ASTRONOMY & SPACE Richard Pearson
September 2014
Comets are colourful fuzz-balls in space that have a small rocky nucleus, a shimmering hallo of dust known as the coma, and long dusty tails stretching thousands of miles in length. They have been seen by sky watchers for centuries, and were once believed to be omens to bad fortune. Over the last decade a number of spacecraft have visited comets, and have helped to reveal their secrets, therefore in this month's program we take a look at their findings. After a ten year journey the European Space Agency spacecraft, Rosetta, has arrived at comet 67P Churyumov-Gerasimenko. The probe successfully entered into orbit around its nucleus on 6 August 2014, and the historic event made headlines worldwide. Lying at a distance of 251 million miles from the earth it takes a radio command 80 minutes to reach us from Rosetta, which lies just inside the orbit of the giant planet Jupiter. The comet is expected to reach perihelion on 13 August 2015 when it will then be 115.5 million miles from the Sun. The comet was discovered in 1969 by Klim Ivanovych Churyumov of the Kiev Astronomical Observatory, who examined a photograph that had been exposed for periodic comet 32P/Comas Solà by Svetlana Ivanovna Gerasimenko on 11 September 1969. Rosetta has now began the task of making science observations of comet 67P. The first temperature measurements of comet 67P/Churyumov–Gerasimenko were made between 13th to 21 July, when Rosetta closed in from 8,700 miles to the comet to just over 3,100 miles. The observations were made by the spacecraft’s visible, infrared and thermal imaging spectrometer, VIRTIS, and revealed an average surface temperature of –70ºC. This implies the surface is predominantly covered in dust rather than ice. Although –70ºC may seem rather cold, it is 20–30ºC warmer than predicted for a comet at that distance covered exclusively in ice. OSIRIS images show that the coma of comet 67P covers an area of 93 by 93 square miles (150x150 square kilometres). This image was taken on 25 July 2014. The centre of the image corresponds to the position of the comet. The central area and the pale, round structure right from the core are optical artefacts. The comet is actually a two-rock system which came together to form the peanut shaped nucleus we see in the new images. The nucleus is small, it measures 2.2×2.5 miles in size, yet careful analysis of the first close up images of the comet taken on 29 July show a jet of material originating from a crack in the surface of the nucleus where the two objects come together. Scientists at the European Space Agency are now busy trying to find a safe landing place for the small Rosetta probe Philae. On 11 November, Rosetta will try to place the Philae Lander on the comet. It must attach to its nucleus, which is travelling through space at 12.5 miles per second. Knowing the location of these jets is of great importance to ensure that the Philae probe does not come to any harm. The science study of comet 67P by the Rosetta space craft will need to be supplemented by ground based observations of comet Churyumov–Gerasimenko by keen amateur astrophotographers located provisionally in the southern hemisphere. With a magnitude of +18.7 it is not an easy object to find. The comet is now located near the double star named Ascella (zeta Sagittarii), moving between Messier's' 54 & M 70. When 67P reaches opposition, its closest point to the Sun on 13 August 2015, the comet will have moved into Gemini (the twins). It can then be found a little above, and to the east, of Epsilon Gminorum, Mebusta to give it its proper name. We believe its magnitude will then be +13.2, so that comet 67P will be visible in 8-inch telescopes. Ground based observations are being co-ordinated by Padma Yanamandra-Fisher who is a senior research scientist at the Space Science Institute Boulder, Colorado in the US.
Gminorum, Mebusta to give it its proper name. We believe its magnitude will then be +13.2, so that comet 67P will be visible in 8-inch telescopes. Ground based observations are being co-ordinated by Padma Yanamandra-Fisher who is a senior research scientist at the Space Science Institute Boulder, Colorado in the US. Padma Yanamandra & Colin Snodgrass has set up a special face-book group named PACA_Rossetta67P, which stands for Pro-Am Collaborative Astronomy, and is also one Rosetta's coordinator for amateur observations of 67P. Padma Yanamandra-Fisher informed me that group members interact by sharing information, methods and techniques. Of course, compared to the fantastic detail observed by Rosetta and (hopefully Philae), the ground-based amateur observervations are a challenge, she said. However, they provide a complementary global perspective of the comet, coma and activity, plus another data set to compare against the legacy data from previous apparitions. With multiple observing windows from both the northern and southern hemispheres. Tony Angel is an observer at the Searchlight Observatory Network at the Observatory Sierra Contraviesa in Italy. He informs me that telescopes used by the group's amateurs range from small to 72 inches in size, and then there are the professionals with access to larger ones. When 67P gets bright enough for spectroscopy then there are skills here for that as well he said. In a future program I will showcase the various astronomers, including Rolando Ligustri, who observer 67P last apparition, and Alan Hale, there facilities and observations. So, if you would like to be involved in observing and imaging comet Churyumov– Gerasimenko, do lookup the group PACA_Rossetta67P on face book. So what are comets and where do they come from? Our solar system is made up of the Sun in the centre, and eight planets. Near to the Sun are the rocky planets Mercury, Venus, Earth & Mars, followed by a wide gap filled with over 3,000 chunks of rock known as the Asteroid belt. Further, away we come to the gas giants of Jupiter, Saturn, Uranus and Neptune. Pluto is a dwarf planet lying in the region astronomers call the Kuiper Belt 3,666 million miles from the Sun. Astronomers use the Earth Sun distance of 93.5 million miles as a unit of measurement they call the Astronomical Unit. On this scale, the most distant planet, Neptune, lies 30 AU from the Sun, which is about 2.8 billion miles. Comets are members of the Solar system that astronomers believe are the left over remnants from the formation of the planets 4.5 billion years ago. The best known of these is Halley's Comet, which orbits the Sun in a narrow elliptical path once every 76 years. It last came to perihelion on 9 February 1986. Comets are actually dirty big snowballs of rock and dust, and you can actually make a replica comet here on Earth. Video Comets come in two classes. Long period comets have orbits measuring thousands of years, of which many may never return to the inner Solar System once past perihelion, there closest point to the Sun. They originate in the furthest abyss of our area of space known as the Oort cloud. Comet C/2012 A1 Siding Spring is a long period comet that originates from the Oort Cloud. It has a period of 7 million years. On October 19 the comet will pass to within 26,000 miles of the planet Mars six days before perihelion, its closest point to the Sun. It will then be of magnitude +7.7 lying in the constellation of Pavo, and visible in the southern hemisphere.
From the surface of Mars it will be a spectacular sight in Martian skies shining at magnitude 9.0. Named after the Dutch astronomer Jan Oort, this is a spherical cloud of comets believed to surround the Sun occupying a vast space from somewhere between 2,000 to 5,000 AU, to as far as 50,000 AU (over 4.6 trillion miles) from the Sun. If you recall that Neptune lies 30 AU from the Sun, you can see just how distant the Oort cloud is. Some estimates place the outer edge at between 1.5 and 3 light years. One light year measures about 6 million million miles, so again you have an idea of the vast distances involved. The region can be subdivided into a spherical outer Oort cloud, and a doughnut-shaped inner Oort cloud. All of the eight comets visited by spacecraft over the last ten years come within the class of Short Period comets. They are named Halley, Giacobini–Zinner, Grigg–Skjellerup, Borrelly, Wild 2, Tempel 1, Hartley and now Comet Churyumov–Gerasimenko. Short period comets originate from the area of the Solar System astronomers call the Kuiper Belt. This is a region extending from the orbit of Neptune, the eighth planet, to approximately 50 AU from the Sun, and is very similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. The Kuiper belt is home to the dwarf planets Pluto. Some of the Solar System's moons, such as Neptune's Triton and Saturn's moon Phoebe are also believed to have originated in the area. In order to better understand the origin of our Solar System, and possibly life on Earth, planetary scientists are keen to carryout observations of these comets close up. The latest spacecraft from the European Space Agency, Rosetta, will do just that as it follows Comet Churyumov–Gerasimenko along its 6.4-year orbit around the Sun. Astronomer Fred Whipple was born on November 5, 1906, in Iowa. In 1945, he became a Harvard professor of astronomy specialising in the study of comets. In 1950, he put forward a theory that comets are composed of ice and dust which has become known as the 'Dirty Snowball model.' In his paper printed in the Astrophysical Journal, he said that: The nucleus is visualized as ices, and other possible materials, unstable at room temperature, gathered together into a compact mass, and combined with meteoric materials, all at extremely low temperatures of about -50° Centigrade. Vaporization of the ices by externally applied solar radiation leaves an outer crust of nonvolatile insulating meteoric material on the comet's nucleus. He added that heat transfer through thin meteoric layers in a vacuum was mainly by radiation, and that the heat transfer is inversely proportional to the effective number of layers. Fred Whipple concluded that an appreciable time lag in heat transfer could occur for a rotating cometary nucleus. The momentum transfer from the velocity of the emitted gas would propel the nucleus, reduce the mean motion, and increase the eccentricity of the comet's orbit he said. As it turns out, this is an accurate description of a comet. The first successful comet probe to test the Dirty Snowball theory was Giotto of the European Space Agency. It was launched towards Halley's Comet on 2 July 1985, and was a fly by mission. The spacecraft was originally a research satellite built by British Aerospace, and modified with the addition of a dust shield as proposed by Fred Whipple himself, which comprised a thin aluminium sheet separated by a space, and a thicker Kevlar sheet. Giotto was named after the Early Italian Renaissance painter Giotto di Bondone. It was equipped with a camera known as HMC or Halley Multi-colour Camera to view the Nucleus, instruments to investigate the composition of Halley's Comet, and detectors counting the impact of cometary debris on the craft's protective shield.
On 13 March 1986, the mission succeeded in approaching Halley's nucleus at a distance of 370 miles, and there were some surprises. First, the nucleus was much warmer than had been predicted. In shape, it was likened variously to a baked bean, or an avocado pear, and it was about 9 miles long and 5 miles wide. The low density indicated that the comet was made up of ‘fluffy’ material, with water ice as the main constituent, together with other substances such as carbon dioxide, formaldehyde, nitrogen and ammonia. There was considerable surface structure, including a bright region, which seems to have been a hill almost a mile high; there were craters, and jets, and three main areas of activity. Yet all the jets seemed to issue from one small area on the sunlit side of the nucleus, and the remaining 85 per cent of the surface was inactive. The rotation period was given as 7.3 hours, and there was a ‘processional’ period of 53 hours, so that the nucleus was behaving rather in the manner of a spinning gyroscope. It was also estimated that the comet would have lost about 300 million tons of material by the time that it moved back into the remote parts of the Solar System. Only 10% of the surface was active, with at least three out gassing jets seen on the sunlit side. Analysis showed Halley's Comet formed 4.5 billion years ago from volatiles (mainly ice) that had condensed onto interstellar dust particles. It had remained practically unaltered since its formation. The measured volume of material ejected by Halley was 80% water, 10% carbon monoxide and a 2.5% mix of methane and ammonia. Other hydrocarbons, iron, and sodium were also detected in tiny amounts. However, the real shock was the colour of the nucleus. It was not ice-bright, as most people had expected; it was black reflecting no more than 2 to 4 per cent of the sunlight falling upon it. Astronomer Fred Whipple & Sir Patrick Moore were in the German Control centre in Darmstadt as the science results came in, and Professor Whipple had been proven correct. Halley's Comet was indeed a dirty snowball. The plasma and ion mass spectrometer instruments showed Halley has a carbon-rich surface, so there must be a layer of Carbon which protects the ice below from evaporating quickly, and is an effective - insulator. The nucleus's surface was rough and of a porous quality, with the density of the whole nucleus as low as 0.3 grams per cubic centimetre. The quantity of material ejected was found to be three tonnes per second for seven jets, and these caused the comet to wobble over long time periods, which was also predicted by Professor Fred Whipple. The dust ejected was mostly the size of cigarette smoke particles. Two kinds of dust were seen: one with carbon, hydrogen, nitrogen and oxygen. The other with calcium, iron, magnesium, silicon and sodium. The ratios of abundances of the comet's light elements were the same as the Sun's. The implication is that the constituents of Halley comet are among the most primitive in the solar system. Following its successful fly-by of comet Halley, Giotto went into sleep mode ready for its extended mission. It was commanded to wake up on 2 July 1990 when it flew by Earth in order to sling shot to its next cometary encounter. The probe then flew by the Comet GriggSkjellerup on 10 July 1992, which it approached to a distance of about 322 miles (200 km). Here again the Giotto spacecraft successfully confirmed Fred Whipple's Dirty Snowball theory was correct. The flyby conditions were different from those during the Halley encounter. Since GriggSkjellerup would approach Giotto at an angle of 68 degrees instead of head-on, the bumper
shield would afford no protection. However, Giotto worked well and sent back some nice images of comet Grigg-Skjellerup close up. The eight operational experiments (including the radio science investigation) provided a surprising wealth of exciting data. Conditions inside the comet's plasma (ionised gas) cloud were much different. The first cometary ions (charged particles) were detected 708,000 miles from the nucleus, about 12 hours before its closest approach. Scientists were surprised when an abrupt shock wave was detected on Giotto's outbound leg, but not clearly identified on the inward journey. This is caused when the supersonic solar wind slammed into the plasma around the comet. The Magnetic field strength was slightly higher than at Halley's Comet, however, as predicted, no activity was detected around the much smaller nucleus. Strangest of all was the discovery of unusual magnetic waves, each about 1,610 miles apart, near the comet. Activity rose and fell over a period of about 70 seconds and increased in strength as time went by. The waves were generated by 'pick-up ions,' charged particles created from the breakup of water molecules around the comet, as they moved in the magnetic field created by the solar wind. Following the cometary encounter, the science experiments on board Giotto were switched off for the last time on 23 July 1992. The next comet to be visited by a spacecraft was Borrelly. NASA's Deep Space 1 flew past the comet's nucleus on September 21, 2001. It was steered toward the comet during the extended mission of the craft. The flyby of Comet Borrelly was also a great success and returned extremely detailed images of the comet's surface. The images were of higher resolution than the previous pictures of Halley's Comet taken by the Giotto. The Plasma Experiment by the Planetary Exploration instrument, or PEPE, reported that the comet's magnetic field was offset from the nucleus. This is believed to be due to the emission of jets, which were not distributed evenly across the comet's surface. Despite having no debris shields, the spacecraft survived the comet passage intact. One question that planetary scientists wanted answers to was whether comets carried organic chemicals which may help kick start life here on Earth. Two British Astronomers, Fred Hoyle and Nalin Chandra Wickramasinghe, then enter the frame. Chandra Wickramasinghe, a former student of Sir Fred Hoyle, is a distinguished astronomer who made important contributions to the theory of cosmic dust. In 1974, he proposed the theory that dust in interstellar space and in comets was largely organic. In his later years, Hoyle became a staunch critic of theories used to explain the origin of life on Earth. With Chandra Wickramasinghe, Hoyle promoted the hypothesis that the first life on Earth began in space, spreading through the universe via what astronomers call Panspermia, and that evolution on earth is influenced by a steady arrival of viruses arriving via comets. His belief that comets had a significant percentage of organic molecules was well ahead of his time, as the dominant views in the 1970s and 80s were that comets largely consisted of water ice, so the presence of organic matter was considered highly controversial. What was needed was a spacecraft to test the idea by visiting a comet, collect a sample of cometary dust, and return it safely to the earth for analysis. On 7 February 1999, NASA launched its spacecraft named Stardust towards comet Wild 2. It carried four science instruments, along with the Stardust Sample Collection system or SSE. The particle collector uses aerogel, a low-density, inert, micro porous, silica-based substance, to capture dust grains as the spacecraft passed through the coma of Wild 2. After the experiment was complete, the collector would recede into the Sample Return Capsule
for entering the Earth's atmosphere. The capsule with the enclosed samples would then be retrieved from the Earth's surface and studied. On January 2, 2004, Stardust encountered Comet Wild 2. The relative velocity between the comet and the spacecraft was such that the comet actually overtook the spacecraft from behind as they travelled around the Sun. During the encounter, the spacecraft was on the sun-lit side of the nucleus, approaching at a speed of 4 miles a second at a distance of 147 miles. On January 16, 2006, the Sample Return Capsule successfully separated from Stardust and re-entered the Earth's atmosphere. Here is Senior Research scientist Hermann Bohnhardt. Video Glycine has been detected in meteorites before, and there have been astronomical observations, which have detected it in interstellar gas clouds. However, the Stardust discovery is described as a first in cometary material. Analysis indicates that the Late Heavy Bombardment of the young planets 4.6 billion years ago included cometary impacts after the Earth formed, and before life evolved. Carl Pilcher, who leads NASA's Astrobiology Institute, commented, "The discovery of Glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the Universe may be common rather than rare." One surprise, which was announced in Mid August 2014 by the Stardust team, is that several of the tiny dust grains actually originate from outside the Solar System in inter stellar space, although it is not clear if they come from within the Oort cloud. This brings us back to Rosetta, which is now orbiting comet 67P/Churyumov–Gerasimenko. Rosetta’s mass spectrometer COSIMA, managed by the Max Planck Institute for Solar System Research in Germany, is now beginning to detect cometary dust. On Sunday 10 August, COSIMA exposed the first of 24 target holders aiming to collect single dust particles. The number is likely to be small now because of the comet's great distance from the Sun. However, as 67P travels closer to the Sun along its orbit, the comet’s activity will increase and more dust will be within reach. For now, scientists are planning to keep the target exposed for one month, and they will check on a weekly basis to see if small dust grains have attached to the detector. The detector consists of a gold plate covered by a 30 µm thick layer of “metal black” (gold in this case), which has a very low light reflecting ability. Tests in the laboratory have shown that this layer should decelerate and capture cometary dust particles impacting with velocities of 225 mph, five times faster than a speeding car. An international team of scientists using the Atacama Large Millimetre/submillimeter Array (ALMA) at the European Southern Observatory in Chile has made incredible 3D images of the ghostly atmospheres surrounding comets ISON and Lemmon. These new observations provide important insights into how and where comets forge new chemicals, including intriguing organic compounds. ALMA'S high-resolution observations provided a tantalizing 3D perspective of the distribution of the molecules within these two cometary atmospheres, or comas. The critical 3D component of the ALMA observations was made by combining highresolution, two-dimensional images of the comets with high-resolution spectra obtained from three important organic molecules—hydrogen cyanide (HCN), hydrogen isocyanide (HNC), and formaldehyde (H2CO). These spectra were taken at every point in each image. They
identified not only the molecules present but also their velocities, which provided the third dimension, indicating the depths of the cometary Comas. The new results reveal that hydrogen cyanide gas flows outward from the nucleus quite evenly in all directions, whereas hydrogen isocyanide is concentrated in clumps and jets. ALMA'S exquisite resolution could clearly resolve these clumps moving into different regions of the cometary comas on a day-to-day and even hour-to-hour basis. These distinctive patterns confirm that the hydrogen isocyanide, and formaldehyde molecules, actually form within the coma and provide new evidence that hydrogen isocyanide may be produced by the breakdown of large molecules or organic dust. Michael Mumma, director of the Goddard Centre for Astrobiology and a co-author on the study explained: "Understanding organic dust is important, because such materials are more resistant to destruction during atmospheric entry, and some could have been delivered intact to the early Earth, thereby fuelling the emergence of life. These observations open a new window on this poorly known component of cometary organics." Therefore, not only does ALMA let us identify individual molecules in the coma, it also gives us the ability to map their locations with great sensitivity. If planetary scientists of the European Space Agency are lucky, Rosetta will also be able to detect genetic material such as Glycine in the collected dust grains of comet 67P, with confirmation coming from the Rosetta Lander Philae. So there is much to look forward to over the next 12 months, and I am quite sure there will be a number of surprises along the way, as Rosetta follows comet 67P around the Sun. If you would like to be involved in observing comet Churyumov–Gerasimenko, please visit the PACA_Rosetta67P group on Face Book. That is all we have time for this month. Please visit our web site Vimeo, where you can watch all of the past shows of Astronomy & Space. In addition, if you like this program, please share it with your friends, and members of your local astronomical society. Until we come back next month, Good Evening.
Comet temple 1
Comet Wild 2