and geostationary Earth design occurred in 2014. Manfred Memorial Moon orbit, is anyone trying for the Moon? Indeed, Mission (4M) by LuxSpace, a child company of there already have been an accidental and inGerman OHB System, was sent to the Moon in tended lunar flyby by commercial spacecraft. And honor of OHB System’s founder, Professor Manthe first commercial lunar landing is expected to fred Fuchs, who died in 2014. It was carried on occur in the very near future. the Chinese Chang’e 5-T1 spacecraft. The Moon flyby took place on 28 October 2014, after which 9.5.1 First Commercial Robotic the spacecraft entered an elliptical Earth orbit and continued transmission until 11 November 2014. Lunar Flyby There were two commercial lunar flybys. The payload weighed 14 kg and contained One was unintended, the other was intended; both two scientific instruments. The first instrument claim to have been the first private lunar flyby. was a radio beacon to test a new approach for loPAS-22, also known as AsiaSat 3 and then HGS-1, cating spacecraft. Amateur radio operators were was a geosynchronous communications satellite encouraged via prize incentives to receive the which was salvaged from an unusable geosynchrotransmissions and send results back to LuxSpace. nous transfer orbit by means of the Moon’s gravThe second instrument, a radiation dosimeter proity. AsiaSat 3 was owned by AsiaSat Ltd. of Hong vided by the Spanish company iC-Málaga, continuKong to provide communications and television 192

ously measured radiation levels throughout the satellite’s circumlunar path.

9.5.2 First Commercial Human Lunar Flyby (ca. 2017?) In chapter 1, you learned about Dennis Tito, who paid for a ride to and a stay on the ISS. The first space tourist, Tito was a customer of Space Adventures Ltd., a Virginia-based commercial spaceflight company, who, together with the now defunct MirCorp, brokered Tito’s journey with the Russian Space Agency. Tito’s flight occurred in 2001, and since then, seven other people have flown to the ISS with Space Adventures. Space Adventures is now proposing to take space tourists to the Moon by 2017.

Figure 9-14. Dennis Tito, the first private citizen to visit the International Space Station, shares his experiences with visitors at the 40th Space Congress. (Wikipedia/NASA)

“The thing I have taken away from it is a sense of completeness for my life — that everything else I would do in my life would be a bonus.” – Dennis Tito

Deep Space Expedition Alpha (DSE-Alpha) is the name given to the mission proposed in 2005 to take the first space tourists to fly around the Moon. The mission plans involve a modified Soyuz capsule docking with a booster rocket in Earth orbit which then sends the spacecraft on a free return circumlunar trajectory that circles around the Moon once. The DSE-Alpha proposal is to launch the Soyuz with one crew member and two passengers aboard. While the ticket price was originally announced in August 2005 to cost US$100 million per seat, Space Adventures founder Eric Anderson publicized in January 2011 that one of the two available seats had been sold for $150 million. Launch dates were targeted for 2017/2018.

9.5.3 First Private Robotic Lunar Landing (ca. 2017?) The Google Lunar XPRIZE is a $30 million competition to land a privately funded robot on the Moon. The mission of the Google Lunar XPRIZE is to incentivize space entrepreneurs to create a new era of affordable access to the Moon and beyond. According to the XPRIZE website, the competition’s $30 million prize purse will be awarded to teams who are able to land a privately funded rover on the Moon, travel 500 meters, and transmit back high definition video and images. The first team that successfully completes this mission will be awarded the $20 million dollar Grand Prize. The second team to successfully complete the mission will be awarded $5 million dollars. To win either of these prizes, teams must prove that 90% of their mission costs were funded by private 193

Figure 9-15. Carnegie Mellon University’s Andy rover is a four-wheeled robot designed to scramble up steep slopes and survive the temperature swings and high radiation encountered while exploring the Moon. (CMU NEWS) sources. Teams were given until the end of 2016 to announce a verified launch contract to remain in the competition and complete their mission by the end of 2017. By the end of 2015, there were four teams in the running which had booked a launch. Astrobiotic Technology Inc., a Pittsburgh-based company with ties to Carnegie Mellon University, had a flight on a SpaceX Falcon 9; Hakuto, a Japanese team, had arranged to piggyback on that ride. SpaceIL, an Israeli nonprofit organization, also booked a Falcon 9. The fourth competitor, Moon Express Inc., another American company, signed a rocket launch contract with Rocket Lab, a New Zealand startup company. Under the launch services contract, Rocket Lab will use its Electron rocket system. Astrobotic is an American privately held company that is developing space robotics technology for lunar and planetary missions. It was founded in 2008 by Carnegie Mellon Professor Red Whittaker and his associates, with the goal of winning the Google Lunar XPRIZE. Astrobotic has current plans to conduct a lunar landing and rover mission to win the Google Lunar XPRIZE and deliver commercial payload. Their lunar lander, Griffin, has flexible payload mounts which can accommodate a variety of rovers and other payloads to support robotic missions like skylight exploration, sample return, regional prospecting, and polar volatile characterization.

Figure 9-16. Lacus Mortis, Latin for “Lake of Death,” is a plain of basaltic lava flows in the northeastern part of the Moon. In March 2014 Astrobotic announced Lacus Mortis will be the target destination for its first Moon mission as part of the Google Lunar XPRIZE competition. Their intention is to land next to a pit located in the Lacus Mortis plain then circumnavigate the pit with the rover. (Wikipedia/NASA) The first mission will carry the Andy rover developed by Carnegie Mellon University. Andy is named after the University founder, Andrew Carnegie, and the Pittsburgh banker, Andrew Mellon. Andy will be released from the Griffin Lander to visit a pit in the Lacus Mortis region. Andy is 33 kg and contains unique pivoting axle suspension that allows it to drive faster in rugged terrain. The name Hakuto means “White Rabbit,” and comes from a Japanese folktale wherein the shape of a rabbit can be seen in the dark areas of the lunar surface. Their rover development is led by Professor Kazuya Yoshida (Department of Aerospace Engineering at Tohoku University), who has contributed to numerous Japanese space missions. Hakuto’s operations are performed entirely by volunteer members. The team consists of many professionals from various fields, including those outside of aerospace and scientific fields. Team Hakuto will attach their two rovers, Moonraker and Tetris, to Astrobotic’s Griffin lander. The rovers will be released to line up and

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Figure 9-17. Moon Express Inc. is developing the multipurpose MX-1 spacecraft. (Moon Express Inc.) compete against Andy, to see which rover will traverse 500 meters and transmitting HDTV images back to Earth in real time to win the Google Lunar XPRIZE. Hakuto’s ultimate target is to explore holes that are thought to be caves or “skylights” into underlying lava tubes, for the first time in human history. These lava tubes could prove to be very important scientifically, as they could help explain the Moon’s volcanic past. They could also become candidate sites for long-term habitats, able to shield humans from the Moon’s hostile environment. The two rovers are linked by a tether, so that Tetris can be lowered into a suspected skylight. SpaceIL is an Israeli nonprofit organization working to land the first Israeli spacecraft on the Moon. The organization was founded in 2011, when three young Israeli engineers undertook to enter the Google Lunar XPRIZE competition. Today, SpaceIL is led by CEO Eran Privman together with the three co-founders: Yariv Bash, Kfir Damari and Yonatan Winetraub. SpaceIL has nearly 30 full-time staff and dozens of volunteers. SpaceIL is using a hop concept instead of a rover, because it required less mass. SpaceIL developed the idea of a space hop, in which the spacecraft

lands on the surface of the Moon and then takes off again with the fuel left in its propulsion system. Then it will perform another landing, 500 meters away, according to the XPRIZE criteria. SpaceIL’s goal is to make an educational impact and to inspire a whole new generation in Israel and around the world to take interest in science, technology, engineering, and math; to recreate an “Apollo Effect” in Israel. The team is committed to using the potential prize money to promote science and scientific education in Israel, to ensure that Israel will continue to live up to its reputation for excellence in these fields. Moon Express Inc. is an American privately held company formed by a group of Silicon Valley and space entrepreneurs, with the goal of winning the Google Lunar XPRIZE, and ultimately mining the Moon for natural resources of economic value. Moon Express is combining best practices of traditional aerospace know how with the innovation and entrepreneurial culture of Silicon Valley. Moon Express is developing its MX-1 lander spacecraft as a space vehicle capable of a multitude of applications, including delivering scientific and commercial payloads to the Moon, at a fraction of the cost of conventional approaches. About the size of a large coffee table, the MX-1 uses a

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Figure 9-18. This image of Dale A. Gardner holding up a â&#x20AC;&#x153;For Saleâ&#x20AC;? sign has been widely used to symbolize the commercial space sector. (NASA) completely self-contained single stage spacecraft that can reach the surface of the Moon from a geosynchronous transfer orbit.

9.5.4 Commercial Business On The Moon According to the Outer Space Treaty (see Chapter 2), the exploration and the use of Outer Space, including the Moon and other celestial bodies, shall be for the benefit of, and in the interests of, all countries and shall be province of all mankind. In this sense, the exploitation of the Moon and other celestial bodies by commercial businesses should not face any problems. However, the details of what exactly will be the property rights of companies establishing businesses on the Moon, or elsewhere in the Solar System for that matter, have still to be worked out. Assuming that spaceflight enables companies to conduct business on the Moon, where might there be opportunity for profit? Space tourism is an obvious venue. Another burgeoning space industry is that of space burials. Private companies such as Celestis Inc and Elysium Space offer space burial services on the Moon. Only a sample of cremains is launched so as to make the service affordable. Celestis is collaborating with Astrobiotics and Moon Express. In addition to human burials in space, they are also offering space burials for pets. Elysium is also partnering with Astrobiotis.

Lunar mining is another avenue to make profit off lunar resources. It is not clear, however, whether or not the Moon offers materials of such rarity that their extraction on the Moon and return to Earth would indeed be profitable. When you study lunar mining, the first material that usually comes up as potentially viable is Helium-3. Helium-3 (He-3) is a light isotope of helium with two protons and one neutron, in contrast with two neutrons in common helium. The abundance of He-3 is thought to be greater on the Moon than on Earth, having been embedded in the upper layer of regolith by the solar wind over billions of years, though still lower in quantity than in the Solar Systemâ&#x20AC;&#x2122;s gas giants. The interest in He-3 arrises from its potential use in fusion power plants. Other than Moon Express, companies that have considered mining the Moon are Shackleton Energy Company, and Golden Spike. If there is going to be a human colony on the Moon, there will be many opportunities for commercial businesses to supply communications, habitats, and logistics. This is an opportunity for public-private partnerships between countries that are considering permanent lunar outposts, and private companies that can provide mission support in cislunar space. Bigelow Aerospace is an American space technology startup company, based in North Las Vegas, NV, that is pioneering work on expandable space station modules, and is considering building a lunar habitat.

9.6 Private Spaceflight Beyond the Moon (ca. mid-2020s?) While to date only governments have launched rockets that have reached other bodies in the Solar System, lunar flights by commercial spaceflight companies are expected to take place within the next few years. If successful, these rocket flights will open up the entire Solar System for commercial exploration and exploitation. Entrepreneurs such as Elon Musk and Dennis Tito have been outspoken in favor of private missions to Mars. Mars Colonial Transporter is the name of the privately funded development pro-

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Figure 9-19. NASA’s NEXT ion thruster on the test stand (NASA) ject by Musk’s company SpaceX, to design and build a spaceflight system of reusable rocket engines, launch vehicles and space capsules to transport humans to Mars, and return to Earth. SpaceX began development of the large Raptor rocket engine for the Mars Colonial Transporter, but it will not be operational earlier than the mid-2020s. Inspiration Mars Foundation is an American nonprofit organization founded by Dennis Tito that proposes to launch a crewed mission to fly by Mars in 2021. The mission would launch on

NASA’s Space Launch System heavy-lift booster, and use an Orion capsule for the crew of two, probably a married couple. There are other companies who seek to find and utilize resources beyond the Moon, and are not focussed on a mission to Mars. One example is Deep Space Industries. According to their website, Deep Space Industries is a startup that wants to mine asteroids for materials, and to use those materials for manufacturing on space stations. Another company interested in asteroids is Planetary Resources Inc. This company has already flighttested its Arkyd spacecraft, which will eventually survey near Earth asteroids for valuable resources. Refresh your knowledge of asteroids by re-reading chapter 3.5.

9.6.1 Ion Thrusters All spacecraft launched from Earth have used chemical rockets. Only multi-stage chemical rockets are powerful enough to overcome the pull of Earth’s gravity. But once a rocket has taken a spacecraft into space, there exist other means to accelerate it. If they can be built and operated by commercial spaceflight companies, ion thrusters

Figure 9-20. Schematic of the ion thruster used for the Deep Space 1 mission (Wikipedia/NASA) 197

Figure 9-21. Boeing 702 satellites now use ion thrusters to change Earth orbits after launch. Notice the large solar arrays. (Boeing)

ing constant thrust trajectories in Chapter 7. The fuel of the ion engine is an electrically neutral gas, and of course, there is no oxidizer. The energy for the thruster comes from solar radiation. It is harnessed using on-board solar arrays, and stored in power supplies. The power is then used in two ways. Firstly, it charges an electron gun that shoots electrons at the fuel gas. Most commonly, the fuel gas that is used is xenon (Xe). The electrons collisionally ionize the xenon gas, turning it into a plasma of positive ions. Secondly, the power electrically charges two grids, one with a positive charge, and another with a negative charge. Similarly to a parallel-plate capacitor, the two grids create an electric field. The positively charged xenon ions are attracted to the negatively charged grid owing to the electrostatic, or Coulomb force (cf. Chapter 3.2.4). This accelerates, and then, due to the holes in the grid, expels the gas ions as the thruster’s reaction mass. Electrons are injected into the beam after acceleration to maintain a neutral plasma and to avoid giving the entire spacecraft an electric charge. A schematic of the Deep Space 1 ion thruster is shown in Figure 9-20. Ion thrusters have been considered commercially in the quest for interplanetary missions. In 2012, HyperV Technologies Corporation succeeded in obtaining funding, via a Kickstarter campaign, to demonstrate a prototype electric pulsed

may be a less expensive alternative for reaching interplanetary space. Chemical rockets achieve their large thrust with a high mass-flow rate but low exhaust velocity. An ion thruster is an example of a nonchemical rocket engine. An ion thruster creates thrust by accelerating ions to high exhaust velocity in an electric field, but has a low mass-flow rate, and hence, a lower thrust than a chemical rocket engine. However, ion engines need less reaction mass. Ion propulsion systems were first demonstrated in space by the NASA Lewis (now Glenn Research Center) missions Space Electric Rocket Test (SERT) I and II. The first was SERT-1, launched 20 July 1964; it successfully proved that the technology operated as predicted in space. These were electrostatic ion thrusters using mercury and cesium as the reaction mass. Many ion thruster have since been flown by a va- Figure 9-22. The Planetary Society’s LightSail solar riety of international space agencies. We first en- sailing spacecraft was, in 2015, scheduled to ride a SpaceX Falcon Heavy rocket to orbit in 2016 with its countered such an electric thruster on NASA’s parent satellite, Prox-1. (Josh Spradling / The Planeinterplanetary Dawn spacecraft when considertary Society) 198

Figure 9-23. Forces on a solar sail (SolarSailWiki) plasma jet thruster. Their goal is for this thruster to enable highly reliable, high performance, low cost interplanetary space transportation. But as of 2015, this thruster had not been flight proven. Companies that have successfully engineered and flown versions of the ion thruster include the European Airbus Defence & Space, a division of Airbus Group. They used ion propulsion to shift ESA’s ARTEMIS spacecraft from a low to a high Earth orbit in 2002/2003. The US aerospace company Boeing launched two communications satellites in 2015, equipped with ion thrusters for in-space flight. Boeing used the thrusters to change the orbits of the satellites. Neither company, however, has announced plans to use their flight-proven ion thrusters for interplanetary spaceflights.

9.6.2 Solar Sails An entirely different class of spacecraft propulsion is propellant-less propulsion. In this class of spacecraft propulsion, the vehicle carries no reaction mass with it. An example of this kind of propulsion is a solar sail. Solar sails use solar radiation to push large, ultra-thin mirrors to high speeds. On 21 May 2010, Japan Aerospace Exploration Agency (JAXA) launched the world’s first interplanetary solar sail spacecraft IKAROS (Inter-

planetary Kite-craft Accelerated by Radiation Of the Sun) to Venus. You might think that solar sails work by using the solar wind, but that is not true. The force that the solar wind exerts on a one square kilometer area at the average Earth-Sun distance is at best 6 × 10−3 N. Compare that with the force that sunlight exerts at the same distance and on the same area, 9 N, more than a thousand times stronger. This radiant force per area is also called the radiation pressure. A photon of radiation that interacts with a solar sail exerts a force. When the photon is absorbed by the solar sail, it pushes on the sail because of the action-reaction law. When additionally, the solar sail is made of shiny material, like a mirror, then the photon is reflected by the sail. The reflection, or ejection, of the photon from the sail creates a second action-reaction pair. Combined, the two forces give the resultant total force of radiation on the solar sail (see Figure 9-23). The force on the solar sail is larger the closer the sail is to the Sun, because the strength of the solar radiation decreases inversely with the distance squared (see Chapter 3). And shorter wavelength radiation, which carries larger energy (see Chapter 2), can in principle provide a larger force. The force on the sail is also larger the larger the sail is. And if you can make the sail very shiny, this will also increase its efficiency, by producing a larger reflected force. The lighter the material, the better. A light-yet-strong material is Mylar. Solar sailing does not incur propellant costs, as the spacecraft, once launched, does not need to carry propellant to generate thrust. Solar sails have to be tacked, just like normal sails on sailboats, to generate the thrust and to steer the vehicle. Solar sails can be tacked to fly on spiraling, constant thrust orbits to the inner or outer Solar System. Despite having a force component pushing away from the Sun, the tangential force allows a sail to both travel towards and away from the sun. Like a rocket, if the tangential force is in the same direction as the spacecraft’s orbit, the orbit will expand and the sail will travel out

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May 2015 the Society’s technology demonstrator, LightSail-A, was launched, and deployed its solar sail on 7 June 2015. Thanks to a highly successful Kickstarter campaign, LightSail1 is funded for four months of orbital operations in Earth orbit. In 2015, LightSail-1 was anticipated to be launched in September 2016 with its partner spacecraft, Prox-1, aboard the first operational flight of the SpaceX Falcon Heavy. If LightSail-1 is successful, The Planetary Society will execute two more solar Figure 9-24. By tacking back and forth into the Sun’s rays like a sailsail projects with more complex boat, LightSail gradually raises the apogee of its orbit. (Jason Davis goals. LightSail-2’s goal would / The Planetary Society) be to collect scientific data and from the sun. If the sail is tilted the other direcimprove solar sailing control. LightSail-3’s goal tion so that the tangential force goes against the would be to travel to the Earth-Sun L1 Lagrangian orbit, the orbit will contract and the sail will spiral point (cf. Figure 2-18), which is almost four times in closer to the Sun. as far as the Moon. There, it would be used to deA private non-profit working on using solar tect solar eruptions, which can damage power and sail technology is The Planetary Society. The Plane- communication systems on Earth and orbiting tary Society has been working on delivering a prispacecraft. Such detection will provide earlier vate solar sail into orbit as far back as 1999. On 20 warnings of geomagnetic storms potential power failures. Space Services Inc., the company that launched the first privately funded rocket, declares an interest, on their website, in developing solar sail technology for lunar and planetary missions. They were well positioned in a publicprivate partnership with NASA, on NASA’s Sunjammer. They were involved in this solar sail demonstration test mission through their key partner L’Garde, the sail’s manufacturer. However, the Sunjammer mission was cancelled in 2014. It is therefore unclear how Space Services is planning to move forward with solar sails. On 23 October 2015, NASA solicited proposals for a CubeSat-scale solar sail for space propulsion. Accordingly, a solar sail propulsion system is Figure 9-25. A solar sail can be tacked to fly misbeing developed at NASA Marshall Space Flight sions to the inner or outer Solar System. The total force from the Sun is shown by the blue arrows. Center to provide propulsion for the Near Earth (SolarSailWiki) 200

Asteroid Scout (NEAS) project. A variety of potential target asteroids would be identified based upon launch date, time of flight, and rendezvous velocity. NEAS is planned to perform a slow flyby of an asteroid within approximately two years of launch, scheduled to occur in 2018. The deployed sail area is targeted to be approximately 86 square meters and has a tentative mass allocation of 2.5 kg. NASA desires for the solar sail technology and design being developed for the NEAS mission to be commercially available after the completion and delivery of the flight system hardware in 2018. This public-private partnership has the potential to assist and stimulate future, commercial spaceflight to asteroids.

Figure 9-26. NASAâ&#x20AC;&#x2122;s Commercial Crew Program is working with the aerospace industry through phases and contracts to facilitate the development of safe, reliable and cost-effective human spaceflight capabilities for low Earth orbit transportation. (NASA) 201

II. R ESEARCH THE A NSWER .

9 Test Your Understanding I. A NSWER IN A FEW SENTENCES . 1. What is meant by space commercialization? 2. What are the three major sectors of the space industry? 3. What makes Telstar 1 a perfect example of a public-private partnership? 4. What are some current US needs that drive space commercialization? 5. What are the areas in which NASA sees a potential for public-private partnerships? (Give at least three examples.) 6. What was the Ansari XPRIZE? 7. Which was the first private crewed spaceflight, and who piloted it? 8. What kind of a launch did the first private crewed spacecraft use? 9. Why is the Falcon 1 rocket such an important contribution to spaceflight? 10.Which US companies have launched spacecraft that have docked with the ISS? 11. What are some accidents that have contributed to set-backs to the private spaceflight industry? (Give at least three examples.) 12.Where could you go if you had the means of becoming a space tourist? 13.What is the Google Lunar XPRIZE? 14.How was Astrobiotic Technology Inc. founded, and what are the company’s goals for the Google Lunar XPRIZE mission? 15.Who has the legal rights to lunar resources? 16.What are some business opportunities that companies envision to have on the Moon? 17.Who is planning private spaceflights to Mars? 18.How does an ion thruster work? Include a drawing. 19.What is meant by no reaction mass? 20.What generates the thrust of a solar sail? 21.Which locations in the Solar System will generate the largest thrust for solar sails? 22.What qualities of a solar sail will make it deliver a high thrust?

1. When you fly across the US, say, New York to LA, you pay a certain ticket price. Research and then work out how much the ticket price is on a per pound basis. Describe how you came up with your estimate. Compare this to per pound prices for space launches by Virgin Galactic and SpaceX. Finally, find out the intended and actual price per pound for a Space Shuttlelaunched payload. 2. In this chapter, we have focused on US spaceflight companies. Who are the major international commercial competitors to US commercial spaceflight companies which have successfully launched satellites to orbit? Describe three major competitors. Include information about the companies’ founding and their locations, and the launch vehicles that they use. 3. What is OneWeb? What is the purpose of OneWeb? Describe the satellites, and the orbits that they are planned to be placed in. 4. What is a CubeSat? What does it have in common with a toaster? How many CubeSat’s has NASA launched? And what are the CubeSats headed for Mars intended to accomplish? 5. How would you tack a solar sail, and what trajectory would it follow, for a flight to the inner Solar System? Include a drawing. Describe the space mission that has flown a solar sail to Venus. 6. What is an electric sail, and how does it differ from from a solar sail? What are some pros and cons of the electric sail? What applications are envisioned for the electric sail? 7. What is a space elevator? How does it work? What are some of the reasons why it is difficult to build a space elevator?

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A painting of a human Mars mission by Les Bossinas (Wikipedia/NASA/Lewis Research Center)

SPACE COLONIZATION Where might humans set up space colonies? And why? What would be some of the difficulties in colonizing Outer Space?

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10.1 What is Space Colonization? Humans have colonized throughout history. A colony could take many forms, as a military or civilian outpost, or a permanent settlement. A settlement created by people migrating from a central region to an outlying one became the modern definition of colonization. Space colonization, then, is the establishment of a permanent human settlement in Outer Space to which people from the Earth migrate for a variety of purposes.

10.2 Why colonize space? Space and survival refers to a position stating that the long-term survival of the human species and civilization require proper use of the resources of Outer Space, and in particular space colonization, as failing to do so could lead to human extinction. Proponents of space colonization make the case that humanity currently has all of its “eggs in one basket,” so to speak. If instead some people were to move off the Earth, than no matter what might happen to make Earth uninhabitable, some of the independent colonies of Earth might survive. While increasing the number of places

“Once the threshold is crossed when there is a self-sustaining level of life in space, then life’s long-range future will be secure irrespective of any of the risks on Earth. . . . Will this happen before our technological civilization disintegrates, leaving this as a might-have-been? Will the selfsustaining space communities be established before a catastrophe sets back the prospect of any such enterprise, perhaps foreclosing it forever? We live at what could be a defining moment for the cosmos.” Sir Martin Rees, England’s Astronomer Royal, in his book “Our Final Hour,” 2003

where humans live would help to prevent extinction, expanding the distance between human colonies might also be a good strategy. People closest to a disaster event are most likely to be killed or injured; people farthest away are most likely to survive. The human race as a whole would survive longer if we spread out to multiple colonies with a range of Earth distances, rather than if we stayed on our central planet alone. A particular aspect of space colonization is that, unlike colonization on Earth, the colo204

10.2.1 Why Solar System Colonization?

Figure 10-1. Comet Shoemaker–Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. This HST image shows the sites where pieces of the comet impacted Jupiter’s atmosphere. (NASA/STScI) nists would expand into territories that are uninhabited by indigenous peoples. The second argument for space colonization is one which favors it as a form of exploitation of space resources. It is based in the notion that Outer Space hosts vast resources, in terms of real estate and natural resources. As discussed in chapter 8, such resources may exist, and are the target of future commercial and governmental mining operations. Mining for material on the Moon or asteroids might allow us, for example, to move some manufacturing off planet. An ancillary effect of space manufacturing is that we could do it without increasing the pollution on Earth. The two regions of Outer Space which are considered for space colonization are broadly divided by inside versus outside of our Solar System. The first has humans migrating off the Earth and populating the Solar System. The other is concerned with humans migrating out of the Solar System and away from our Sun. There are reasons why both of these ideas are considered.

Colonizing the Solar System means that some humans move and start to live permanently and autonomously off the Earth, but remain within the confines of our Solar System. The two most common arguments made in favor of Solar System colonization are the survival of human civilization in case of a planetary-scale disaster (natural or man-made), and the vast resources of the Solar System for expansion of human society. One potential natural disaster is the collision of Earth with a large comet or asteroid. Such a collision could kill billions of people. Large collisions have occurred in the past, destroying many species. Future collisions are inevitable, although we don’t know when they will happen. If there were a major collision today, not only would billions of people die, but recovery would be difficult since everyone would be affected. If major space settlements were built before the next collision, the unaffected space settlements could provide aid, much as we offer help when disaster strikes another part of the world. Other disasters that could make the Earth uninhabitable are due to human activity. This could be through acts of war, especially, a global war. World War III is generally considered a hypothetical successor to World War II and is often suggested to be nuclear, devastating in nature, and likely much more violent than both WWI and WWII combined. This war is anticipated and planned for by military and civil authorities in many countries. Concepts range from purely conventional scenarios, or a limited use of nuclear weapons, to the destruction of the planet. Global warming and climate change are terms for the observed century-scale rise in the average temperature of the Earth’s climate system and its related effects. Industrial activities have created large emissions of so-called greenhouse gases which are changing the way the Earth responds as a whole to temperature changes. Climate change over the 21st century is likely to adversely affect hundreds of millions of people

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through increased coastal flooding, reductions in water supplies, increased malnutrition, and negative impacts on health. It is possible that human-induced climate change could trigger abrupt, large-scale changes with severe global impacts, but the probabilities of triggering such events are as yet poorly understood. Yet another human made disaster could be due to superbugs. Superbugs are strains of bacteria that are resistant to several types of antibiotics. For nearly a century, bacteria-fighting drugs Figure 10-2. An artistâ&#x20AC;&#x2122;s illustration of the Earth as the Sun starts to known as antibiotics have helped turn red giant. (Wikipedia) to control and destroy many of 10.2.2 Why Interstellar Colonizathe harmful bacteria that can make us sick. Howtion? ever, the overuse and misuse of antibiotics has In approximately 5.4 billion years the Sun helped to create drug-resistant bacteria. Drugwill to turn into a red giant star. The Sun will beresistant forms of tuberculosis, gonorrhea, and come sufficiently large to engulf the current orbits staph infections are just a few of the dangers we of the Solar Systemâ&#x20AC;&#x2122;s inner planets, including now face. A common superbug that is increasingly Earth (see chapter 3.9.1). Therefore, a very long infecting people is known as MRSA. time in the future, the Solar System will no longer The resources of the Solar System may, in be a good place to live. the future, have many uses, especially if human Proponents of interstellar colonization arcolonization comes to pass. Some have declared gue that humans must find a way for interstellar asteroid mining the next gold rush. Asteroids conspaceflight in the long run, if we want to survive as tain hydrogen and oxygen, useful as rocket fuel; a species. As we have found via space exploration, some may have water or water ice, a precious comthere are many stars in the Milky Way Galaxy, and modity for human settlers. These chemicals were many Earth-like planets, at least in terms of size, found via remote sensing and by space probes. in orbit around other stars. This opens up the opThere are asteroids which are thought to be made portunity to colonize an extrasolar planet. predominantly of metal, but it is less clear what metals they contain. In any event, the metals could be harvested for space-manufacturing and construction purposes. Some asteroids have been hyped up by asteroid mining companies to contain abundant quantities of elements that are rare on Earth, such as platinum, and other elements in the platinum group. These metals would be very valuable for trading with Earth. Lunar mining has also been considered profitable (cf. 9.5.4).

10.3 Space Colony Types Space colonies are divided into two broad types:

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I. Surface-based settlements that would exist on or below the surfaces of planets, moons, etc.;

Figure 10-3. This is a design sketch of a spacecraft called Nautilus-X, which includes a rotating torus of sufficient scale for the demonstration of artificial gravity. It was a 2011 NASA proposal for a long-duration crewed space transport vehicle. The Nautilus-X design concept did not advance beyond the initial drawings and proposal. (Wikipedia/NASA) II. Space habitats — free-floating space stations that would orbit the Earth, another planet, moon, etc. or be in an independent orbit around the Sun. There is considerable debate among space settlement advocates as to which type (and associated locations) represents the better option for expanding humanity throughout the Solar System. Current proposals for space colonies center on space stations which, for starters, are in Earth orbit, and on settlements on the Moon and on Mars. The International Space Station has provided the most recent, temporary, non-autonomous, human presence in low Earth orbit. To exist as an independent colony, the habitat would have to supply all of the things that humans need for survival. Foremost amongst the colonists’ survival needs are gravity and a life support system. To furnish these continuously, the colonists would need materials and energy. Orbital colonies have unique and desirable properties. Building and maintaining orbital colonies should be quite a bit easier than constructing similarly sized homesteads on the Moon and Mars. Compared to other locations, Earth orbit has substantial advantages.

Orbits close to Earth can be reached in hours, whereas the Moon is days away and trips to Mars take months. There is ample continuous solar power in high Earth orbits. The amount of gravity can be controlled at any desired level by rotating an orbital colony. Colonies in orbit are better positioned to provide goods and services to Earth. For these reasons, orbital colonies will almost certainly come first, with lunar and martian colonization later. The main, but solvable, disadvantage of orbital colonies is lack of materials. These may be expensively imported from the Earth, or more cheaply from extraterrestrial sources, such as from asteroids. Surface-based settlements would have the advantage in terms of on-site usable materials.

10.3.1 Gravity In A Space Colony The creation of artificial gravity is considered desirable in long-term space travel or habitation, for ease of mobility, for in-space fluid management, and to avoid the adverse long-term health effects of weightlessness. The minimal gforce required to avoid bone loss is not known—nearly all current experience is with g-forces of 1 g (on the surface of the Earth) or 0 g in orbit. There has not been sufficient time spent on the Moon to determine whether lunar gravity is sufficient. The Moon and Mars have a surface gravity much less than Earth normal. The lunar surface is at roughly 1/6 g and Mars’ is 1/3 g. It has been suggested that the descendants of colonists born in low-g colonies would adapt to the low-g environment. While this is good for their life in the colony, the adaptation might make it very difficult for them to ever visit the Earth. It is in principle possible to create artificial gravity of 1 g on a space station. You previously learned about apparent weightlessness on an orbiting space station (cf. 5.5.3). A spacecraft that rotates about its own axis can produce the opposite effect, an apparent weight. This could be achieved by building a spinning spacecraft of a toroidal shape, as shown in Figures 10-3 and 10-4. The ro-

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Figure 10-4. Cut-away view of a spinning torus with human experiencing artificial gravity on the hull (NASA) tation drives any object inside the spacecraft toward the hull; this is a manifestation of the object’s attempt to travel in a straight line due to inertia. The spacecraft’s hull provides a normal force with acts as the centripetal force required for the objecs to travel in a circle. Thus, the apparent weight felt by the object is simply the reaction force of the object on the hull reacting to the force of the hull on the object, in accordance with Newton’s third law. In chapter 6, you first learned about centripetal force, centripetal acceleration, and angular velocity, when we discussed orbits. The centripetal acceleration can be written as a

v2 = r

ω2 × r

where, for a spinning space station, a is the acceleration at any point of its perimeter or hull, r is its radius, v is the velocity of a point on the outer hull, and ω is the angular velocity of the station. To find the specifications for the space station, set the centripetal acceleration equal to 1 g, Earth gravity. The above equation shows that many combinations of radius and angular velocity yield the same, 1 g. For example, you can either spin a small station really fast, or you can make the station very big, and spin it slowly. Typical space settlement designs are roughly one half to a few kilome-

ters (a few miles) across. A few designs are even larger. Practical Outer Space applications of artificial gravity for humans have not yet been built and flown, principally due to the large size of the spacecraft required. The Gemini 11 mission attempted to produce artificial gravity by rotating the capsule around the Agena Target Vehicle to which it was attached by a 36-meter tether. They were able to generate a small amount of artificial gravity, about 0.00015 g, by firing their side thrusters to slowly rotate the combined craft like a slow-motion pair of bolas. The resultant force was too small to be felt by either astronaut, but objects were observed moving towards the “floor” of the capsule.

10.3.2 Life Support In A Space Colony Components of the life support system are life-critical, because failure would result in death or severe injury to the colonists. The life support system must supply air, water and food. It must also maintain the correct body temperature, an acceptable pressure on the body and deal with the body’s waste products. Shielding against harmful external influences such as radiation and dust particle impacts would also be necessary. There is no breathable air in space, on the Moon, or on Mars. Settlements must be air tight to hold a breathable atmosphere. Space life support systems must maintain atmospheres composed, at a minimum, of oxygen, water vapor and carbon dioxide. The partial pressure of each component gas adds to the overall barometric pressure. Most modern crewed spacecraft use conventional air (nitrogen/oxygen) atmospheres and use pure oxygen only in pressure suits during extravehicular activity where acceptable suit flexibility mandates the lowest inflation pressure possible. Space colonists will consume water for drinking and many other uses. Water must be stored, used, and reclaimed (from waste water) efficiently when no on-site sources exist, such as on a space station. Space exploration has shown the

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existence of water on the Moon and on Mars, which may conceivably be extracted for the use of the space colonists. Life support systems would need to include a plant cultivation system which allows food to be grown within buildings and/or vessels. NASA has been studying crop support systems for more than 30 years, with a goal of designing a selfsustaining life support system for crew members. But no large-scale crop production tests have yet been conducted in space. The ISS has Figure 10-5. Biosphere 2 is an Earth systems science research fabeen used to grow hydroponic letcility located in Oracle, Arizona. (Wikipedia) tuce. A future system could be delems including low amounts of food and oxygen, signed so that it reuses most (otherwise lost) nutridie-offs of many animal and plant species, and ents. This is done, for example, by composting toipeople and management issues. lets which reintegrate waste material (excrement) An environmental concern for any kind of back into the system, allowing the nutrients to be long-duration space travel and for space habitats taken up by the food crops. The food coming from is radiation protection. Life on Earth is shielded the crops is then consumed again by the system’s from lethal doses of electrically charged particles, users and the cycle continues. the cosmic rays, by the magnetosphere (cf. 2.4.1), A closed ecological system is generally proas well as from energetic electromagnetic radiaposed for life support. This has been attempted on tion, by the atmosphere (cf. 8.2.1). Solar flares creEarth. Biosphere 2 is an Earth systems science reate a particularly hazardous particle environment search facility located in Oracle, Arizona. Biooutside of Earth’s magnetosphere. sphere 2 was originally constructed between 1987 Travelers spending any amount of time on a and 1991 by Space Biosphere Ventures. It was spaceship outside of geospace, as well as settlers named “Biosphere 2” because it was meant to be on a space habitat outside of geospace, would be the second fully self-sufficient biosphere, after the negatively affected by these particles. Neither the Earth itself. Biosphere 2 is a 3.14-acre Moon, nor Mars, have a magnetic field, to shield (1.27-hectare) structure built to be an artificial, the settlers from cosmic rays. The best way to promaterially closed ecological system, or vivarium. tect the colonists is to surround their ship or habiIt remains the largest closed system ever created. tat by a large amount of material which absorbs It encloses a rainforest, an ocean with a coral reef, the incoming particle radiation. This is also rea mangrove wetland, a savannah grassland, a fog ferred to as mass shielding. desert, a agricultural system, a human habitat, An additional concern is collision with and a below-ground infrastructure. Biosphere 2 small dust particles. Though small in mass, dust was only used twice for its original intended purparticles can have a high velocity relative to the poses as a closed-system experiment with huspacecraft, and thus a dangerously high momenmans: once from 1991 to 1993, and the second tum. Mass shielding would protect the settlement time from March to September 1994. Both atfrom dust particle/micrometeorite impacts. Howtempts, though heavily publicized, ran into prob209

Figure 10-6. A design sketch of the inflated BEAM module (balloon structure at top center) berthed to the ISS. Astronauts are expected to enter the module a few times a year to gather performance data and inspect the structure. Following its test period, the module is jettisoned from the station, burning up on re-entry. (NASA) ever, one could build orbital settlements next to asteroids, which could be mined for materials. Fortunately, there are tens of thousands of suitable asteroids in orbits near that of Earth alone, and far more in the asteroid belt.

ever, such massive bulk is a significant problem for accelerating and maneuvering space vessels for the transit to the settlement, or for maintaining rotation in a spinning space station.

10.3.3 Material Supply For A Space Colony Colonizing space is extraordinarily expensive because launch costs are so high. Launching payloads from Earth is very expensive, as you know from chapter 9, where we considered the cost per pound to Earth orbit today. To colonize space will require much lower launch costs than exist today. Else, colonists can take very little in the way of material goods with them. In any case it would be best if the colony could harvest materials and manufacture goods locally. There is a clear difference between space and surface settlements with respect to materials. Colonies on the Moon or on Mars will be able to have mining operations, to get sufficient material for shielding the settlement, and to obtain materials for building and running the station. Earthorbiting stations do not have this advantage. How-

10.3.4 Energy Supply For A Space Colony Solar energy is abundant in the Solar System, is reliable and is commonly used to power satellites today. There is no night in free space, and no clouds or atmosphere to block sunlight. Massive structures will be needed to convert sunlight into large amounts of electrical power for settlement use. Energy may be an export item for space settlements, using microwave/laser beams to send power to Earth. We already garner solar power on Earth and on satellites, by using solar arrays. Solar arrays contain many solar panels which are packed with solar cells. Solar cells are made of semiconductor materials, usually silicon. Such materials exhibit a property known as the â&#x20AC;&#x153;photovoltaic effectâ&#x20AC;? that causes them to absorb photons of light

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and release electrons. When these free electrons are captured, an electric current results that can be used as electricity. The photovoltaic effect was first observed by French physicist A. E. Becquerel in 1839. Incidentally, solar cells gained prominence with their incorporation onto the 1958 Vanguard I satellite. Hence, solar power is a technology which advanced by spaceflight. Another way to gain energy from solar radiation is via a solar thermal collector that supplies heat by absorbing sunlight, for the purpose of either direct heating or indirect electrical power generation from heat. Radiation obeys an inverse-square law (chapter 3). Thus, the closer the settlement is to the Sun, the more solar energy per unit area it can harvest. In the outer Solar System, there is far less solar energy available. So, the distance from the Sun matters for the available solar energy supply. Large solar power photovoltaic cell arrays or thermal power plants would be needed to meet the electrical power needs of the settlersâ&#x20AC;&#x2122; use. These power plants could be at a short distance from the main structures if wires are used to transmit the power, or much farther away with wireless power transmission. A major export of the initial space settlement designs is anticipated to be large solar power satellites that would use wireless power transmission to send power to locations on Earth, or to colonies on the Moon or other locations in space. For locations on Earth, this method of getting power is extremely benign, with zero emissions and far less ground area required per watt than for conventional solar panels. Once these satellites are primarily built from lunar or asteroid-derived materials, the price of solar power satellite electricity could be lower than energy from fossil fuel or nuclear energy; replacing these would have significant benefits such as elimination of greenhouse gases and nuclear waste from electricity generation. A full day on the Moon lasts an Earth month (cf. 4.4.2). Hence the Moon has nights of two Earth weeks in duration. Any settlement would have to store solar energy for use during nighttime, or rely on other energy sources. Mars

has nights, as well as an atmosphere with dust storms to cover and degrade solar panels. Also, its greater distance from the Sun translates into smaller amounts of solar energy compared with Earth orbit or the Moon. For these reasons, nuclear power is sometimes proposed for colonies in more distant locations from the Sun.

10.4 Colonization locations There are some locations that are being seriously considered as space settlements by governments and private spaceflight companies. Other locations offer promise, but are not yet considered with the same amount of interest. Space stations are generally considered as a stepping stone to surface colonization.

10.4.1 Colonizing A Space Habitat A space habitat is a type of space station, but is intended as a permanent settlement. No space habitats have yet been constructed but many design proposals have been made, with varying degrees of realism. In 2009, Russian and European space officials started consultations on possible goals for human spaceflight after the end of the ISS program. At the forefront of the talks were Russian plans to

Figure 10-7. NASA has partnered with four companies to develop habitat concepts. Lockheed Martin plans to accomplish this by â&#x20AC;&#x201C; in part â&#x20AC;&#x201C; using heritage subsystems from the Orion capsule itself to help reduce the cost of the initial habitat design. (NASA)

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Figure 10-8. This artist’s concept of a lunar base and extra-base activity was revealed during a 1986 Summer Study on possible future activities for NASA. A roving vehicle similar to the one used on three Apollo missions is depicted in the foreground. The artwork was done by Dennis Davidson. (NASA) replace the ISS with a new outpost in low Earth orbit in 2020-2025. However unlike the ISS, which was designed to serve primarily as a research lab, the new station was conceived as an assembly point for missions to the Moon and Mars. Russian and European officials said they hoped that NASA would also be interested in the project. However, it appears that NASA expects that a successor to the ISS will be a commercially operated facility. In an effort to stimulate deep space capability development across the aerospace industry, NASA in 2014 announced the Next Space Technologies for Exploration Partnerships (NextSTEP). Accordingly, the next step for US human spaceflight is the development of capabilities to support more extensive missions to deep-space destinations. The Orion capsule is the first component of NASA’s human exploration beyond low Earth orbit and will have a capability of sustaining a crew of four for 21 days in deep space and returning them safely to Earth. Later, the Orion could connect to an Exploration Augmentation Module.

The module would serve as a foundational component of a future in-space habitat. NextSTEP partners will provide concept studies and technology investigation. In total, four companies – Lockheed Martin, Bigelow, Orbital ATK, and Boeing – were awarded contracts to address habitat concept development. Bigelow Aerospace is an American company that has plans to build the Bigelow Commercial Space Station. Currently, Bigelow Expandable Activity Module (BEAM) is an expandable space station module being developed under contract to NASA, for use as a temporary module on the ISS from 2016 to 2017. BEAM is a radical departure from existing metallic designs, using expandable space habitat technology (cf. Figure 10-6). The Chinese space program includes plans for are a large modular space station, “Tiangong Space Station.” It is a modular space station, with comparable size and weight to the Soviet/Russian Mir, which was the predecessor of the ISS. Operations will be controlled from the Beijing Aerospace Command and Control Center in China. The

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Figure 10-9. An artist’s rendition of ESA’s proposed multi-dome lunar base being built. Once assembled, the inflated domes are covered by robots with a layer of lunar regolith to help protect the occupants against space radiation and micrometeoroids. (ESA) planned launching date is around 2020. China has solicited international participation, in the form of modules that would attach to their station.

10.4.2 Colonizing The Moon The colonization of the Moon is the proposed establishment of permanent human communities or robot industries on the Moon. Because of its proximity to Earth, the Moon has been seen as the most obvious natural expansion target. Recent indication that water might be present in noteworthy quantities at the lunar poles has renewed interest in the Moon. There are many pros and cons that people have come up with when debating a permanent lunar outpost. A lunar base could be a site for launching rockets with locally manufactured fuel to distant planets such as Mars. The energy required to send objects from Earth to the Moon is lower than for most other bodies. Transit time is comparably low. The Apollo astronauts made the trip in three days and future technologies could improve on this time. The short transit time would also allow emergency supplies to quickly reach a

Moon colony from Earth, or allow a human crew to evacuate relatively quickly from the Moon to Earth in case of emergency. This could be an important consideration when establishing the first human colony. If the Moon were colonized then it could be tested if humans can survive in low gravity. Those results could be utilized for a viable Mars colony as well. Launching rockets from the Moon would be easier than from Earth because the Moon’s gravity is lower, requiring a lower escape velocity. A lower escape velocity would require less propellant. The round trip communication delay to Earth is less than three seconds, allowing near-normal voice and video conversation, and allowing some kinds of remote control of machines from Earth that are not possible for any other celestial body. The delay for other Solar System bodies is minutes or hours. This, again, could be particularly valuable in an early colony, where life-threatening problems requiring Earth’s assistance could occur. There is also a long list of disadvantages to a lunar colony. The long lunar night would impede reliance on solar power and require a colony to be

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Figure 10-10. A proposed Mars mission profile, designed by NASA in 2009 (NASA) designed that could withstand large temperature extremes. The lack of an atmosphere must be dealt with, and increases the chances of the colony being hit by meteoroids. Even small pebbles and dust (micrometeoroids) have the potential to damage or destroy insufficiently protected structures. Moon dust is an extremely abrasive glassy substance formed by micrometeoroids and unrounded due to the lack of weathering. It sticks to everything and can damage equipment, and it may be toxic. Plants would need to be grown in sealed chambers. A large portion of the Moon’s orbit is outside the protection of the Earth’s magnetosphere (chapter 2.4.2), making radiation shielding mandatory. Some suggest building the lunar colony underground, which would give protection from radiation and micrometeoroids. This would also greatly reduce the risk of air leakage, as the colony would be fully sealed from the outside except for a few exits to the surface. It is uncertain whether the low gravity on the Moon is strong enough to prevent detrimental effects to human health in the long term.

NASA is not currently planning a return to the Moon. However, many other countries, including China, Russia, Japan, and India do. The most serious project at this time is the European Space Agency’s (ESA) permanently manned Lunar base, to be built by 2025. The new head of ESA, Johann-Dietrich Wörner, in 2015 as one of his first acts, announced that he wants to build a lunar station on the Moon’s far side. International partners are welcome. Incidentally, the far side of the Moon is also the location of choice for a radio telescope for space exploration. Its location on the far side would shield it from all of the interfering radio signals created by human civilization on Earth.

10.4.3 Colonizing Mars Mars colonization has been advocated by several non-governmental groups for a range of reasons and with varied proposals. One of the oldest groups is the Mars Society who promotes a NASA program to accomplish human exploration of Mars and has set up Mars analog research stations in Canada and the United States. Mars to

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Stay advocates recycling emergency return vehicles into permanent settlements as soon as initial explorers determine permanent habitation is possible. Mars One aims to establish a fully operational permanent human colony on Mars by 2023 with funding coming from a reality TV show and other commercial exploitation, although this approach has been widely criticized as unrealistic and infeasible. MarsPolar intends to establish a human settlement, around 2029, on Mars’ polar region, the part of the planet with abundant quantities of water ice. They intend to finance this project with donations. Elon Musk founded SpaceX with the long-term goal of developing the technologies that will enable a self-sustaining human colony on Mars. In 2015 he stated “I think we’ve got a decent shot of sending a person to Mars in 11 or 12 years.” Before any people are transported to Mars on the notional Mars Colonial Transporter envisioned by SpaceX, a number of Figure 10-11. Conjunction (top) and opposition (bottom) class robotic cargo missions would be unmissions to Mars (NASA) dertaken first in order to transport the requisite equipment, habitats and supplies. Sir continues utilizing the ISS until 2024; validating Richard Branson, in his lifetime, is “determined to deep space technologies and studying the effects be a part of starting a population on Mars. I think of long duration space missions on the human it is absolutely realistic. It will happen... I think body. The second stage, “Proving Ground,” moves over the next 20 years, we will take literally hunaway from Earth reliance and ventures into cisludreds of thousands of people to space and that will nar space for most of its tasks. This is when NASA give us the financial resources to do even bigger plans to capture an asteroid (planned for 2020), things.” test deep space habitation facilities, and validate NASA also has a vision to land humans on capabilities required for human exploration of Mars, and Mars is a major driver for what the Mars. Finally, phase three is the transition to indeagency is doing in the next decades. In 2015, pendence from Earth resources. The “Earth IndeNASA published its official plan for human explopendent” phase includes long term missions, and ration and colonization of Mars. The plan operates the harvesting of martian resources for fuel, wathrough three distinct phases leading up to fully ter, and building materials. NASA is aiming for husustained colonization. The first stage, already unman missions to Mars in the 2030s, though Earth derway, is the “Earth Reliant” phase. This phase independence could take decades longer. 215

Colonizing Mars will face some of the same difficulties as colonizing the Moon. Mars (cf. chapter 3.3.4) has no global magnetic field comparable to Earth’s geomagnetic field, requiring mass shielding of any Mars station. The atmospheric pressure on Mars is far below the limit at which people can survive without pressure suits. Martian air is toxic, being composed of nearly 96% carbon dioxide, and the thin atmosphere does not filter out ultraviolet sunlight. The relatively strong gravity and the presence of aerodynamic effects make it difficult to land heavy, crewed spacecraft with thrusters only, as was done with the Apollo Moon landings, yet the atmosphere is too thin for aerodynamic effects to be of much help in aerobraking and landing a large vehicle (cf. chapter 6.8.2). Landing piloted missions on Mars will require braking and landing systems different from anything used to land crewed spacecraft on the Moon or robotic missions on Mars. Communication with Earth is relatively straightforward when Earth is above the martian horizon. NASA and ESA included communications relay equipment in several of the Mars orbiters, so Mars already has communications satellites. While these will eventually wear out, additional orbiters with communications relay capability are likely to be launched before any colonization expeditions are mounted. As you may recall from chapter 1, the one-way light time for Mars ranges from about 4 minutes at closest approach to about 24 minutes at the largest possible superior conjunction. Real-time communication, such as telephone conversations or Internet chat, between Earth and Mars would be highly impractical due to the long time lags involved. But first and foremost is the concern that there has never been a crewed mission to Mars. As you know, a Hohmann trajectory to Mars takes several months (cf. 7.4.1), and launch opportunities arise only when the planets are in a proper alignment. NASA has considered a variety of possibilities. Each mission profile possesses unique characteristics such as flight trajectories, delta v requirements, travel times, and surface stay times.

The Short-Stay Mission, which is often referred to as an opposition-class mission, provides Mars stay times of 30 to 90 days with a round trip total time of 400 to 650 days. This mission class requires a large delta v, even after taking advantage of a Venus gravity assist. As you know from the rocket equation (chapter 5.6), an increase in delta v results in a large increase in propellant mass. Short-Stay Missions could be the stepping stone for human exploration of Mars. The Long-Stay Mission, which is usually referred to as a conjunction-class mission, provides Mars stay times up to about 600 days with a round trip total time of about 900 to 1000 days. The delta v is lower than for the Short-Stay Mission. This trajectory provides an opportunity to send a more massive spacecraft (i.e. more cargo) at the same cost as a smaller spacecraft following a higher delta v trajectory. Maximizing the payload of each launch vehicle minimizes the number of launches necessary to transport the required surface equipment. The disadvantage of this trajectory for a crewed Mars launch vehicle is the crew’s long exposure to the in-space environment. Opposition class trajectories also allow for a “fast transit” solution, in which the delta v is increased to reduce both the inbound and outbound journey to around 120 days; however, the total mission duration still remains long. Mission planners consistently adopt a splitmission design, in which the crew and the cargo are sent on separate vehicles. The cargo would be sent ahead of the crew on a low delta v, conjunction class trajectory. With the release of the motion picture “The Martian” in 2015 another trajectory that became widely publicized is ballistic capture. The ballistic capture trajectory, first proposed for a Mars mission in 2014, does not put the spacecraft on a path to meet Mars, but rather, on one that merely aims for Mars’ orbit. There, the spacecraft waits for Mars to approach, and for Mars’ gravity to capture it (cf. chapter 7). The ballistic capture trajectory would use less propellant than a Hohmann trajectory and thus save money; and it can work with larger launch windows. The

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ballistic capture maneuver was successfully executed in 1991 at the Moon with the Japanese “Hiten” probe.

10.4.4 Colonies In Other Solar System Locations There are several other bodies of the Solar System which are being considered as potential colonization targets. Lagrange point (cf. Figure 218) colonization is the colonization of the five equilibrium points in the orbit of a planet or its primary moon. The most obvious such points for colonization are those in the Earth–Moon and in the Sun–Earth systems. There is a suggestion that Mercury (cf. chapter 3.3.1) could be colonized using the same technology, approach and equipment that would be used in colonizing the Moon. Such colonies would almost certainly be restricted to the polar regions due to the extreme daytime temperatures elsewhere on the planet. Observations of Mercury’s polar regions by radar from Earth and the observations of the MESSENGER probe (chapter 8.4.3) have been consistent with water ice and/or other frozen volatiles being present in permanently shadowed areas of craters in Mercury’s polar regions. Measurements of Mercury’s exosphere, which is practically a vacuum, revealed more ions derived from water than scientists had expected. All of these observations are consistent with water ice and/or other volatiles being available to hypothetical future colonists of Mercury. Ceres (cf. chapter 3.4) is a dwarf planet in the asteroid belt, comprising about one third the mass of the whole belt and being the sixth largest body in the inner Solar System by mass and volume. Ceres has a surface area somewhat larger than Argentina. Being the largest body in the asteroid belt, Ceres could become the main base and transport hub for future asteroid mining infrastructure, allowing mineral resources to be transported further to Mars, the Moon and Earth. Most of the larger moons of the outer planets contain water ice, liquid water, and organic compounds that might be useful for sustaining hu-

Figure 10-12. An artist’s drawing of a rocket lifting off from one of Saturn’s moons (pixabay/R. Bertrams) man life. The Jovian system (cf. chapter 3.3.5) in general poses several disadvantages for colonizing because of its severe radiation environment and its particularly deep gravity well. The Artemis Project designed a plan to colonize Europa. Scientists are to inhabit igloos and drill down into the europan ice crust, exploring any subsurface ocean. It also discusses use of air pockets for human inhabitation. Due to its distance from Jupiter’s powerful radiation belt, Callisto is subject to less exposure. When NASA carried out a study called HOPE (Revolutionary Concepts for Human Outer Planet Exploration) regarding the future exploration of the Solar System, the target chosen was Callisto. It could be possible to build a surface base that would produce fuel for further exploration of the Solar System. On 9 March 2006, NASA’s Cassini space probe found possible evidence of liquid water on Enceladus, a moon of Saturn (cf. chapter 3.3.6). This means liquid water could be collected much more easily on Enceladus than on, for instance,

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Europa. Discovery of water, especially liquid water, generally improves a celestial bodyâ&#x20AC;&#x2122;s consideration for colonization dramatically. There would be many difficulties to overcome in the colonization of the outer Solar System. The outer planets are much farther from Earth than the inner planets, and would therefore be harder and more time-consuming to reach. In addition, return voyages may well be prohibitive considering the time and distance. Solar radiation is many times less concentrated in the outer Solar System than the inner Solar System. It is unclear as to whether solar power would be usable there, using some form of concentration mirrors, or if nuclear power would be necessary.

10.5 Interstellar colonization Colonization of the Solar System is quite feasible with present-day technology. The main obstacle is the cost. Colonizing another star system is quite a different story. The principle obstacle here is that with current propulsion technology, the travel times are extremely long. Interstellar colonization also depends on the existence of habitable exoplanets qualifying for colonization within a reachable distance. As you will recall from chapter 1, the distances to the stars are measured in lightyears. Since no material object can travel at the speed of light, transit times cannot in principle be at the highest of speeds, the speed of light. According to special relativity, an object that has nonzero rest mass cannot travel at the speed of light. As the object approaches the speed of light, the objectâ&#x20AC;&#x2122;s energy and momentum increase without bound. Thus our current understanding of the world through physics tells us that times to the nearest stars will never be less than many years, even if we garner the resources to travel at some significant percentage of the speed of light. In fact, reaching even the nearest star, Proxima Centauri, with current rockets, would take tens of thousands of years. Therefore, the time required for interstellar travel given current propulsion methods far exceeds a human lifespan.

Figure 10-13. Scientists using data from NASAâ&#x20AC;&#x2122;s Kepler mission (chapter 8) have in 2015 confirmed the first near-Earth-size planet orbiting in the habitable zone of a Sun-like star. The habitable zone is the region around a star where temperatures are just right for water to exist in its liquid form. The artistic concept compares Earth (left) to the new planet, called Kepler-452b, which is about 60 % larger. (NASA Ames/JPL-Caltech/ T. Pyle & W. Stenzel) The 100 Year Starship project aims at making interstellar travel possible sometime in the 2100s. It originated form a study, which seeded grant money to a private entity. Funded jointly by NASA and the US Defense Advanced Research Projects Agency, the goal was to create an organization that could last 100 years and help foster the research needed for interstellar travel. Announced in 2004, the endeavor was meant to excite several generations to commit to the research and development of breakthrough technologies to advance the eventual goal of interstellar space travel. American physician and former NASA astronaut Dr. Mae Jemison made the winning bid as leader of her own foundation, the Dorothy Jemison Foundation for Excellence. The Foundation was awarded a $500,000 grant for further work. The new organization maintained the name, 100 Year Starship. 100 Year Starship has hosted annual symposia where papers on a number of subjects related to interstellar flight are presented.

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defects or development abnormalities. Given the principal problem of long duraEmbryo space colonization is a theoretical tion transit times, here are some solutions that interstellar space colonization concept that inhave have been suggested. A generation ship, or volves sending a robotic mission to a habitable tergeneration starship, is a hypothetical type of interrestrial planet transporting frozen early-stage hustellar ark starship. Since a spaceship might take man embryos or the technological or biological thousands of years to reach even nearby stars, the means to create human embryos. The 2014 movie original occupants of a generation ship would “Interstellar” is a recent science fiction film which grow old and die, leaving their descendants to conuses this concept. In contrast to the sleeper ship tinue traveling. Such a ship would have to have exproposal, embryo space colonization does not retraordinarily reliable systems that could be mainquire the more technically challenging “freezing” tained by the ship’s inhabitants over long periods of fully developed humans. Upon arrival of the of time. Generation ships would also have to solve embryo-carrying spacecraft at the target planet, major biological, social, and moral problems and fully autonomous robots would build the first setwould also need to deal with complex matters of tlement on the planet and start growing food. self-worth and purpose for the various crews inThereafter the first embryos could be unfrozen. volved. As an example, a moral quandary exists The embryos would need to develop in artificial regarding how intermediate generations, those uteri until a large enough population existed to destined to be born and die in transit without actuprocreate by natural biological means. Intelligent ally seeing tangible results of their efforts, might machines would raise and teach the children. feel about their forced existence on such a ship. Whether it will be possible to develop fully Estimates of the minimum viable population vary, autonomous robots that can build the first settlebut are usually a few ten thousand. ment on the target planet and raise the first huA sleeper ship is a hypothetical type of mans, is unclear. Because the initial probe must spaceship in which most or all of the crew spend be maximally compact, the robots that build the the journey in some form of hibernation or sushabitat would themselves have to be built autonopended animation. This can also be useful to remously from local materials. Though such technolduce the consumption of life support system reogy does not yet exist, there are strong economic sources by crew members who are not needed durincentives to develop it, which are unrelated to ing the trip. Putting the astronauts in cryogenic space colonization. sleep, a favorite of science fiction writers, would probably involve whole body vitrification. In ordinary freezing, large ice crystals form which damage cells. Vitrification is cryogenic freezing with a substance that keeps the ice crystals small. Vitrification of people is not reversible with current technology. Tiny organisms (e.g. embryos up to eight cells) can be cryogenically preserved and revived. Pregnancies have been reported from embryos stored for 16 years. Many studies have evaluated the children born from frozen embryos, or “frosties.” The result has been uniformly positive with no increase of birth Figure 10-14. Illustration from a presentation about the Voyager spacecraft (NASA) 219

10 Test Your Understanding I. A NSWER IN A FEW SENTENCES . 1. What is meant by space colonization? 2. What are the main arguments in favor of space colonization? 3. What are some of the reasons why humans should colonize the Solar System? 4. What is the main reason why humans should colonize exoplanets? 5. What are the two types of space colony that are being considered? 6. In broad terms, what does a colony need to be independent of the Earth? 7. What is the difference between a space station and a space habitat? 8. How can artificial gravity be generated on a spacecraft or space habitat? Make a sketch of the spacecraft, include a person, and labels for the angular velocity of the station, linear velocity of the person due to inertia, centripetal acceleration of the person, and the normal force. 9. What was the purpose of the Biosphere 2 experiment? 10.Why is particle radiation a concern for space travel and space colonization? Are there differences between in-space habitats to those on the Moon or Mars? 11. What is the photovoltaic effect and how is it used in space? 12.What are some advantages to using solar arrays in space versus on Earth? 13.What other means, other than the photovoltaic effect, exists to gain energy from sunlight? 14.What are the salient differences between a conjunction and opposition class mission to Mars? 15.What is a ballistic capture trajectory? 16.What is meant by a split-mission design? 17.Why are certain moons of Jupiter and Saturn being considered as colonization targets? 18.What is the principle obstacle for interstellar colonization? 19.What is the 100 Year Starship project?

20.What are some current ideas of how interstellar colonization could be achieved? 21.What is vitrification? 22.What is meant by â&#x20AC;&#x153;frosties?â&#x20AC;? 23.Why is the development of artificial intelligence important for interstellar colonization? II. R ESEARCH THE A NSWER . 1. With what frequency have mass extinction events occurred on Earth? What are some of reasons for these events? Discuss at least three. 2. When the Sun begins its evolution into a red giant star, how might this affect the Earth? What might become of the planets in the inner Solar System? What will happen to the planets in the outer Solar System? 3. What are some of the advantages and disadvantages of colonizing the five Lagrange points of the Earth-Moon system? Include a sketch. Discuss differences in uses, station keeping and energy generation between different L points. 4. The Biosphere 2 was built to study an autonomous ecosystem that could serve as the analog of a space habitat. What are terrestrial analog sites and how have they been used? Has anyone explored a Mars analog habitat on Earth? 5. What is meant by the habitable zone? Where is the habitable zone in our Solar System? How does the habitable zone vary from star to star? 6. Research if there are other organizations, other than 100 Year Starship, that are dedicated to developing interstellar travel. Where are these organizations located, who funded them, and what is their stated purpose? 7. What is the Bussard ramjet? 8. How does an antimatter rocket work?

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CREDITS AND LICENSING

This section provides a list of text and image sources. Here, text and image sources are credited explicitly, via a listing of the hyperlink of the source. A briefer credit for each image was also included with each image caption. The most frequently used source is Wikipedia. Text on Wikimedia Commons is owned by the original writer and licensed under the Creative Commons Attribution-ShareAlike 3.0 license and the GNU Free Documentation License. Images and other media on Wikimedia Commons are almost all under some kind of free license (usually CC-BY, CC-BY-SA, or GFDL; see Commons:Licensing) or in the public domain. Each media file has its licensing specified on its file description page. Most Wikipedia text content that has been used for this book has been edited for consistency. For more information about crediting and licensing Wikipedia content, please refer to https://commons.wikimedia.org/wiki/Commons :Reusing_content_outside_Wikimedia and https://commons.wikimedia.org/wiki/Commons :Reusing_content_outside_Wikimedia/licenses. The second most frequently used source is NASA websites. Text and images on NASA websites are not copyrighted. For more information, please refer to http://www.nasa.gov/multimedia/guidelines/ind ex.html. In addition, some material comes from other websites, and is usually in the public domain or not copyrighted. When reusing the material published in this book, please, be aware of the original credits and attached licenses.

Front and Back Matter Image on the front cover: Photo by Dr. Donald Roy Pettit, NASA JSC, Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0) https://www.flickr.com/photos/nasa_jsc_photo/7197236836/sizes/ o/ Copyleft symbol on page i: "Copyleft" by Zscout370, Sertion, e.a. Own work. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Copyleft.svg Image on page v: "Ap4-s67-50531" by NASA - High-resolution image from Apollo Image Archive. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Ap4-s67-50531.jpg

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http://en.wikipedia.org/wiki/Impact_event

Chapter 1

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http://en.wikipedia.org/wiki/Space_and_survival

http://en.wikipedia.org/wiki/Portal:Spaceflight

http://history.msfc.nasa.gov/rocketry/index.html

http://en.wikipedia.org/wiki/Spaceflight

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http://en.wikipedia.org/wiki/Interstellar_travel

http://en.wikipedia.org/wiki/Space_Race

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http://en.wikipedia.org/wiki/Sergei_Korolev

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Images

http://en.wikipedia.org/wiki/Apollo_11 http://www.nasa.gov/mission_pages/apollo/missions/apollo11.html

Chapter Cover NASA - S116-E-05983 (12 Dec. 2006) --- Backdropped by a colorful Earth, including land mass that covers parts of New Zealand, astronaut Robert L. Curbeam Jr. (left) and European Space Agency (ESA) astronaut Christer Fuglesang, both STS-116 mission specialists, participate in the mission's first of three planned sessions of extravehicular activity (EVA) as construction resumes on the International Space Station. http://spaceflight.nasa.gov/gallery/images/shuttle/sts-116/html/s11 6e05983.html

http://en.wikipedia.org/wiki/Neil_Armstrong http://voyager.jpl.nasa.gov/ http://en.wikipedia.org/wiki/Spacefaring http://en.wikipedia.org/wiki/Dennis_Tito http://en.wikipedia.org/wiki/Orbiting_Astronomical_Observatory

Figure 1-1 http://www.apolloarchive.com/apg_thumbnail-test.php?ptr =497&imageID=KSC-69P-632

http://en.wikipedia.org/wiki/Space_exploration http://en.wikipedia.org/wiki/Commercialization_of_space

Figure 1-2 Bundesarchiv, RH8II Bild-B2055-44 / CC-BY-SA [CC-BY-SA-3.0-de via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Bundesarchiv_R H8II_Bild-B2055-44,_Peenem%C3%BCnde,_Raketenrampe_mit_V 2.jpg

http://www.nasa.gov/topics/technology/features/telstar.html http://en.wikipedia.org/wiki/Space_warfare http://en.wikipedia.org/wiki/Intercontinental_ballistic_missile

Figure 1- 3 http://commons.wikimedia.org/wiki/File:V-2_rocket_diagram_(wit h_English_labels).svg

http://en.wikipedia.org/wiki/Strategic_Defense_Initiative http://en.wikipedia.org/wiki/Overview_effect

Figure 1-4 http://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1957-001B

http://www.overviewinstitute.org/

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Figure 1-5 http://grin.hq.nasa.gov/ABSTRACTS/GPN-2000-001004.html

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Figure 1-6 "Gagarin in Sweden" by Arkiv: Sydsvenskan sydsvenskan.se. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Gagarin_in_Sweden.jpg

https://en.wikipedia.org/wiki/Electromagnetic_radiation https://en.wikipedia.org/wiki/Electromagnetic_spectrum https://en.wikipedia.org/wiki/Black-body_radiation

Figure 1-7 "RIAN archive 612748 Valentina Tereshkova" by Alexander Mokletsov / Александр Моклецов - RIA Novosti archive, image #612748. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:RIAN_archive_612748_V alentina_Tereshkova.jpg

https://en.wikipedia.org/wiki/Redshift https://en.wikipedia.org/wiki/Stellar_parallax http://map.gsfc.nasa.gov/cosmology/cosmology.html http://en.wikipedia.org/wiki/Cosmic_microwave_background

Figure 1-8 "Ap11-s69-31740". Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Ap11-s69-31740.jpg

http://en.wikipedia.org/wiki/Age_of_the_universe http://en.wikipedia.org/wiki/Observable_universe

Figure 1-9 http://www.hq.nasa.gov/alsj/a11/AS11-40-5886.jpg http://map.gsfc.nasa.gov/ Figure 1-10- http://hubblesite.org/gallery/spacecraft/05/ http://en.wikipedia.org/wiki/Dark_matter Figure 1-11 "Telstar". Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Telstar.jpg

http://en.wikipedia.org/wiki/Earth

Figure 1-12 "Peacekeeper missile" by United States Air Force - Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Peacekeeper_missile.jpg

http://en.wikipedia.org/wiki/Universe http://en.wikipedia.org/wiki/Abundance_of_the_chemical_element s

Figure 1-13 http://www.hq.nasa.gov/office/pao/History/alsj/a410/AS8-14-2383 HR.jpg

http://en.wikipedia.org/wiki/Composition_of_the_human_body http://en.wikipedia.org/wiki/Outer_space

Figure 1-14 http://history.msfc.nasa.gov/rocketry/03.html http://en.wikipedia.org/wiki/Moon Figure 1-15 Public domain: http://www.wpclipart.com/space/rocketry/rocket_action_r eaction.png.html

http://en.wikipedia.org/wiki/Lagrangian_point http://en.wikipedia.org/wiki/Lagrange_point_colonization

Figure 1-16 http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA17793

http://voyager.jpl.nasa.gov/ http://en.wikipedia.org/wiki/Heliosphere

Figure 1-17 Edited NASA image http://commons.wikimedia.org/wiki/File:PaleBlueDot.jpg

http://en.wikipedia.org/wiki/Oort_cloud Figure 1-18 http://photojournal.jpl.nasa.gov/catalog/PIA00451 http://en.wikipedia.org/wiki/Alpha_Centauri Figure 1-19 NASA/Paolo Nespoli http://spaceflight.nasa.gov/gallery/images/station/crew-27 /html/iss027e036759.html

http://en.wikipedia.org/wiki/Spiral_galaxy http://en.wikipedia.org/wiki/Irregular_galaxy http://en.wikipedia.org/wiki/Elliptical_galaxy

Chapter 2

http://en.wikipedia.org/wiki/Virgo_Cluster

Text

http://en.wikipedia.org/wiki/Void_%28astronomy%29

http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line

http://en.wikipedia.org/wiki/Laniakea_Supercluster

http://en.wikipedia.org/wiki/Theodore_von_K%C3%A1rm%C3%A1 n http://en.wikipedia.org/wiki/Space_law

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Chapter Cover NASA astronaut photograph ISS028-E-020072 http://visibleearth.nasa.gov/view.php?id=765342

Figure 2-14 "Van Allen radiation belt" by Original uploader was Booyabazooka at en.wikipedia - Originally from en.wikipedia; description page is/was here. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Van_Allen_radiation_belt. svg

Figure 2-1 "Atmosphere layers-en" by NOAA & User:Mysid - Vectorized by Mysid on a NOAA picture with the Kármán line added by Latitude0116. Image renamed from Image:Atmosphere layers.svg. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Atmosphere_layers-en.svg

Figure 2-15 Polarlicht 2" by United States Air Force photo by Senior Airman Joshua Strang. This Image was released by the United States Air Force with the ID 050118-F-3488S-003. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Polarlicht_2.jpg

Figure 2-2 NASA GRIN archive, JPL photohttp://grin.hq.nasa.gov/ABSTRACTS/GPN-2000-001500.html

Figure 2-16 "Aurora-SpaceShuttle-EO" by NASA (Crew of STS-39) Original uploader was Seth Ilys at en.wikipedia. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Aurora-SpaceShuttle-EO.j pg

Images

Figure 2-3 Adapted from "Hubble constant" by Brews ohare - Own work. Licensed under CC BY-SA 3.0 via Commons https://commons.wikimedia.org/wiki/File:Hubble_constant.JPG

Figure 2-17 "Earth-Moon" by Earth-image from NASA; arrangement by brews_ohare. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Earth-Moon.PNG

Figure 2-4 "EM Spectrum Properties edit" by Inductiveload, NASA self-made, information by NASABased off of File:EM_Spectrum3-new.jpg by NASAThe butterfly icon is from the P icon set, P biology.svgThe humans are from the Pioneer plaque, Human.svgThe buildings are the Petronas towers and the Empire State Buildings, both from Skyscrapercompare.svg. Licensed under CC BY-SA 3.0 via Commons https://commons.wikimedia.org/wiki/File:EM_Spectrum_Propertie s

Figure 2-18 “Lagrange 2 mass” by NASA - NASAApollo 15 Solo Orbital Operations. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Lagrange_2_mass.gif Figure 2-19 NASA. Adapted from http://spaceplace.nasa.gov/review/solar-tricktionary/heliosphere.en. jpg

Figure 2-5 Adapted from NASA graphic at https://www.spacetelescope.org/images/opo1308e/

Figure 2-20 "PIA17046 - Voyager 1 Goes Interstellar" by NASA / JPLCaltech - Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:PIA17046_-_Voyager_1_ Goes_Interstellar.jpg

Figure 2-6 http://www.eso.org/public/images/ann11014a/ Figure 2-7 "Universe expansion2" by Gnixon at English WikipediaLater version(s) were uploaded by Papa November at en.wikipedia.(Original text: en:User:Gnixon) - Created by uploader from public domain source. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Universe_expansion2.pn g

Figure 2-21 "Milky Way Night Sky Black Rock Desert Nevada" by Steve Jurvetson - Flickr. Licensed under Creative Commons Attribution 2.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Milky_Way_Night_Sky_B lack_Rock_Desert_Nevada.jpg

Figure 2-8 Adapted from http://map.gsfc.nasa.gov/media/ContentMedia/990004b.jpg

Figure 2-22 http://www.eso.org/public/images/eso1118a/

Figure 2-9 NASA/WMAP Science Team http://commons.wikimedia.org/wiki/File%3ACMB_Timeline75.jpg

Figure 2-23 Magellanic Clouds ― Irregular Dwarf Galaxies" by ESO/ S. Brunier - ESO. Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Magellanic_Clouds_%E2% 80%95_Irregular_Dwarf_Galaxies.jpg

Figure 2-10 http://map.gsfc.nasa.gov/media/121236/index.html Figure 2-11 "Observable universe logarithmic illustration" by Unmismoobjetivo - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Observable_universe_loga rithmic_illustration.png

Figure 2-24 "Messier 089 Hubble WikiSky" by NASA, STScI, WikiSky - Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Messier_089_Hubble_Wi kiSky.jpg

Figure 2-12 Earth’s magnetic field, public domain http://www.sciencelearn.org.nz/Contexts/Dating-the-Past/Sci-Medi a/Images/Earth-s-magnetic-field

Figure 2-25 NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA http://hubblesite.org/gallery/album/galaxy/cluster/pr2003 001a/

Figure 2-13 Earth’s magnetosphere. NASA/Goddard/Aaron Kaase http://www.nasa.gov/mission_pages/sunearth/multimedia/magneto sphere.html

Figure 2-26 "Large-scale structure of light distribution in the universe" by Andrew Pontzen and Fabio Governato - Licensed under

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Creative Commons Attribution 2.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Large-scale_structure_of_l ight_distribution_in_the_universe.jpg

Images Chapter Cover A full-disk multiwavelength extreme ultraviolet image of the sun taken by SDO on March 30, 2010. False colors trace different gas temperatures. Reds are relatively cool (about 60,000 Kelvin, or 107,540 F); blues and greens are hotter (greater than 1 million Kelvin, or 1,799,540 F). Credit: NASA/Goddard/SDO AIA Team/Goddard/SDO AIA http://www.nasa.gov/mission_pages/sdo/news/first-light.html

Figure 2-27 NASA and The Hubble Heritage Team (STScI/Aura) http://hubblesite.org/newscenter/archive/releases/2001/10/

Chapter 3

Figure 3-1 "Planets2008" by Edits by Farry. Source image on English Wikipedia credited by original uploader to "Martin Kornmesser", and later an anonymous edit re-credited it to "zaria mayers" . - Edit of en:Image:Planets2006.jpg. Original was at: http://sse.jpl.nasa.gov/planets/index.cfm. Licensed under Public domain via Wikimedia Commons -http://commons.wikimedia.org/wiki/File:Planets2008.jpg

Text http://en.wikibooks.org/wiki/General_Astronomy/The_Solar_Syste m http://en.wikipedia.org/wiki/Solar_System http://en.wikipedia.org/wiki/Planet

Figure 3-2 "Solarsys" by Rursus - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Solarsys.svg

http://en.wikipedia.org/wiki/Dwarf_planet http://en.wikipedia.org/wiki/Small_Solar_System_body

Figure 3-3 "Sun poster" by Kelvinsong - Own work. Licensed under CC BY-SA 3.0 via Commons https://commons.wikimedia.org/wiki/File:Sun_poster.svg

http://en.wikipedia.org/wiki/Sun http://en.wikipedia.org/wiki/Watt

Figure 3-4 Adapted from Wikipedia. "Solar spectrum en" by Nick84 Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons -http://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg

http://en.wikipedia.org/wiki/Sunlight http://en.wikipedia.org/wiki/Solar_activity

Figure 3-5 Adapted by http://www.apnphotographyschool.com/lighting/take-control-of-pho tographic-lighting-with-lighting-ratios-the-inverse-square-law/ from Wikipedia"Inverse square law" by Borb - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0-2.5-2.0-1.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Inverse_square_law.svg

http://en.wikipedia.org/wiki/Solar_cycle http://en.wikipedia.org/wiki/Solar_wind http://en.wikipedia.org/wiki/Stellar_nucleosynthesis http://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_react ion http://en.wikipedia.org/wiki/Nuclear_fusion

Figure 3-6 NASA/SDO http://www.nasa.gov/content/goddard/giant-january-sunspots/#.VG ZNAkhM7u1http://www.nasa.gov/sites/default/files/latest_4096_hmii_0.jpg

http://en.wikipedia.org/wiki/Thermonuclear_fusion Figure 3-7 "Sunspot Numbers". Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Sunspot_Numbers.png

http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=169 https://solarsystem.nasa.gov/planets/index.cfm

Figure 3-8 "FusionintheSun" by Borb - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0-2.5-2.0-1.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:FusionintheSun.svg

http://www.eso.org/public//outreach/eduoff/vt-2004/mt-2003/mtmercury-orbit.html http://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motio n

Figure 3-9 NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington https://solarsystem.nasa.gov/multimedia/display.cfm?Category=Plan ets&IM_ID=7543

http://en.wikipedia.org/wiki/Conic_section http://en.wikipedia.org/wiki/Nebular_hypothesis http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Sola r_System

Figure 3-10 Adapted from the NSSDC Photo Gallery https://solarsystem.nasa.gov/multimedia/display.cfm?Category=Plan ets&IM_ID=112 Figure 3-11 https://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=9643

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Figure 3-12 NASA/JPL-Caltech/MSSS https://solarsystem.nasa.gov/multimedia/display.cfm?Category=Plan ets&IM_ID=15823

Figure 3-27 Adapted from NASA/Space Place http://spaceplace.nasa.gov/solar-system-formation/en/ Figure 3-28 "Solar Life Cycle" by Oliverbeatson - Own work. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Solar_Life_Cycle.svg

Figure 3-13 NASA/JPL/University of Arizona https://solarsystem.nasa.gov//multimedia/display.cfm?IM_ID=9523 Figure 3-14 NASA/JPL https://solarsystem.nasa.gov/multimedia/display.cfm?Category=Plan ets&IM_ID=18286

Figure 3-29 "Sun red giant" by Oona Räisänen (User:Mysid), User:Mrsanitazier. - Vectorized in Inkscape by Mysid on a JPEG by Mrsanitazier (en:Image:Sun Red Giant2.jpg).. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Sun_red_giant.svg

Figure 3-15 https://solarsystem.nasa.gov/multimedia/display.cfm?Category=Plan ets&IM_ID=13583

Figure 3-30 NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO) http://www.nasa.gov/multimedia/imagegallery/image_feature_77.ht ml

Figure 3-16 https://solarsystem.nasa.gov/multimedia/display.cfm?Category=Plan ets&IM_ID=15663 Figure 3-17 http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Plane ts&IM_ID=11943

Chapter 4

Figure 3-18 NASA/JPL-Caltech/UCLA/MPS/DLR/IDA http://www.jpl.nasa.gov/spaceimages/details.php?id=pia14317

Text

Figure 3-19 "Comet-Hale-Bopp-29-03-1997 hires adj" by Philipp Salzgeber - Philipp Salzgeber's website. Licensed under Creative Commons Attribution-Share Alike 2.0-at via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Comet-Hale-Bopp-29-03-1 997_hires_adj.jpg

http://en.wikipedia.org/wiki/Guidance,_navigation_and_control

Figure 3-20 ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/ IAA/SSO/INTA/UPM/DASP/IDA https://www.flickr.com/photos/europeanspaceagency/14862775903/ in/set-72157638315605535/

http://en.wikipedia.org/wiki/Dead_reckoning

http://en.wikipedia.org/wiki/Navigation http://en.wikipedia.org/wiki/Pilotage

http://en.wikipedia.org/wiki/Spacetime http://en.wikipedia.org/wiki/Celestial_navigation

Figure 3-21 http://www.eso.org/public//outreach/eduoff/vt-2004/mt-2003/mtmercury-configurations.jpg

http://www.grc.nasa.gov/WWW/k-12/airplane/coords.html http://www2.jpl.nasa.gov/basics/bsf2-1.php

Figure 3-22 "Conic Sections" by http://commons.wikimedia.org/wiki/User:Magister_Mathematicae http://commons.wikimedia.org/wiki/File:Secciones_c%C3%B3nicas.s vg. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Conic_Sections.svg

http://en.wikipedia.org/wiki/Pittsburgh https://www.grc.nasa.gov/www/k-12/TRC/glossary.htm http://en.wikipedia.org/wiki/Horizontal_coordinate_system

Figure 3-23 "OrbitalEccentricityDemo" by ScottAlanHill - ScottAlanHill 600×480 (24,403 bytes) (Examples of orbital trajectories with various eccentricities. Created by submitter.) English Wikipedia. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:OrbitalEccentricityDemo.sv g

http://en.wikipedia.org/wiki/Ecliptic

Figure 3-24 NASA/STScI http://amazing-space.stsci.edu/news/archive/2006/01/ill-01.php

http://en.wikipedia.org/wiki/Ecliptic_coordinate_system

http://en.wikipedia.org/wiki/Equatorial_coordinate_system http://en.wikipedia.org/wiki/Attitude_control http://en.wikipedia.org/wiki/Ecliptic

http://en.wikipedia.org/wiki/International_Atomic_Time

Figure 3-25 NASA/JPL-Caltech/STScI http://www.nasa.gov/multimedia/imagegallery/image_feature_693. html

http://www.nist.gov/pml/div688/utcnist.cfm

Figure 3-26 ALMA (ESO/NAOJ/NRAO) http://www.eso.org/public/images/eso1436a/

http://en.wikipedia.org/wiki/Gregorian_calendar

http://time.gov/widget/

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http://en.wikipedia.org/wiki/0_%28year%29

http://en.wikipedia.org/wiki/Solar_time

Figure 4-6 "Mercator projection SW" by Strebe - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Mercator_projection_SW.j pg

http://en.wikipedia.org/wiki/Lunar_phase

Figure 4-7 NASA/JPL http://www2.jpl.nasa.gov/basics/bsf2-1.php

http://en.wikipedia.org/wiki/Tide

Figure 4-8 http://history.nasa.gov/ap15fj/pics/lvlh.gif

http://en.wikipedia.org/wiki/Tidal_force

Figure 4-9 JAXA http://global.jaxa.jp/countdown/f13/schedule/img/sequence_e.gif

http://en.wikipedia.org/wiki/Day

http://en.wikipedia.org/wiki/Tidal_acceleration Figure 4-10 "Azimuth-Altitude schematic" by TWCarlson http://commons.wikimedia.org/wiki/File:Azimut_altitude.svg. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Azimuth-Altitude_schemati c.svg

http://en.wikipedia.org/wiki/Earth%27s_orbit http://en.wikipedia.org/wiki/Equinox http://en.wikipedia.org/wiki/Solstice

Figure 4-11 "Ursa Major - Ursa Minor - Polaris" by BonÄ? - based on file:Ursa Major and Ursa Minor Constellations.jpg. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Ursa_Major_-_Ursa_Mino r_-_Polaris.jpg

http://en.wikipedia.org/wiki/Spacecraft_Event_Time http://www2.jpl.nasa.gov/basics/bsf13-1.php http://www.gps.gov/systems/gps/ http://en.wikipedia.org/wiki/Trilateration

Figure 4-12 "Meridian on celestial sphere" by Tfr000 (talk) 16:56, 18 June 2012 (UTC) - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Meridian_on_celestial_sph ere.png

http://history.nasa.gov/ap08fj/03day1_green_sep.htm

Images

Figure 4-13 "Diurnal motion of polaris and northern starts over half a day" by W.s.w.p. - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Diurnal_motion_of_polaris _and_northern_starts_over_half_a_day.jpg

Chapter Cover http://history.nasa.gov/ap08fj/03day1_green_sep.htm Figure 4-1 http://history.nasa.gov/diagrams/apollo.html

Figure 4-14 http://spotthestation.nasa.gov/images/astro_horizon.jpg Figure 4-2 "US Navy 101207-N-2055M-272 Senior Chief Quarter Master Jonathan Myers teaches Command Master Chief April Beldo how to use a marine sextant during a" by U.S. Navy photo by Mass Communication Specialist 3rd Class Travis K. Mendoza - This Image was released by the United States Navy with the ID 101207-N-2055M-272 (next).This tag does not indicate the copyright status of the attached work. A normal copyright tag is still required. See Commons:Licensing for more information Via Wikimedia Commons http://commons.wikimedia.org/wiki/File:US_Navy_101207-N-2055 M-272_Senior_Chief_Quarter_Master_Jonathan_Myers_teaches_C ommand_Master_Chief_April_Beldo_how_to_use_a_marine_sexta nt_during_a.jpg Figure 4-3 http://www.grc.nasa.gov/WWW/k-12/airplane/coords.html Figure 4-4 http://commons.wikimedia.org/wiki/File:Latitude_and_longitude_g raticule_on_a_sphere.svg Figure 4-5 "Mollweide projection SW" by Strebe - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Mollweide_projection_SW. jpg

Figure 4-15 "Ra and dec rectangular" by Tfr000 (talk) 19:24, 23 April 2012 (UTC) - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Ra_and_dec_rectangular.p ng Figure 4-16 "Ecliptic path" by TauĘťolunga - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Ecliptic_path.jpg Figure 4-17 "Ra and dec on celestial sphere" by Tfr000 (talk) 15:34, 15 June 2012 (UTC) - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Ra_and_dec_on_celestial_ sphere.png Figure 4-18 http://www.nasa.gov/multimedia/imagegallery/image_feature_445.h tml# Figure 4-19 http://history.nasa.gov/ap15fj/pics/starmap1.jpg Figure 4-20 http://nmp.jpl.nasa.gov/st6/TECHNOLOGY/star_camera.html

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Figure 4-21 "International date line" by Another Matt (talk) - I (Another Matt (talk)) created this work entirely by myself, incorporating a map from http://commons.wikimedia.org/wiki/File:Blankmap-ao-090N-north_ pole.png , which is stated to be public domain. Licensed under CC0 via Wikipedia http://en.wikipedia.org/wiki/File:International_date_line.png

http://en.wikipedia.org/wiki/Gyroscope http://en.wikipedia.org/wiki/Drag_%28physics%29 http://en.wikipedia.org/wiki/Energy http://en.wikipedia.org/wiki/Kinetic_energy

Figure 4-22 "AxialTiltObliquity" by I, Dennis Nilsson. Licensed under CC BY 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:AxialTiltObliquity.png

http://quest.arc.nasa.gov/space/teachers/suited/6work.html http://en.wikipedia.org/wiki/Lift_%28force%29

Figure 4-23 "Lunar-Phase-Diagram" by Original uploader was Minesweeper at en.wikipedia. Later version(s) were uploaded by Philip mc laughlin at en.wikipedia. - Transferred from en.wikipedia; transferred to Commons by User:Andrei Stroe using CommonsHelper.. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Lunar-Phase-Diagram.png

http://en.wikipedia.org/wiki/Gravitation http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravita tion http://en.wikipedia.org/wiki/Weight

Figure 4-24 NOAA http://oceanservice.noaa.gov/education/yos/resource/JetStream/oce an/bulge_max.htm

http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html http://en.wikipedia.org/wiki/Free_fall

Figure 4-25 "Apollo 11 Lunar Laser Ranging Experiment" by NASA NASA Apollo Archive http://www.hq.nasa.gov/office/pao/History/alsj/a11/AS11-40-5952.j pg. Licensed under Public Domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Apollo_11_Lunar_Laser_R anging_Experiment.jpg

http://en.wikipedia.org/wiki/Potential_energy http://en.wikipedia.org/wiki/Chemical_energy http://en.wikipedia.org/wiki/Hill_sphere

Figure 4-26 http://www.ncdc.noaa.gov/sites/default/files/styles/431px_width/pu blic/NASA-seasonalvariations.jpg?itok=YwxYPMIY

http://en.wikipedia.org/wiki/Sphere_of_influence_%28astrodynami cs%29 http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

Figure 4-27 Top: "Earth-lighting-equinox EN". Licensed under CC BYSA 2.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Earth-lighting-equinox_EN .png Bottom:"Earth-lighting-summer-solstice EN". Licensed under CC BY-SA 2.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Earth-lighting-summer-sols tice_EN.png

http://en.wikipedia.org/wiki/Konstantin_Tsiolkovsky http://en.wikipedia.org/wiki/Spacecraft_propulsion

Images

Figure 4-28 http://www.gps.gov/multimedia/poster/ Figure 4-29 Public domain clipart by tmjbeary https://openclipart.org/detail/191659/gps-satellites-trilateration

Chapter Cover NASA/jim Campbell, Aero-News Network http://mediaarchive.ksc.nasa.gov/detail.cfm?mediaid=31873

Figure 4-30 http://history.nasa.gov/ap08fj/pics/p23.gif

Figure 5-1 Adapted from NASA http://exploration.grc.nasa.gov/education/rocket/Images/rktfor.gif

Chapter 5

Figure 5-2 "3D Gyroscope". Licensed under Public Domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:3D_Gyroscope.png

Text http://en.wikipedia.org/wiki/Stephen_Hawking http://exploration.grc.nasa.gov/education/rocket/TRCRocket/rocket _principles.html http://en.wikipedia.org/wiki/Force http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

Figure 5-3 http://wiki.ssm-fans.info/rcs Diagram taken with permission from the SSM2007 Commander's Reference Manual Figure 5-4 "DBP 1993 1646 Isaac Newton" by Deutsche Bundespost scanned by NobbiP. Licensed under Public Domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:DBP_1993_1646_Isaac_Ne wton.jpg Wikipedia comments indicate this file may not be in the public domain.

http://en.wikipedia.org/wiki/Accelerometer

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Figure 5-5 "Frank De Winne on treadmill cropped" by NASA http://spaceflight1.nasa.gov/gallery/images/station/crew-21/html/iss 021e028204.html. Licensed under Public Domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Frank_De_Winne_on_trea dmill_cropped.jpg Figure 5-6 “Skaters showing newtons third law". Licensed under Public Domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Skaters_showing_newtons _third_law.png Figure 5-7 Adapted from NASA http://www.grc.nasa.gov/WWW/K-12/airplane/newton3.html

http://en.wikipedia.org/wiki/Gravity_drag http://en.wikipedia.org/wiki/Gravity_turn http://exploration.grc.nasa.gov/education/rocket/TRCRocket/practic al_rocketry.html http://en.wikipedia.org/wiki/Combustion http://www.grc.nasa.gov/WWW/k-12/airplane/combst1.html http://en.wikipedia.org/wiki/Robert_H._Goddard https://www.nasa.gov/returntoflight/system/system_STS.html

Figure 5-8 "Orbital Equations Figure 1". Licensed under Public Domain via Wikibooks http://en.wikibooks.org/wiki/File:Orbital_Equations_Figure_1.png

http://en.wikipedia.org/wiki/Specific_impulse

Figure 5-9 Adapted from NASA http://exploration.grc.nasa.gov/education/rocket/ffall.html

http://en.wikipedia.org/wiki/Liquid_rocket_propellant

http://en.wikipedia.org/wiki/Solid-fuel_rocket

http://en.wikipedia.org/wiki/Liquid_hydrogen Figure 5-10 "Tsiolkovsky" by Unknown http://www.nmspacemuseum.org/halloffame/images.php?image_id= 27. Licensed under Public Domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Tsiolkovsky.jpg Figure 5-11 "Var mass system" by Skorkmaz at en.wikipedia. Later version(s) were uploaded by PAR at en.wikipedia. - Transferred from en.wikipedia; transferred to Commons by User:Logan using CommonsHelper.. Licensed under โดเมนสาธารณะ via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Var_mass_system.PNG Figure 5-12 "Tsiolkovsky rocket equation" by Krishnavedala - Own work. Licensed under CC0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Tsiolkovsky_rocket_equati on.svg

Chapter 6

http://en.wikipedia.org/wiki/Liquid_oxygen http://www.nasa.gov/topics/technology/hydrogen/hydrogen_fuel_of _choice.html http://en.wikipedia.org/wiki/Propellant_mass_fraction http://www-pao.ksc.nasa.gov/kscpao/nasafact/pdf/ssp.pdf http://en.wikipedia.org/wiki/Single-stage-to-orbit http://en.wikipedia.org/wiki/Multistage_rocket http://en.wikipedia.org/wiki/Saturn_V http://history.nasa.gov/SP-4029/Apollo_18-19_Ground_Ignition_W eights.htm http://en.wikipedia.org/wiki/Air_launch_to_orbit

Text

http://en.wikipedia.org/wiki/Pegasus_%28rocket%29

http://en.wikipedia.org/wiki/Exploration_Flight_Test_1

http://en.wikipedia.org/wiki/Lander_%28spacecraft%29

http://en.wikipedia.org/wiki/Escape_velocity http://en.wikipedia.org/wiki/Potential_energy

https://www.faa.gov/other_visit/aviation_industry/designees_delega tions/designee_types/ame/media/Section%20III.4.1.7%20Returning %20from%20Space.pdf

http://en.wikipedia.org/wiki/Circular_motion http://en.wikipedia.org/wiki/Aerobraking http://en.wikipedia.org/wiki/Low_Earth_orbit http://en.wikipedia.org/wiki/Escape_velocity

http://en.wikipedia.org/wiki/Atmospheric_entry

http://en.wikipedia.org/wiki/Flight_airspeed_record

http://www.hq.nasa.gov/pao/History/SP-4209/ch3-4.htm

http://en.wikipedia.org/wiki/Jules_Verne

http://www.orbiterwiki.org/wiki/Reentry

http://en.wikipedia.org/wiki/Space_gun

http://webcache.googleusercontent.com/search?q=cache:RaGaST4w Q5gJ:www.researchgate.net/publictopics.PublicPostFileLoader.html% 3Fid%3D5412cd62d11b8be1268b45ee%26key%3D3e951a19-48ba-468 6-8bd7-8e2cf00c8847+&cd=11&hl=en&ct=clnk&gl=us

http://en.wikipedia.org/wiki/G-force http://quest.nasa.gov/saturn/qa/new/Effects_of_speed_and_acceler ation_on_the_body.txt

http://www.grc.nasa.gov/WWW/BGH/hihyper.html

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Figure 6-17 Adapted from NASA http://www.nasa.gov/vision/space/features/saturn_V.html

http://en.wikipedia.org/wiki/Ablation http://en.wikipedia.org/wiki/Communications_blackout

Figure 6-18 Adapted from NASA http://spaceflight.nasa.gov/gallery/images/apollo/apollo11/html/s69 _39958.html

Images

Figure 6-19 http://dayton.hq.nasa.gov/IMAGES/LARGE/GPN-2003-00045.jpg

Chapter Cover NASA http://www.nasa.gov/sites/default/files/thumbnails/image/15766576 390_99849c8ca3_o.jpg

Figure 6-20 Adapted from NASA http://mediaarchive.ksc.nasa.gov/detail.cfm?mediaid=28352

Figure 6-1, 6-2 Adapted from NASA http://www.nasa.gov/sites/default/files/full_eft1_infographic.jpg

Figure 6-21 http://www.nasaspaceflight.com/wp-content/uploads/2014/07/2014 -07-20-11_12_25-CSM-toward-LM-Apollo-Google-Search-350x238.jp g

Figure 6-3 Deutsches Zentrum fĂźr Luft- und Raumfahrt (DLR). The website is published under the CC BY 3.0 German license. This particular figure is licensed to GDFL. http://www.dlr.de/en/Portaldata/1/Resources/portal_news/newsarc hiv2009_1/2_newton_ariane.jpg Figure 6-4 http://commons.wikimedia.org/wiki/File:FETMlaunch.jpg

Figure 6-7 "StappSled". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:StappSled.jpg Figure 6-8 NASA/MSFC https://mix.msfc.nasa.gov/abstracts.php?p=2775 Figure 6-9 http://exploration.grc.nasa.gov/education/rocket/TRCRocket/IMAG ES/gimbaled.gif

http://www.nasa.gov/sites/default/files/images/269792main_GPN-2 000-001210_full.jpg Figure 6-22 Adapted from NASA https://www.hq.nasa.gov/alsj/S66-05097.jpg

Figure 6-5 http://commons.wikimedia.org/wiki/File:Uniform_circular_motion.s vg Figure 6-6 http://sites.wff.nasa.gov/code810/images/edu_newton_1stlawapp.gif

http://www.nasa.gov/centers/langley/images/content/69664main_A pollo-fig6.gif

Figure 6-23 "Skip reentry trajectory" by Clem Tillier; Earth graphic based on NASA image of Earth seen from Apollo 17. - Own work of Clem Tillier. Licensed under CC BY-SA 2.5 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Skip_reentry_trajectory.sv g

Figure 6-24 http://www.jpl.nasa.gov/images/msl/20120809/edl20120809-full.jp g Figure 6-25 http://microgravity.grc.nasa.gov/Orion/images/OrioReturns2.jpg

Figure 6-10 http://wright.nasa.gov/airplane/Images/combst1.gif

Figure 6-26 â&#x20AC;&#x153;STS-3 landing". Licensed under Public Domain via Wikimedia Commons https://commons.wikimedia.org/wiki/File:STS-3_landing.jpg

Figure 6-11 Adapted from NASA http://exploration.grc.nasa.gov/education/rocket/TRCRocket/IMAG ES/solidrocket.gif

Figure 6-27 US Navy via NASA http://www.nasa.gov/sites/default/files/20141205-awg0001_0228.jp g

Figure 6-12 Adapted from NASA http://exploration.grc.nasa.gov/education/rocket/TRCRocket/IMAG ES/liquidrocket.gif

Chapter 7

Figure 6-13 NASA/Wikipedia http://en.wikipedia.org/wiki/Robert_H._Goddard

Text

Figure 6-14 https://www.nasa.gov/returntoflight/system/system_STS.html

http://history.nasa.gov/conghand/traject.htm http://en.wikipedia.org/wiki/Trajectory

Figure 6-15 Adapted from NASA http://exploration.grc.nasa.gov/education/rocket/Images/rktstage.gif Figure 6-16 Adapted from NASA http://exploration.grc.nasa.gov/education/rocket/Images/rktstage.gif

http://en.wikipedia.org/wiki/Isaac_Newton http://en.wikipedia.org/wiki/Perturbation_%28astronomy%29 http://en.wikipedia.org/wiki/Delta-v_budget

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http://en.wikipedia.org/wiki/Sub-orbital_spaceflight Figure 7-4 NASA illustration by Robert Simmon http://earthobservatory.nasa.gov/Features/OrbitsCatalog/images/orb ital_inclination.png

http://en.wikipedia.org/wiki/Ansari_X_Prize http://en.wikipedia.org/wiki/Retrograde_and_prograde_motion

Figure 7-5 "Comparison satellite navigation orbits" by Cmglee, Geo Swan - Own work, Earth bitmap is File:North_pole_february_ice-pack_1978-2002.png by Geo Swan.. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Comparison_satellite_navi gation_orbits.svg

http://en.wikipedia.org/wiki/Orbital_inclination http://en.wikipedia.org/wiki/Ground_track http://fas.org/spp/military/docops/army/ref_text/chap5im.htm http://science.nasa.gov/science-news/science-at-nasa/2000/ast01dec _1/

Figure 7-6 USGS http://earthshots.usgs.gov/earthshots/sites/all/files/earthshots/earth shot/about/satellite-orbit.jpg

http://www.au.af.mil/au/awc/space/primer/ Figures 7-7 & 7-8 US Air Force, Air University, Space Primer, Chapter 8 http://www.au.af.mil/au/awc/space/primer/

http://history.nasa.gov/shuttleoverview1988/part1.htm http://en.wikipedia.org/wiki/Orbital_maneuver

Figure 7-9 http://science.nasa.gov/media/medialibrary/1999/04/07/ast06may9 9_1_resources/StationCoverage1.gif

http://en.wikipedia.org/wiki/Orbital_inclination_change http://en.wikipedia.org/wiki/Space_rendezvous

Figure 7-10 US Air Force, Air University, Space Primer, Chapter 8 http://www.au.af.mil/au/awc/space/primer/

http://www.orbiterwiki.org/wiki/GPIS_5:%20Dancing%20In%20The %20Dark

Figure 7-11 Adapted from NASA http://history.nasa.gov/shuttleoverview1988/p3.jpg

http://en.wikipedia.org/wiki/Automated_Transfer_Vehicle Figure 7-12 Adapted from NASA http://history.nasa.gov/afj/pics/lor2.gif

http://www.esa.int/Education/Space_In_Bytes/ATV_a_very_special _delivery_-_Lesson_notes

Figure 7-13 "Phase Shuttle" by Esburgos - Own work. Licensed under CC BY-SA 4.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Phase_Shuttle.jpg

http://en.wikipedia.org/wiki/Exploration_of_Mars http://www2.jpl.nasa.gov/basics/bsf4-1.php

Figure 7-14 Adapted from NASA http://spaceflight.nasa.gov/gallery/images/shuttle/sts-124/hires/s12 4e009982.jpg

http://en.wikipedia.org/wiki/Gravity_assist http://en.wikipedia.org/wiki/Gravity_turn

Figure 7-15 http://www2.jpl.nasa.gov/basics/launch_still.gif

http://en.wikipedia.org/wiki/Free_return_trajectory http://en.wikipedia.org/wiki/Nuclear_pulse_propulsion

Figure 7-16 http://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Orbit al_Hohmann_Transfer.svg/1024px-Orbital_Hohmann_Transfer.svg. png?1430838853406

Images

Figure 7-17 "Hohmann transfer orbit" by Leafnode - Own work based on image by Hubert Bartkowiak. Licensed under CC BY-SA 2.5 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Hohmann_transfer_orbit.s vg

Chapter Cover ESA/NASA https://www.nasa.gov/sites/default/files/chmielewski-3_0.jpg Figure 7-1 "SatelliteCentripetalForce" by Jfmelero - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:SatelliteCentripetalForce.sv g

Figure 7-18 http://mars.nasa.gov/mer/technology/bb_propulsion-01.html Figure 7-19 "Gravity assist still Jupiter". Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Gravity_assist_still_Jupite r.svg

Figure 7-2 Adapted from "Flightpath02" by copenhagensuborbitals http://www.copenhagensuborbitals.com/press.php - free press pictures. Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Flightpath02.jpg Figure 7-3 Adapted from ESA http://blogs.esa.int/rocketscience/files/2015/02/IXV_flight_profile.p ng

Figure 7-20 "Gravity assist moving Jupiter". Licensed under CC BY-SA 3.0 via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Gravity_assist_moving_Ju piter.svg

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https://en.wikipedia.org/wiki/Curiosity_%28rover%29 Figure 7-21 http://solarsystem.nasa.gov/multimedia/gallery/Voyager_Path.jpg

https://en.wikipedia.org/wiki/Sample_return_mission

Figure 7-22 http://history.nasa.gov/afj/launchwindow/figs/Fig%2015.png

Images

Figure 7-23 "Circumlunar-free-return-trajectory" by NickFr - Own work. Licensed under Public Domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Circumlunar-free-return-tr ajectory.png Figure 7-24 http://www.jpl.nasa.gov/images/blog/20130501/dawnorbit-600.png

Chapter 8 Text http://en.wikipedia.org/wiki/Space_exploration http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=2205 http://www.nasa.gov/topics/universe/features/planet-colors.html https://en.wikipedia.org/wiki/Apache_Point_Observatory_Lunar_La ser-ranging_Operation https://en.wikipedia.org/wiki/Gran_Telescopio_Canarias https://en.wikipedia.org/wiki/Color_photography https://en.wikipedia.org/wiki/Gustav_Kirchhoff https://en.wikipedia.org/wiki/Doppler_effect http://www.nasa.gov/mission_pages/kepler/spacecraft/index.html https://en.wikipedia.org/wiki/Kepler_%28spacecraft%29 http://solarsystem.nasa.gov/basics/bsf9-1.php https://en.wikipedia.org/wiki/Luna_1 https://en.wikipedia.org/wiki/Grand_Tour_program https://en.wikipedia.org/wiki/Flyby_anomaly

Chapter Cover NASA https://www.nasa.gov/sites/default/files/thumbnails/image/pia1914 2_malhi-mojave.jpg Figure 8-1 Adapted from NASA http://solarsystem.nasa.gov/images/splash-dsn.jpg Figure 8-2 https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=10679 Figure 8-3 "Remote Sensing Illustration" by Arkarjun - Own work. Licensed under CC BY-SA 3.0 via Commons https://commons.wikimedia.org/wiki/File:Remote_Sensing_Illustrat ion.jpg Figure 8-4 http://hubblesite.org/the_telescope/hubble_essentials/image.php?i mage=light-path Figure 8-5 "Atmospheric electromagnetic opacity" by NASA (original); SVG by Mysid. - Vectorized by User:Mysid in Inkscape, original NASA image from File:Atmospheric electromagnetic transmittance or opacity.jpg.. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Atmospheric_electromagn etic_opacity.sv Figure 8-6 http://sci.esa.int/education/50369-the-orion-nebula-in-multiwavelen gth/ Figure 8-7 Adapted from NASA http://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/images /carbon-solution.jpg Figure 8-8 "Spectral lines en" by User:Jhausauer. Original uploader was Jhausauer at en.wikipedia - Transferred from en.wikipedia; transferred to Commons by User:Magnus Manske using CommonsHelper.(Original text : Author). Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Spectral_lines_en.PNG

https://en.wikipedia.org/wiki/Planetary_cartography

Figure 8-9 "Hydrogen transitions" by A_hidrogen_szinkepei.jpg: User:Szdoriderivative work: OrangeDog (talk â&#x20AC;˘ contribs) A_hidrogen_szinkepei.jpg. Licensed under CC BY 2.5 via Commons https://commons.wikimedia.org/wiki/File:Hydrogen_transitions.svg

https://en.wikipedia.org/wiki/MESSENGER

Figure 8-10 http://imagine.gsfc.nasa.gov/science/toolbox/atom.html

https://www.nasa.gov/mission_pages/viking/viking30_fs.html

Figure 8-11 "Emission spectrum-Fe" by User:nilda - Own work. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Emission_spectrum-Fe.sv g

https://en.wikipedia.org/wiki/Pluto

https://en.wikipedia.org/wiki/Carl_Sagan http://mars.nasa.gov/msl/mission/overview/ https://en.wikipedia.org/wiki/Mars_Science_Laboratory https://en.wikipedia.org/wiki/Sample_return_mission

Figure 8-12 "Doppler effect diagrammatic". Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Doppler_effect_diagramm atic.png

232

Figure 8-13 http://imagine.gsfc.nasa.gov/educators/hera/spectroscopy/black_hol e/accretion.html

Figure 8-31 http://mars.nasa.gov/msl/mission/science/results/ Figure 8-32 http://www.nasa.gov/multimedia/imagegallery/image_feature_2129. html

Figure 8-14 "Kepler orbit" by DKG http://lasp.colorado.edu/kepler-launch/docs/314125main_Kepler_pr esskit_2-19_smfile.pdf - page 17. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Kepler_orbit.png

Chapter 9

Figure 8-15 http://planetquest.jpl.nasa.gov/page/methods

Text

Figure 8-16 http://kepler.nasa.gov/files/mws/aas2010-1wbLightCurves.jpg

https://en.wikipedia.org/wiki/Commercialization_of_space

Figure 8-17 http://planetquest.jpl.nasa.gov/page/methods

http://www.space.commerce.gov/policy/national-space-policy/

Figure 8-18 http://www.nasa.gov/ames/kepler/kepler-planet-candidates-july-201 5

https://en.wikipedia.org/wiki/Telstar

Figure 8-19 JHU/APL http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto/images/MissionPathtoPluto-MissionTimeline-TenYears.jpg

https://en.wikipedia.org/wiki/List_of_communication_satellite_com panies

http://www.wired.com/2012/07/50th-anniversary-telstar-1/

http://www.nasa.gov/connect/ebooks/historical_analogs_detail.html

Figure 8-20 JHU/APL http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto/images/trajector yImage_lg.jpg

http://www.space.commerce.gov/

Figure 8-21 JHU/APL http://pluto.jhuapl.edu/News-Center/Press-Conferences/July-24-201 5.php

https://www.nasa.gov/sites/default/files/files/NASA_Partnership_R eport_LR_20140429.pdf

Figure 8-22 http://photojournal.jpl.nasa.gov/jpeg/PIA19947.jpg

https://en.wikipedia.org/wiki/Private_spaceflight

Figure 8-23 http://www.nasa.gov/sites/default/files/images/119541main_image_ feature_358_ys_full.jpg

https://en.wikipedia.org/wiki/Conestoga_%28rocket%29

Figure 8-24 "MESSENGER trajectory" by NASA; Vectors by Mysid Vectorized by User:Mysid in Inkscape, based on Image:Traj73004 helio ecldto 52005.jpg which was originally uploaded on en-wp by en:User:Miranche who stated http://messenger.jhuapl.edu/news_room/MessengerQuadrafold.pdf as its source.. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:MESSENGER_trajectory.s vg

https://en.wikipedia.org/wiki/The_Spaceship_Company

Figure 8-25 http://photojournal.jpl.nasa.gov/jpeg/PIA19449.jpg

https://en.wikipedia.org/wiki/Timeline_of_first_orbital_launches_b y_country

Figure 8-26 http://www.nasa.gov/sites/default/files/thumbnails/image/messenge r-global-coverage_0.jpg

https://en.wikipedia.org/wiki/SpaceX

Figure 8-27 https://www.nasa.gov/multimedia/imagegallery/image_feature_599. html

https://en.wikipedia.org/wiki/Falcon_9

https://en.wikipedia.org/wiki/Commercial_Spaceflight_Federation

https://en.wikipedia.org/wiki/Cygnus_CRS_Orb-3

https://en.wikipedia.org/wiki/Virgin_Galactic http://www.virgin.com/richard-branson/the-end-of-ntsbs-investigati on-and-the-future-of-virgin-galactic https://en.wikipedia.org/wiki/OneWeb_satellite_constellation

https://en.wikipedia.org/wiki/SpaceX_CRS-5

Figure 8-28 http://msl-scicorner.jpl.nasa.gov/Instruments/

https://en.wikipedia.org/wiki/Dragon_%28spacecraft%29

Figure 8-29 http://www.nasa.gov/mission_pages/msl/multimedia/pia16089.html Figure 8-30 http://photojournal.jpl.nasa.gov/feature/19912

https://en.wikipedia.org/wiki/Commercial_Crew_Development

https://en.m.wikipedia.org/wiki/Heinlein_Prize_for_Advances_in_S pace_Commercialization https://en.wikipedia.org/wiki/SpaceX_private_launch_site

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https://en.wikipedia.org/wiki/PAS-22 http://www.spaceservicesinc.com https://en.wikipedia.org/wiki/International_Launch_Services https://prod.nais.nasa.gov/cgibin/eps/sol.cgi?acqid=167975 https://en.wikipedia.org/wiki/Manfred_Memorial_Moon_Mission https://en.wikipedia.org/wiki/Chang%27e_5-T1

Images

https://en.wikipedia.org/wiki/Space_burial

Chapter Cover NASA/JPL http://www.jpl.nasa.gov/images/blog/20141204/climate_site_16.jpg

https://en.wikipedia.org/wiki/DSE-Alpha http://www.spaceadventures.com/13-years-later-reflecting-on-the-wo rlds-first/

Figure 9-1 https://spinoff.nasa.gov/Spinoff2008/ipp_1.html

http://lunar.xprize.org/about/overview

Figure 9-2 From https://www.nasa.gov/sites/default/files/files/NASA_Partnership_R eport_LR_20140429.pdf

http://lunar.xprize.org/teams/astrobotic https://www.astrobotic.com/

Figure 9-3 "Conestoga-1620" by NASA - Scanned from a NASA Brochure. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Conestoga-1620.jpg

http://lunar.xprize.org/teams/hakuto http://team-hakuto.jp/en/

Figure 9-4 "Flight 16P taxi pre launch photo D Ramey Logan" by D. Ramey Logan (WPPilot) - Own work. Licensed under CC BY-SA 4.0 via Commons https://commons.wikimedia.org/wiki/File:Flight_16P_taxi_pre_laun ch_photo_D_Ramey_Logan.jpg

http://lunar.xprize.org/teams/team-spaceil http://www.spaceil.com/ http://lunar.xprize.org/teams/moon-express

Figure 9-5 https://commons.wikimedia.org/wiki/File:White_Knight_Two_and_ SpaceShipTwo_from_directly_below.jpg

http://www.moonexpress.com/index.html https://en.wikipedia.org/wiki/Space_burial

Figure 9-6 http://www.spacex.com/gallery/thales-mission

https://en.wikipedia.org/wiki/Helium-3

Figure 9-7 http://www.spacex.com/gallery/crs-5-mission-january-2015

https://en.wikipedia.org/wiki/Mars_Colonial_Transporter

Figure 9-8 http://www.nasa.gov/images/content/638594main_05_demonstrati on_maneuvers.jpg

https://en.wikipedia.org/wiki/Inspiration_Mars_Foundation https://en.wikipedia.org/wiki/Bigelow_Aerospace

Figure 9-9 http://www.spacex.com/sites/spacex/files/fhgraphic_copy.jpg

http://deepspaceindustries.com/ https://en.wikipedia.org/wiki/Planetary_Resources

Figure 9-10 http://www.spacex.com/gallery/crew-dragon-pad-abort-test

https://en.wikipedia.org/wiki/Ion_thruster

Figure 9-11 http://www.spacex.com/media-gallery/detail/100831/3261

http://cs.astrium.eads.net/sp/spacecraft-propulsion/ion-propulsion/i ndex.html http://boeing.mediaroom.com/2015-09-10-Boeing-World-s-First-AllElectric-Propulsion-Satellite-Begins-Operations https://www.kickstarter.com/projects/2027072188/plasma-jet-electri c-thrusters-for-spacecraft/description https://en.wikipedia.org/wiki/Solar_sail http://wiki.solarsails.info/index.php/Solar_Sailing_101 http://sail.planetary.org/ http://www.planetary.org/blogs/jason-davis/2015/20150904-lightsai l-km-per-day.html

Figure 9-12 "SpaceX private-launch facility location--TexasProposal-201304" by Federal Aviation Administration - US government, FAA agency, April 2013 Draft Environmental Impact Statement for SpaceX Texas Launch Site, April 2013. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:SpaceX_private-launch_fa cility_location--TexasProposal--201304.jpg Figure 9-13 "Rima Ariadaeus-1". Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Rima_Ariadaeus-1.jpg Figure 9-14 "Dennis Tito" by NASA http://nix.ksc.nasa.gov/info;jsessionid=2ooqgoj4jqqjb?id=KSC-03PD

234

-1489&orgid=5. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Dennis_Tito.jpg

https://en.wikipedia.org/wiki/World_war https://en.wikipedia.org/wiki/Effects_of_global_warming

Figure 9-15 http://www.cmu.edu/news/stories/archives/2014/november/novemb er24_lunarroverandy.html

http://www.spacequotations.com/colonization.html https://newsinhealth.nih.gov/issue/feb2014/feature1

Figure 9-16 "Lacus-mortis-clem1" by Clementine probe http://lpod.org/coppermine/displayimage.php?pos=-486. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Lacus-mortis-clem1.jpg

https://en.wikipedia.org/wiki/Asteroid https://en.wikipedia.org/wiki/Asteroid_mining

Figure 9-17 http://www.moonexpress.com/images/mx-1%20spacecraft%20-%20i phone%20of%20space-u3101-fr.jpg

https://en.wikipedia.org/wiki/Artificial_gravity

Figure 9-18 http://grin.hq.nasa.gov/ABSTRACTS/GPN-2000-001036.html

https://en.wikipedia.org/wiki/Life_support_system

https://en.wikipedia.org/wiki/Nautilus-X

https://en.wikipedia.org/wiki/Biosphere_2 Figure 9-19 http://www.nasa.gov/sites/default/files/images/712983main_NEXT _LDT_Thrusterhi-res_full.jpg Figure 9-20 "Ion engine" by Vectorization: Chabacano - Retrieved from NASA - DS1: How the Ion Engine Works by NASA Glenn Research Center on 2006-07-24.Link is broken, what about this one: [1]Vectorized by Chabacano. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Ion_engine.svg

https://en.wikipedia.org/wiki/Solar_cell http://science.nasa.gov/science-news/science-at-nasa/2002/solarcell s/ http://www.nasa.gov/mission_pages/station/research/experiments/ 863.html https://en.wikipedia.org/wiki/Plants_in_space

Figure 9-21 http://www.boeing.com/history/products/702-satellite.page

https://en.wikipedia.org/wiki/Space_habitat

Figure 9-22 http://sail.planetary.org/gallery/lightsail.html

https://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module

Figure 9-23 http://wiki.solarsails.info/index.php/File:Sail_forces.png

http://bigelowaerospace.com/beam/

Figure 9-24 http://www.planetary.org/multimedia/space-images/charts/lightsailorbit-raising.html

https://en.wikipedia.org/wiki/Bigelow_Expandable_Activity_Module

Figure 9-25 http://wiki.solarsails.info/index.php/File:Sailorbtransfer.png

https://en.wikipedia.org/wiki/Chinese_large_modular_space_statio n

Figure 9-26 Adapted from http://www.nasa.gov/images/content/713016main_CCP-New-Ride-5 x7.jpg

http://www.nasa.gov/feature/next-space-technologies-for-exploratio n-partnerships-nextstep-projects

http://www.russianspaceweb.com/opsek.html

https://en.wikipedia.org/wiki/Colonization_of_the_Moon

Chapter 10

https://en.wikipedia.org/wiki/Colonization_of_the_Moon https://en.wikipedia.org/wiki/Colonization_of_Mars

Text

https://en.wikipedia.org/wiki/Atmosphere_of_Mars

https://en.wikipedia.org/wiki/Colony

https://www.nasa.gov/content/nasas-journey-to-mars

https://en.wikipedia.org/wiki/Colonialism

https://en.wikipedia.org/wiki/Human_mission_to_Mars

https://en.wikipedia.org/wiki/Space_colonization

http://nssdc.gsfc.nasa.gov/planetary/mars/marsprof.html

https://en.wikipedia.org/wiki/Space_and_survival

https://en.wikipedia.org/wiki/Lagrange_point_colonization

https://en.wikipedia.org/wiki/Comet_Shoemaker

https://en.wikipedia.org/wiki/Colonization_of_the_outer_Solar_Sys tem

http://settlement.arc.nasa.gov/Basics/wwwwh.html

https://en.wikipedia.org/wiki/Interstellar_travel

235

https://en.wikipedia.org/wiki/Mass_in_special_relativity

Figure 10-10 http://www.nasa.gov/centers/johnson/exploration/graphics_marsbri efing_112311.html

https://en.wikipedia.org/wiki/Interstellar_travel https://en.wikipedia.org/wiki/100_Year_Starship https://en.wikipedia.org/wiki/Generation_ship

Figure 10-11 extracted from http://ntrs.nasa.gov/search.jsp?R=20150001240 Figure 10-12 https://pixabay.com/en/rocket-launch-saturn-rocket-437218/

https://en.wikipedia.org/wiki/Sleeper_ship https://en.wikipedia.org/wiki/Suspended_animation https://en.wikipedia.org/wiki/Cryonics https://en.wikipedia.org/wiki/Cryopreservation

Figure 10-13 Adapted from http://planetquest.jpl.nasa.gov/image/282 Figure 10-14 http://www.nasa.gov/images/content/532136main_voyager-042811.j pg

Images Chapter Cover "Mars mission" by Les Bossinas of NASA Lewis Research Center. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Mars_mission.jpg Figure 10-1 http://hubblesite.org/newscenter/archive/releases/1994/1994/34/im age/a/ Figure 10-2 â&#x20AC;&#x153;Red Giant Earth" by Fsgregs at the English language Wikipedia project. Licensed under CC BY-SA 3.0 via Commons https://commons.wikimedia.org/wiki/File:Red_Giant_Earth.jpg Figure 10-3 "Nautilus-X Global-view-1" by Mark L Holderman - NASA Technology Applications Assessment Team. Licensed under Public Domain via Commons https://commons.wikimedia.org/wiki/File:Nautilus-X_Global-view-1. png Figure 10-4 http://settlement.arc.nasa.gov/75SummerStudy/figure3.1.gif Figure 10-5 "Wiki bio2 sunset 001" by Johndedios - Own work. Licensed under CC BY 3.0 via Commons https://commons.wikimedia.org/wiki/File:Wiki_bio2_sunset_001.jp g Figure 10-6 http://www.nasa.gov/mission_pages/station/research/experiments/1 804.html Figure 10-7 http://www.nasaspaceflight.com/2015/11/nasa-progress-habitat-deve lopment-deep-space-exploration/ Figure 10-8 http://spaceflight1.nasa.gov/gallery/images/exploration/lunarexplora tion/html/s86_27256.html Figure 10-9 http://www.esa.int/spaceinimages/Images/2013/01/Multi-dome_bas e_being_constructed

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SECTION INDEX

SUBJECT

SECTION

A Accelerometers Altitudes, orbital Astronomical objects

5.2.1 7.3.4, 7.3.8 2.3.3

B Big Bang

2.2.4

C Calendar Case Study SpaceX Virgin Galactic Cislunar space Colonies in other Solar System locations Colonization interstellar locations space why why interstellar why Solar System Colonizing a space habitat Mars the Moon Commercial business on the Moon first communications satellite Conestoga 1 Constant thrust trajectory Coordinates

4.4 9.4 9.3 2.4.2 10.4.4 10.5 10.4 10 10.2 10.2.2 10.2.1 10.4.1 10.4.3 10.4.2 9.5.4 9.1.1 9.2.1 7.4.4 4.2

237

Coordinate systems ecliptic equatorial geographic horizontal Cosmic Microwave Background radiation Curiosity rover

4.3 4.3.4 4.3.3 4.3.1 4.3.2 2.2.4 8.5.3

Day Deep space trajectories Direct sensing Distance primer Distance, time, and velocity Doppler effect Drag Dwarf planets

4.4.1 7.4 8.1 2.2.3 1.4.1 8.2.5 5.3.1 3.4

Earth fate space Eccentricity, orbital Electromagnetic radiation primer Energy content of the Universe generation in the Sun kinetic potential Escape velocity Exhaust velocity

3.3.3 3.9.1 2.4.1 7.3.1 2.2.2

2.3.1 3.2.4 5.3.1 5.5.2 6.2.1 6.5.2

F Flyby anomaly first commercial human lunar (ca. 2017?) first commercial robotic lunar space probes Forces flight glaunch Free fall Free return trajectory

5.1 6.3.1 6.3 5.5.2 7.4.3

Geospace g-force

2.4.1 6.3.1

8.3.3 9.5.2 9.5.1 8.3

238

Global Positioning System (GPS) Gravity assist trajectory in a space colony Newton’s law of turn Ground tracks Guidance, navigation and control Gyroscopes

4.5 7.4.2 10.3.1 5.5 6.4.1 7.3.6 4.1 5.2.1

Hill sphere Hohmann trajectory Hubble’s Law

5.5.4 7.4.1 2.2.1

I Ideal rocket equation Images Impulse, specific Inclinations, orbital Intergalactic space Interplanetary space Interstellar colonization space Ion thrusters

5.6 8.2.3 6.5.2 7.3.3, 7.3.9 2.4.5 2.4.3 10.5 2.4.4 9.6.1

J Jupiter

3.3.5

Kármán Line legal issues at Kepler mission Kepler’s laws Kinetic energy Kirchhoff’s laws

2.1.1 2.1.2 8.2.6 3.7 5.3.1 8.2.4

Lander space probe Landing first private robotic lunar (ca. 2017?) on bodies that have atmospheres on bodies without atmospheres Launch air launch to orbit alternatives azimuth forces

8.5 6.8 9.5.3 6.8.2 6.8.1 6 6.7.1 6.7 7.3.7 6.3

239

window

7.3.7

Law Hubble’s Newton’s first Newton’s gravity Newton’s second Newton’s third space Life support in a space colony Lift Lift Off

2.2.1 5.2 5.5 5.3 5.4 2.1.2 10.3.2 5.4.1 6

M Mars colonizing Science Laboratory & Curiosity mission Mass Mercury MESSENGER mission Mission profile Month Moon colonizing the commercial business on the private spaceflight beyond the (ca. mid-2020?) space Motion, Newton’s laws

3.3.4 10.4.3 8.5.3 5.5.1 3.3.1 8.4.3 6.1 4.4.2 2.4.2, 4.4.2 10.4.2 9.5.4 9.6 2.4.2 5.2, 5.3, 5.4

N Navigation Neptune New Horizons mission Newton’s first law law of gravity second law third law

4 3.3.8 8.3.4

Observable Universe Observatory space probes Orbit geostationary geosynchronous Orbital altitudes directions

2.2.5 8.2 7.1 7.3.5 7.3.5

5.2 5.5 5.3 5.4

7.3.4, 7.3.8 7.3.2

240

eccentricity inclinations maneuvers rendezvous Sciences Corporation synchronicity trajectories velocity Orbiter space probes

7.3.1 7.3.3, 7.3.9 7.3.8 7.3.10 9.2.2 7.3.5 7.3 6.2.2 8.4

P Planets configurations dwarf formation Potential energy Prefixes Private spaceflight beyond the Moon (ca. mid-2020s?) Propulsion types

3.3 3.6 3.4 3.8.2 5.5.2 1.4.2 9.2 9.6 6.5 6.5.1

Q R Remote sensing Rendezvous, orbital Rocket equation for launch flight physics principle spaceflight velocities

8.1 7.3.10 1.1.1, 1.3 5.6 6.4 5.1 5 1.3.1 1.3 6.2

S Sample Mission Kepler Mars Science Laboratory & Curiosity MESSENGER New Horizons Satellite first commercial communications Saturn Scientific notation SI units Size of the observable Universe

8.2.6 8.5.3 8.4.3 8.3.4 9.1.1 3.3.6 1.4.2 1.4.2 2.2.5 241

Sensing Active remote Direct Passive remote Remote Small Solar System Bodies Solar sails Solar System arrangement colonies in other locations colonization, why formation future heliocentric objects origin overview small bodies Solar wind Space beginning (def.) cislunar colonization what is why why interstellar why Solar System colony energy supply for a gravity in a life support in a material supply for a types commercial first human lunar flyby (ca. 2017?) first robotic lunar flyby commercialization US process what is contents end exploration other modes of geo habitat, colonizing a 242

8.1.1 8.1 8.1.1 8.1 3.5 9.6.2 3 3.1.3 10.4.4 10.2.1 3.8 3.9 3.1.1 3.1.2 3.8 3.1 3.5 3.2.2 2 2.1 2.4.2 10 10.1 10.2 10.2.2 10.2.1 10.3 10.3.4 10.3.1 10.3.2 10.3.3 10.3 9.5.2 9.5.1 9 9.1.2 9.1 2.3 2.2 8 8.6 2.4.1 10.4.1

intergalactic interplanetary interstellar launch first private: Conestoga 1 first private crewed: SpaceShipOne first private to orbit: Orbital Sciences Corporation law navigation outer probes flyby purpose of why do lander purpose of why do observatory orbiter purpose of why do reaching regions size telescopes purpose of why put in space Spacecraft time trajectories Spaceflight biggest obstacle for commercial uses of in the 21st century introducing political uses of private beyond the Moon (ca. mid-2020s?) to the Moon rockets for scientific uses of types of what is why SpaceShipOne 243

2.4.5 2.4.3 2.4.4 6 9.2.1 9.2.3 9.2.2 2.1.2 4 2 8.3 8.3.1 8.3.2 8.5 8.5.1 8.5.2 8.2 8.4 8.4.1 8.4.2 1.1.1 2.4 2.2.5 8.2 8.2.1 8.2.2 4.4.4 7 1.4 1.2.2 1.1.4 1 1.2.3 9.2 9.6 9.5 1.3 1.2.1 1.1.2 1.1 1.2 9.2.3

SpaceX Specific impulse Spectra Sphere of influence Staging Sub-orbital trajectories Sun activity energy generation formation sunlight wind

9.4 6.5.2 8.2.3 5.5.4 6.6 7.2 3.2 3.2.3 3.2.4 3.8.1 3.2.1 3.2.2

Time

4.4 4.4.4 1.4.1 7.1 7.4.4 7.4 7.4.3 7.4.2 7.4.1 7.3 7.2 5.5.3

T spacecraft velocity, and distance Trajectory constant thrust deep space free return gravity assist Hohmann orbital sub-orbital True weightlessness

U Universe astronomical objects energy content matter content observable ordinary matter content size Uranus

2.3.3 2.3.1 2.3.1, 2.3.2 2.2, 2.2.5 2.3.2 2.2.5 3.3.7

V Velocity distance, and time escape exhaust orbital rocket Venus Virgin Galactic

1.4.1 6.2.1 6.5.2 6.2.2 6.2 3.3.2 9.3

244

W Weight Weightlessness, true

5.5.1 5.5.3

Year

4.4.3

X Y Z

245

A B O U T T H E AU T H O R

Dr. Regina E. Schulte-Ladbeck received her undergraduate degree in physics in 1982 and her graduate degree in astronomy in 1985, both from the University of Heidelberg, Germany. She joined the University of Wisconsin, Madison, in 1987, first, as a postdoctoral scientist in the astronomy department, and later, as a staff scientist of the Space Astronomy Laboratory. Here, she became a member of the Wisconsin Ultraviolet Photo-Polarimeter experiment team, who successfully flew a small telescope on the NASA Space Shuttle during the 199o Astro-1 mission. In 1992, Dr. Schulte-Ladbeck arrived at the University of Pittsburgh as a professor in the department of physics and astronomy. She continued to use space observatories in her research. She was a principal investigator on the Astro-2 Space Shuttle mission, and also with the International Ultraviolet Explorer satellite, and the Hubble Space Telescope. Her research interests have ranged from studying the most massive stars to the stellar populations and star formation in dwarf galaxies. While teaching at the University of Pittsburgh, Dr. Schulte-Ladbeck developed a general education undergraduate course titled â&#x20AC;&#x153;Basics of Spaceflight.â&#x20AC;? This course was first offered at the University of Pittsburgh in 2000, and has been taught every since, with an enrollment of between about 50-120 students each semester. In 2009, Dr. Schulte-Ladbeck founded a small business, RESLscience, for publishing content in space and adventure sciences with a focus on education combined with entertainment.

Dr. Regina E. Schulte-Ladbeck

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