STEM Today

STEM TODAY January 2017, No.16

STEM TODAY January 2017 , No.16

CONTENTS Cancer 11: What are the most effective shielding approaches to mitigate cancer risks?

Part­1 Radiation Environment in Space Apollo Skylab

Part­2 February 2017 , No.17

Editorial Editor: Mr. Abhishek Kumar Sinha Editor / Technical Advisor: Mr. Martin Cabaniss

STEM Today, January 2017, No.16

Cover Page Space Timekeeping: NASA’s SDO Adds Leap Second to Master Clock Images from NASA’s Solar Dynamics Observatory - such as this one showing the sun as it appears in wavelengths of extreme ultraviolet light - have a time stamp showing Universal Time on it. To maintain accuracy, SDO will join official clocks around the world in adding a leap second on Dec. 31, 2016.

Image Credit: NASA/SDO

Back Cover NASA Releases Images of 1st Notable Solar Flare of 2015 An M-class solar flare erupts from the right side of the sun in this image from shortly before midnight EST on Jan. 12, 2015. The image blends two wavelengths of light – 171 and 304 angstroms – as captured by NASA’s Solar Dynamics Observatory.

Image Credit: NASA/SDO

STEM Today , January 2017

Editorial Dear Reader

STEM Today, January 2017, No.16

All young people should be prepared to think deeply and to think well so that they have the chance to become the innovators, educators, researchers, and leaders who can solve the most pressing challenges facing our world, both today and tomorrow. But, right now, not enough of our youth have access to quality STEM learning opportunities and too few students see these disciplines as springboards for their careers.

According to Marillyn Hewson, "Our children - the elementary, middle and high school students of today - make up a generation that will change our universe forever. This is the generation that will walk on Mars, explore deep space and unlock mysteries that we can’t yet imagine". "They won’t get there alone. It is our job to prepare, inspire and equip them to build the future - and that’s exactly what Generation Beyond is designed to do."

STEM Today will inspire and educate people about Spaceflight and effects of Spaceflight on Astronauts.

Editor Mr. Abhishek Kumar Sinha

Editorial Dear Reader The Science, Technology, Engineering and Math (STEM) program is designed to inspire the next generation of innovators, explorers, inventors and pioneers to pursue STEM careers.

According to President Barack Obama, "[Science] is more than a school subject, or the periodic table, or the properties of waves. It is an approach to the world, a critical way to understand and explore and engage with the world, and then have the capacity to change that world..."

STEM Today addresses the inadequate number of teachers skilled to educate in Human Spaceflight. It will prepare , inspire and educate teachers about Spaceflight. STEM Today will focus on NASA’S Human Research Roadmap. It will research on long duration spaceflight and put together latest research in Human Spaceflight in its monthly newsletter.

Editor / Technical Advisor Mr. Martin Cabaniss

STEM Today, January 2017, No.16

SPACE RADIATION Cancer 11: What are the most e ective shielding approaches to mitigate cancer risks?

This gap addresses speci c issues associated with development of shielding optimization methodologies to accurately evaluate radiation protection strategies while accounting for the inherent multiple uncertainties associated with the problem. For most shield design e orts, e ective dose for speci c GCR or SPE radiation design environments has been calculated in order to determine the e ectiveness of di erent shielding con gurations to meet speci c vehicle radiation requirements. However, additional methodologies incorporating probabilistic models are needed to reduce uncertainty in optimization calculations to properly assess the cost (mass savings) vs. bene t of further reducing exposures as required by ALARA. Such integrated optimization methodologies for ALARA do not currently exist.

Special Edition on Space Radiation

STEM Today, January 2017, No.16

The radiation field in space is very different from environmental radiation fields on Earth, both with respect to the various types of radiation involved and their intensities. The term ’space’ generally means the galactic space outside of the Earth’s aviation altitudes. The primary radiation field on the Earth’s surface is composed of low-linear energy transfer (LET) radiations with small high-LET components including neutrons from cosmic radiation and alpha particles emitted from terrestrial radionuclides. In contrast, the primary radiation field in space includes electrons, protons, neutrons, alpha particles, and heavy ions up to very high energies. Additional secondary radiations (e.g. gamma radiation, electrons, muons, neutrons, pions, and collision and projectile fragments) are produced by interactions within the materials of a spacecraft and its equipment and the astronauts themselves.

In manned space flights, astronauts may experience three different exposure conditions. The first situation is in low Earth orbits (LEOs) where they are protected against low-energy particles of galactic and solar origin depending on the inclination of the spacecraft in the Earth’s magnetic field. However, this magnetic field is responsible for the formation of trapped radiation belts, and, in LEOs, astronauts are exposed to albedo radiation particles created through interactions of the solar and galactic particles with the nuclei of the EarthŠs atmosphere. Secondly, when leaving the Earth’s magnetic field to deep space in interplanetary missions, the radiation exposure is due solely to particles of solar and galactic origin, which are directly incident on the spacecraft. Thirdly, in planetary missions, radiation from approximately one hemisphere is shielded by the mass of the planet. If there is no atmosphere, the primary radiation interacts with the nuclei of the soil, which leads to the production of secondary particles with a high contribution of high-LET components. If a thin atmosphere is present, as in the case of Mars, both interactions in the atmosphere and in the soil contribute to secondary radiation. Additionally, astronauts can be exposed sporadically to high-energy electrons and protons from the Sun in solar particle events (SPEs).

Astronauts live and work in LEOs for extended periods of time, and are involved in deep space missions. They live under environmental conditions that are exceptionally different from those encountered on Earth. For missions outside the magnetosphere, ionising radiation is recognised as the key factor through its impact on the crew’s health and performance. Obviously, the radiation environment is quite different from that on Earth; human exposure in space is much higher than on Earth and, apart from SPEs, cannot be avoided by shielding. This is due to extremely high energies of particles in space radiation fields and their high penetration depth in matter, combined with the release of secondary radiations (e.g. fragments, neutrons, and photons) in interactions of the primary radiation with that material. The exposure of astronauts in space is a special case of environmental exposure and is defined as an existing exposure situation by the Commission. In long-term missions, the exposure of astronauts will be higher than the annual limits recommended for exposure of workers on Earth. Publication 103 (ICRP, 2007) stated that ’Exceptional cases of cosmic radiation exposures, such as exposure in space travel, where doses may be significant and some type of control warranted, should be dealt with separately’. Therefore, although astronauts are exposed to ionising radiation during their occupational activities, they are not usually classified as being occupationally exposed in the sense of the ICRP system for radiation protection of workers on Earth and aircraft crew. Thus, for a specific mission, reference values for doses or risks may be selected at appropriate levels, and no dose limits may be applied for the given mission. The more risk-related approach of exposure assessment presented in this report is clearly restricted to the special situation in space, and should not be applied to any other exposure situation on Earth.

Over the last two decades, there has been marked development in activities in space, including an increase in the number of astronauts participating in space missions. Nevertheless, even today, the number of astronauts is small compared with the large number of occupationally exposed persons on Earth and in civil aviation. However, considering the extraordinary exposure situation of this group, radiological protection concepts need to be well defined and realistically implemented with respect to the specific situation found in the space environment and during long-term space missions. The basis for any measure in radiological protection should always be knowledge of the radiation fields involved. Therefore, measurement of environmental radiation and assessment of the exposure of astronauts are very important tasks. Given the discovery of cosmic radiation by Hess in 1912 (Compton, 1936), the study of cosmic radiation and its various components has been underway for a long time.

This has become even more important over the last 50 years, as activities in space have increased and frequently include the presence of astronauts. Obviously, the basic information regarding cosmic radiations and their various components can only be obtained through measurements, and this has been carried out for many years. The specific environmental situation in and around a spacecraft can be estimated either by various measurements at different positions in the specific spacecraft, or by radiation transport calculations when the spacecraft design is sufficiently modelled and the specific composition of the external radiation field, including its variation in time, is appropriately considered within the simulation code applied.

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Special Edition on Space Radiation The specific radiation field in space with its important contribution of heavy ions does not allow simple application of the complete system of dosimetric quantities defined for use in radiological protection on Earth.

The radiation weighting factor of 20 defined for all heavy ions of all energies is not appropriate in space. In addition, the concept of operational dose quantities for external exposure situations is not applicable to the space situation because very-high-energy particles are involved. The concept of operational quantities for external exposure has been introduced by ICRU and ICRP, mainly looking at electron, photon, and neutron radiations of energies up to a few tens of MeV, and has not considered radiation fields in space which include many other particle types with even higher energies.

STEM Today, January 2017, No.16

On Earth, radiological protection of workers and the primary dose limits defined are aimed at limiting the probability of the occurrence of stochastic effects (e.g. risk of cancer or hereditary effects) to an acceptable level compared with other health risks during human life, while at the same time avoiding detriments in humans by deterministic effects (tissue reactions). For practical reasons, the primary limits are defined in terms of doses (effective dose and equivalent dose to the skin, hands, feet, and lens of the eye where specific limits have been defined for avoiding deterministic effects) which can be assessed with sufficient precision for applications in radiological protection, and not in terms of radiation risks, the value of which depends on many individual factors (e.g. age, sex, individual genetic properties). Especially at low levels of exposure, knowledge regarding these risks is very limited and combined with high uncertainties. The value of effective dose is calculated by averaging organ equivalent doses over both sexes and using mean values of weighting factors obtained from epidemiological data, hence from large groups of exposed and unexposed persons. Effective dose should, therefore, not be used for the assessment of individual risks. In addition to ’limitation’ of doses and risks, the principle of optimisation is generally applied in radiological protection, which means that even below exposure limits, optimisation of radiation protection always needs to be considered and might require further measures.

The situation in space is quite different; exposure of astronauts by environmental radiation cannot be avoided in space, and prevention by shielding cannot be completely achieved. Nevertheless, optimisation of radiation protection is an important task, especially because doses to astronauts might exceed 100 mSv in long-term missions. The occurrence of deterministic effects cannot generally be excluded. In addition, the knowledge of radiobiological effects of cosmic radiation, particularly heavy ions, is very limited. The number of individuals involved is small, and hence individual risk assessment is of much higher interest. As a consequence, values of mean absorbed doses in organs and tissues of the human body play an important role, since the weighting factors used in the definition of effective dose or equivalent dose in an organ or tissue are not appropriate in the radiation field in space. In addition, for many years, the use of organ dose equivalent has been preferred by many space agencies instead of the quantity ’equivalent dose in an organ or tissue’.

Radiation monitoring in the spacecraft environment and assessment of doses in the human body of astronauts are important parts of the radiological protection measures for space missions. Due to the complex radiation field and the special requirements for use in space flight, radiation monitoring needs specific measurement devices and procedures. Usually, more than one type of dosimeter is needed for this task, and additional calculations are often necessary to interpret device response.

The calculation of conversion coefficients that relate values of particle fluence or dose external to the human body to values of absorbed dose and mean quality factors in organs and tissues within the body is an important task, often used for the assessment of doses in the body from external measurements. While reference data on conversion coefficients related to the reference voxel phantoms defined in Publication 110 (ICRP, 2009) were published in Publication 116 (ICRP, 2010), data for heavy ions have only become available recently (Sato et al., 2010). In the present report, data are presented for isotropic irradiation of both male and female voxel phantoms. Omnidirectional irradiation (isotropic) is the most realistic exposure situation in space. While shielding effects may result in less isotropic exposure, the movement of the astronauts within the spacecraft balances the situation. Hence, data are presented for isotropic irradiation alone. The use of conversion coefficients is, however, not the only method for assessing organ doses in the body. Based on knowledge of the radiation field outside a spacecraft, calculation of organ doses can be performed, including full radiation transport through the walls and the equipment of a spacecraft and through the body of the astronaut.

On Earth, biological dosimetry is mainly restricted to applications in accidental exposure situations due to the usually low doses of occupationally exposed workers and the difficulty in measuring doses below approximately 50 mSv by this method with acceptable uncertainty; however, the situation in space is quite different. Mission doses may be above this ’threshold’, and biological dosimetry (e.g. study of biological effects on lymphocytes in the human body) allows a very individual assessment if the individual sensitivity of the astronaut is determined in advance and individual calibration is performed.

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Special Edition on Space Radiation The radiation environment in space is a complex mixture of particles of solar and galactic origin with a broad range of energies. For radiological protection, the relevant radiation fields are galactic cosmic radiation (GCR), particles ejected from the Sun during solar particle events (SPE), and secondary radiation produced through interaction with the planet’s atmospheric nuclei. Solar wind particles, even when enhanced due to higher solar activity, do not contribute significantly to radiation exposure of humans due to their relatively low energy and hence their absorption in already very thin shielding materials. Nevertheless, the magnetic field associated with the solar wind modulates the fluence rate of GCR in the energy range below approximately 1 GeV/u. During phases of higher solar activity, the cosmic radiation fluence rate is decreased by a factor of three to four compared with phases of minimum solar activity.

Presently, there is no measurable contribution to radiation exposure by primary electromagnetic ionising radiation, such as from solar x-ray flares as occurred on 4 November 2003 coordinated universal time (UTC) 19:29, or from conspicuous extreme gamma radiation bursts such as occurred on 27 December 2004 UTC 21:30:26.55.

This effect has, therefore, been ignored, although on a geological time scale, its impact on the biosphere may have been significant. Secondary electromagnetic radiation contributes as bremsstrahlung emitted from charged particles upon penetration through matter, and as gamma radiation from the decay of neutral pions π0 created in the Earth’s atmosphere.

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From the point of view of radiological protection, the focus is on the particulate components of primary space radiation of ions and electrons. Other particle types, e.g. neutrons and pions, however, came in from radiation interactions with spacecraft material and the bodies of astronauts. Electrons might become relevant during extravehicular activities (EVAs) or if manned activities in the outer radiation belts become an issue; however, this will not be the case for the foreseeable future.

Exposure to cosmic radiation on the Earth’s surface is reduced to a low level due to the Earth’s magnetic field and an atmospheric shield with a thickness of approximately 1000 g cm−2 . Leaving Earth, astronauts are shielded by the structure of the spacecraft and its interior by an average of approximately 20 g cm−2 , a shielding close to that of the Martian atmosphere; and in LEOs, they are still protected by the Earth’s magnetic field which limits even exposure to solar energetic particles to a level far below the cause of early radiation effects in man.

In the absence of sporadic SPEs, the radiation exposure in LEOs the radiation exposure inside spacecraft is determined by GCR (protons and heavier ions) and trapped protons with a dominant contribution from those inside the South Atlantic anomaly (SAA), an area where the radiation belt comes closer to the Earth’s surface due to displacement of the magnetic dipole axes from the Earth’s centre. In addition, an albedo source of neutrons is produced as the interaction product of the primary galactic particles with the nuclei of the Earth’s atmosphere. Outside the spacecraft, the exposure of astronauts is dominated by the electrons of the horns of the radiation belt, located at approximately 60◦ latitude in Polar regions.

All these radiations from different sources and their interactions by various mechanisms determine the actual field of ionising radiation at any given time and location within the heliosphere. Its complexity is unrivalled by anything known from terrestrial experience. The radiation field inside a spacecraft is even more complex due to the interaction of the high-energy particles with the spacecraft shielding material and the body tissues of the astronauts.

In deep space missions, the Earth’s radiation belts will be crossed in a couple of minutes and therefore their contribution to the astronauts’ radiation exposure is quite small. However, the subsequent protection by the Earth’s magnetic field is then lost, leaving only mission planning and shielding measures as a means of exposure reduction. The following sections describe the radiation field in space and the interaction of the charged particles with the magnetic field and shielding materials. Some numbers are given for radiation exposure in LEOs and in interplanetary missions.

Three major primary sources of radiation can be specified in space: • The solar system with the Sun at its centre is embedded in a complex mixture of ionising radiation, GCR, which enters the heliosphere continuously from all directions. Inside the heliosphere, the GCR fluence rate and particle energy distributions are modulated by the interplanetary magnetic field produced by the charged particles emitted continuously by the Sun, the so-called ’solar wind’.

• In addition to the solar wind, the Sun occasionally emits unusually large pulses of energetic particles, mostly protons and electrons with a small and variable contribution from helium and heavy ions, ejected into space by these solar eruptions. The most significant of these SPEs are produced by the expulsion of large amounts of material in coronal-mass ejections.

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• Celestial bodies equipped with a magnetic moment like the Earth are surrounded by toroidal belts of particulate radiation that are constantly replenished by solar particles, and secondary particles caused by the interaction of solar and galactic particles with the atmosphere. Such radiation belts constitute an important third primary exposure source.

Fig. 2.1 illustrates these three sources of ionising radiation in space, their respective spatial scales, and the dominant role that the Sun plays in modifying their composition. The highest energies measured for GCR particles (Fig. 2.1) are too large to be compatible with their postulated acceleration and containment by intragalactic magnetic fields, thereby giving rise to speculations about extragalactic sources for this part and hence extending the spatial scales even further. The corresponding intensities, however, are too low to contribute substantially to radiation exposures.

In addition to their variation with location in space, the intensity and particulate composition in these fields are subject to temporal variations. As far as space radiation is concerned, two temporal scales of space weather events are relevant. Similar to the annual alternation between summer and winter of ordinary weather on Earth, there is a nearly regular change in solar activity between phases of maximal (’summer’) and minimal (’winter’) solar activity. The solar ’year’ in this case is the Schwabe cycle, a period of approximately 11 years; however, the duration (presently) varies due to as-yet-unknown mechanisms between 9 and 13.6 years. One measure of this activity, for which a continuous observational record exists since 1755, is the Zurich sunspot number.

Apparently, the maximum solar activity is inversely associated with the length of the cycle. In addition to field variation during the regular solar cycle, episodes of extreme solar activity are characterised by explosive releases of magnetic energy that eject giant masses of charged particles from the Sun’s corona into the interplanetary magnetic field. After further acceleration in this field, particle energies up to several GeV can be attained. The impact of these SPEs on the radiation field in space can last for days to weeks.

Further observed solar periodicities such as the magnetic Hale cycle of 22 years, the Gleisberg cycle of approximately 88 years, and the De Vries or Suess cycle of approximately 210 years have not yet been shown to modulate the radiation field substantially, although their impact on the biosphere is likely to be important, as reported in a recent study on glacial climate cycles.

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Special Edition on Space Radiation

GCR originates outside the solar system and is incident isotropically on Earth. There is no conclusive proof regarding the mechanisms accelerating the charged particles, and the astrophysical sites where matter becomes cosmic particle radiation. There is no information about the directional position of their sources as these particles are scrambled by irregular interstellar magnetic fields on their way towards the Earth. Due to their high energies (up to 1020 eV), they most probably originate from supernova explosions, neutron stars, pulsars, or other sources where high-energy phenomena are involved. Detected radiation consists of 98% baryons and 2% electrons. The baryonic component is composed of approximately 85% protons (hydrogen nuclei), with the remainder being alpha particles (approximately 14%) and heavier nuclei (approximately 1%). Fig. 2.2 shows the abundances of these elements up to iron relative to silicon. The ions heavier than alpha particles are termed "HZE particles" [high charge (charge numbers Z > 2) and high energy]. Although iron ions are one-tenth as abundant as carbon or oxygen, their contribution to absorbed dose in tissue is substantial as this dose is proportional to the square of the particle charge. This is indicated in Fig. 2.2 In addition to GCR, a so called "anomalous component" is observed. It consists of originally neutral particles coming from the interstellar gas that become singly ionised by solar radiation after entering the heliosphere.

These particles are then accelerated in collision regions between fast and slow-moving streams of the solar wind. They are able to penetrate deeper into the magnetic field than fully ionised cosmic particles. Their energies are approximately 20 MeV/u, and consequently they can only contribute to radiation effects behind thin shielding. However, it has to be considered that they lose all their electrons after penetration of a very small amount of shielding material, and thus also deposit energy proportional to the square of their charge number Z. Energies of GCR nuclei are presented as kinetic energy per atomic mass unit (amu or u), E. This has the advantage that all nuclei with the same value of energy per amu move with nearly the same velocity regardless of their mass. Using this energy scale, the energy distributions of the different cosmic ray nuclei are very similar. Fluence rate distributions in energy for hydrogen, helium, oxygen, and iron are shown in Fig. 2.3. At

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energies E above some GeV/u, the fluence rate is well represented by a power law N(E) ∼ E−γ with γ of approximately 2.5. Towards lower energies, the distributions get flatter and show a maximum at a few hundred MeV/u.

Fluence rates of cosmic radiation are not constant; they vary between two extremes that correspond in time with maximum and minimum solar activity. Solar activity and cosmic radiation fluence rates are inversely correlated. The slope of the energy distribution in Fig. 2.3 for energies below several GeV/u is affected by this modulation of the cosmic radiation fluence rate. It is caused by the solar magnetic field, which is coupled to the solar wind. The solar wind is a continuous stream of highly-ionised plasma emerging from the Sun. Its intensity depends on solar activity, which can be described by the number of observed sunspots.

During the minimum of the 11-year solar cycle, the solar wind has minimum strength and its effect on the energy distribution is smaller than at maximum solar activity. Cosmic particles incident on the solar system interact with the solar magnetic field and thus lose energy. This leads to flattened energy spectra at lower energies. With increasing solar activity, the maximum fluence rate is shifted to higher particle energies. At 100 MeV/u, the particle fluence rates differ by a factor of approximately 10 between maximum and minimum solar activity conditions, whereas at approximately 4 GeV/u, a variation of approximately 20% is observed.

Monitoring of solar modulation is possible on Earth based on the fluence rate of secondary neutrons produced in the Earth’s atmosphere by interactions of GCR. This fluence rate has been measured over longer periods by different ground-based stations using neutron monitors.

Fig. 2.4 shows an example of data taken over several years with the neutron monitor at Kiel University . It can be seen that details of the modulation seem to be unpredictable statistical fluctuations. However, maxima and minima clearly appear to be inversely correlated to the 11-year solar cycle with a roughly sinusoidal form around an average particle fluence rate. However, the magnitude of the extremes undergoes fluctuations. Predictions for future satellite missions are limited in accuracy within a factor of two or even more based on such unpredictable fluctuations.

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Special Edition on Space Radiation

As well as electromagnetic radiation, the Sun emits particle radiation continuously, mainly consisting of protons and electrons, the solar wind. The intensities of these low-energy particles vary by two orders of magnitude between approximately 1010 and 1012 particles cm−2 s−1 sr−1 . In terms of velocity, this particle stream is characterised by velocities between approximately 300 km s−1 and 800 km s−1 and more. The particle energies, however, are so low (for protons, between 100 eV and 3.5 keV) that the particles will be stopped within the first few microns of unshielded skin. They are, therefore, not of concern for radiation effects in humans.

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Nevertheless, the temporal variation of the solar wind is a major driver that determines radiation exposure from GCR in space, at least within the inner heliosphere. The heliosphere itself can be defined as that domain of the interstellar space which the solar wind which is filled by particles of the solar wind. The magnetic field based on the solar wind provides a similar shielding as the geomagnetic field. The shielding strength can be simulated in terms of a pseudo-electrostatic heliocentric potential against which the charged particles have to work when entering the heliosphere from the local interstellar medium. This potential modifies the GCR energy distributions to the same degree as the interplanetary magnetic field.

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Occasionally, the surface of the Sun releases large amounts of energy in sudden local outbursts of gamma radiation, hard and soft x rays, and radio waves in a wide frequency band (coronal-mass ejection). In these SPEs, large currents and moving magnetic fields in the solar corona accelerate solar matter. Coronal particles with energies up to several GeV escape into the interplanetary space. They spiral around the interplanetary magnetic field lines. Within the ecliptic plane, field lines expand from the Sun into the interplanetary medium like the stream of water from a rotating garden hose. They connect the Earth with a certain spot on the western part of the Sun. The number and energy distribution of particles observed in SPEs on Earth is different from GCR and depends on this connection. SPEs show an enormous variability in particle fluence rates and energy distribution, and have the potential to expose space crew to life-threateningly high doses.

An SPE that is well connected with high particle fluence rates observed on Earth is an infrequent event, most likely to be observed during the period of increasing and decreasing maximum solar activity. Therefore, major SPEs are observed on Earth as random events with low frequency, typically one per month. They last for several hours or days. Events with significant fluence rates of protons with higher energies can be observed as "groundlevel events" (GLEs) by neutron monitors. Fig. 2.5 shows the distribution of GLEs observed over five solar cycles.

Long gaps with no events can be seen during solar minimum activity. Between the last GLE in Cycle 21 and the first GLE in Cycle 22, there was a quiet period of 65 months followed by a sequence of 11 GLEs within 1 year with the approaching maximum of the 23rd solar cycle.

As high-energy particles arrive first and are followed by particles of lower energies, the energy distribution of SPE particles observed on Earth depends on time, t, after onset of the event. Above energies of approximately 10 MeV, SPE particle energy distributions roughly follow the power law I(E) = I0 E−γ , where I0 is the total number of particles of the type considered at time t, E is the energy of a particle, γ is a parameter, and I(E) is the distribution of the number of particles with respect to E. After the onset of the event, the exponent γ decreases with time. This means that the contribution by high-energy particles decreases with time during the event. The constant I0 shows a great deal of structure during the event caused by field irregularities and shock structures in the interplanetary medium. Due to the stochastic occurrence of SPEs, capabilities for the prediction of SPEs and their strength are very limited, and it would be very useful for long-term missions in space if the modelling and forecasting of such events could be improved.

Particles from strong SPEs can induce adverse skin reactions in astronauts if they get caught outside shielding, since above approximately 10 MeV, protons can penetrate spacesuits and reach the skin or the lens of the eye. Depending on the particle intensities, they may induce erythema or trigger late radiation cataracts within the lens of the eye. While the latter take several years to develop and hence pose no threat to safe completion of a mission, severe erythema may well induce performance decrements that could compromise mission success. Anorexia, fatigue, nausea, vomiting, and diarrhoea can also occur. They constitute the symptoms of the prodromic syndrome as the early warning signs (within hours, depending on the dose) that potentially lifethreatening doses may have been incurred. As such, these symptoms will hardly pose a serious threat, unless, for example, emesis occurred in a spacesuit.

Since 1955, five SPEs with intensities and energies large enough to jeopardise crew health behind normal or even enhanced spacecraft shielding have been observed. For these strong events, integral fluence distributions (total number of particles per area above an energy E) have been measured by satellite instruments (see Fig. 2.6). For a further event - that of 23 February 1956 - the fluence distribution has been inferred from an analysis of the count rates of terrestrial neutron monitors that recorded the induced secondary neutrons. Such enhancements of neutron count rates are monitored in a worldwide net of neutron monitor stations, a selected subset of which forms the so-called "Spaceship Earth". GLEs indicate that associated SPE protons with energies above approximately 450 MeV were sufficiently numerous to raise the neutron fluence rate at sea level by at least 5%.

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Special Edition on Space Radiation

A comprehensive list of GLEs observed since 23 February 1956 (GLE No. 5) and 14 July 2000 (GLE No. 59), together with all neutron monitor stations where these events were observed, is kept by the Australian Antarctica Data Centre (http://data.aad.gov.au/aadc/gle/events.cfm). Among all these GLEs, the enhancement by GLE No. 5 measured in Leeds (lat. 53.83 N, long. 358.42 E, alt. 100 m, Pc = 2.20 GV) is approximately 4600% higher than the pre-event count rate, whereas for other SPEs, the enhancement very rarely exceeds a 100% increase.

A small amount of solar particles with low energies also reach the Earth from SPEs at other positions of the Sun which are not fully directed at the Earth. These fluence rates add up to a solar component that dominates over the galactic component at energies below 30 MeV/u. Depending on the conditions of the interplanetary medium, this component undergoes fluctuations that are highly variable and unpredictable. During periods of maximum solar activity, when the fluence rate of GCR is depressed and SPEs are more frequent, the contribution of the solar component is more significant.

For long-term mission planning, in addition to the magnitude that a worstcase event can attain, the frequency of occurrence of events and the proton energy distribution are also important. Fig. 2.7 gives the probability that a particle event with protons of energies above 30 MeV will occur, based on the random nature of SPE occurrence and event size, and based on the records taken for fluence measurements of the last five solar cycles.

The radiation field around the Earth comprises the third radiation source. The particles trapped in the radiation belts discovered by Van Allen are a result of the interaction of GCR and Solar cosmic radiation (SCR) with the Earth’s magnetic field and the atmosphere. The radiation belts consist of electrons, protons, and some heavier ions. Electrons reach energies of up to 7 MeV and protons up to 700 MeV. The energy of heavy ions is less than 50 MeV/u, and because of their limited penetration capacity, they are of no consequence for satellite

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electronics or radiological protection of humans. Charged particles with these energies moving into a dipole field can never enter inner areas of this field.

However, if they are put into this field for any reason, they are restricted to certain positions and cannot escape. They move in spirals along the geomagnetic field lines and are reflected back between the magnetic poles, acting as mirror points. Different processes contribute to fill in particles in the radiation belt, and two main zones of captured particles are observed. The inner belt is mainly formed by decaying neutrons, coming from the atmosphere in which they are produced in cosmic particle interactions, and producing protons and electrons. The outer belt consists mainly of trapped solar particles, and is populated largely by electrons. During disturbance of the magnetosphere by magnetic storms related to solar flares, where the geomagnetic cut-off is usually depressed, particles of lower energies can penetrate from outside towards the inner regions and fill them. The radiation belts extend over a distance from Earth from approximately 200 km to approximately 75,000 km around the geomagnetic equator. Energy loss by cyclotron radiation and by penetration into the upper atmosphere near the geomagnetic mirror points constitutes the major loss mechanisms for the trapped particle population.

Extensive measurements during recent decades with more advanced and dedicated instrumentation on several satellites in well-coordinated orbits yielded the main quantitative database which then became integrated in the AP-8 trapped proton model , which provides energy distributions of average proton fluence rates during quiet magnetospheric conditions. A major application of the AP-8 model is the assessment of radiation exposure from trapped radiation during manned LEO missions such as on the International Space Station (ISS). The AE-8 trapped electron model serves the same purpose of prediction of radiation doses, mainly for the radiation environment in geostationary orbits where energetic electrons constitute the dominant source of ionising radiation.

An improved AE-9/AP-9 model is being developed as part of the Proton Spectrometer Belt Research (PSBR) Program, and is planned to be released in the near future by a consortium of institutions, including the National Reconnaissance Office, Aerospace Cooperation, Air Force Research Laboratory, Los Alamos National Laboratory, Ë? west effect in trapped proton fluence rates. At the bottom and Naval Research Laboratory. There is a strong eastU of their path around the magnetic field lines, protons are travelling eastwards, whereas those at the top of their path are travelling westwards. The westwardtravelling particles have emerged from a region of the atmosphere at lower altitude. Therefore, they encounter a denser atmosphere and are more efficiently removed by interac-

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tions with the nuclei of the atmosphere.

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Fig. 2.8 displays the spatial distribution of electron fluence rate for electron energies above 0.5 MeV (right) and of proton fluence rate for proton energies above 4 MeV (left), at which energy the latter are able to penetrate approximately 1.4 g cm−2 Al (ICRU, 1993a), the shielding provided by lighter spacecraft. Proton fluence rates in the inner belt are intense, and protons reach sufficient energies to penetrate the shielding provided by walls and equipment of spacecraft. Their energy distribution, as shown in Fig. 2.9, needs to be known in order to assess radiation exposures of astronauts. The data in Fig. 2.9 are the results of measurements of the energy distribution of trapped proton fluence rate in the early 1960s. The B,L coordinates are used as a natural coordinate system to specify the satellite position within the geomagnetic field. Here, B denotes the magnetic field strength at a given point, and L denotes the altitude in units of Earth radii at which the magnetic field line through this point intersects the plane through the geomagnetic equator.

Fig. 2.10 shows the fluence rate energy distributions of trapped electrons and protons averaged over the orbit of the Hubble Space Telescope. Electron fluence rates during the solar maximum are greater than during the solar minimum, pointing to the Sun as the dominant primary source that feeds the trapped electron population.

In contrast, the trapped proton fluence rates reflect the (Forbush) modulation of GCR intensity by the solar wind, which results in higher intensities during solar minimum conditions.

The fluence rates and energy distributions shown pertain to quiet magnetic conditions of the terrestrial and interplanetary magnetic fields during minimum and maximum solar activity. In addition to the regular solar cycle variation, both magnetic storms and intensive fluence rates from energetic SPEs significantly shift positions and energies of trapped particle populations so that additional, although transient, radiation belts can be created. The trapped radiation is modulated by the solar cycle; with increasing solar activity, proton intensity decreases, while electron intensity increases. Diurnal variations by a factor of between 6 and 16 are observed in the outer electron belt, and short-term variations due to magnetic storms may raise the average fluence rate by two or three orders of magnitude. The centre of the inner belt is quite stable, especially with respect to protons.

However, at the lower edge of the belt, electron and proton intensities may vary by up to a factor of 5. For the majority of LEO space missions, protons are an important part of the radiation exposure inside spacecraft. Due to their higher energies and correspondingly longer range, their total dose surpasses that of electrons at shielding thicknesses above approximately 0.3 g cm−2 Al. At lower shielding (e.g. in the case of EVAs), the absorbed dose to the skin is dominated by the electron contribution, and may reach up to 10 mGy per day.

Of special importance for LEOs is the South Atlantic Anomaly (SAA), a region over the coast of Brazil where the radiation belt extends down to altitudes of 200 km. This behaviour is due to an 11◦ inclination of the Earth’s geomagnetic dipole axis from its axis of rotation towards North America and a 500 km displacement of the dipole centre towards the western Pacific, with corresponding significantly reduced field strength values. Radiation received in LEOs at low inclinations includes GCR and that due to passages through the SAA. At an orbit

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with 28.5◦ inclination, six orbital rotations per day pass through the SAA, while nine orbital rotations per day do not pass through the SAA. Although traversing the SAA takes less than approximately 15 min and occupies less than 10% of the time in orbit, this region accounts for a significant fraction of total exposure.

In addition to the trapped charged particles in radiation belts, GCR produces secondary neutrons by nuclear reactions in the upper atmosphere of the Earth. Neutrons are produced in two energy regions by two processes.

Neutrons in the region between 1 and 10 MeV are mainly evaporation products of highly excited nuclei with a fairly isotropic angular distribution. High-energy neutrons originate as knock-on neutrons mainly in peripheral collisions, or in charge exchange reactions of high energy protons. Their energy distribution peaks at approximately 100 MeV. They leak into the exosphere and also contribute to the exposure in spacecraft. Measured neutron energy distributions in the Earth’s atmosphere are shown in Fig. 2.11. Their contribution to the radiation field in LEOs is, however, relatively low. A similar neutron field as measured in the atmosphere is produced by interactions of GCR with the spacecraft material and the astronaut’s body . This contribution to the exposure of astronauts is substantial.

To reach spacecraft in LEOs, a charged particle from GCR or the solar cosmic radiation (SCR) has to penetrate the Earth’s magnetic field. Penetrability is a property related to the ion’s magnetic rigidity, which is given by its momentum divided by its charge. All particles with the same rigidity follow a track with the same curvature in a given magnetic field. For each point inside the magnetosphere and each direction from that point, there is a rigidity threshold below which the cosmic particles are not able to reach this point. This rigidity is called the "geomagnetic cutoff rigidity" and is proportional to the magnetic field component perpendicular to the direction of particle motion. For a particle moving towards the centre of the Earth, for example, the cutoff rigidity has a maximum value at the equator, since the particle moves perpendicular to the field lines and the cut-off rigidity vanishes at the pole, since the particle moves in the direction of the field lines. Therefore, geomagnetic shielding is less effective for high inclination orbits than for low inclination orbits. This means that in low inclination orbits, only particles of high energy have access. Towards higher inclinations, additional particles of lower energies are observed. For a geomagnetic latitude λ, the vertical cut-off rigidity Rc can be calculated approximately by Rc = 14.9 cos4 λ/(r/re )2 , where r/re is the ratio of the distance r from the dipole centre to the Earth’s radius, re . The rigidity for particles arriving from directions other than vertical is dependent on the angle of incidence. Due to latitude-dependent shielding, the number of particles incident in the altitude

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of orbiting spacecraft increases from lower inclinations to higher inclinations.

In LEOs, as well as at aviation heights, a second shielding mechanism has to be incorporated into the transport of the primary GCR or SPE ions. Whereas the geomagnetic field, on the one hand, is responsible for the added radiation exposure in LEOs from trapped radiation, it also causes a fairly substantial reduction in radiation exposure, at least near the geomagnetic equator (which differs from the geographic equator).This stems from the deflection due to the Lorentz force of charged particles by the geomagnetic field, as illustrated in Fig.2.12.

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Fig. 2.13 provides a global map of the vertical cut-off rigidities for the geomagnetic field model of Epoch (2000.0). Epoch is a moment in time used as a reference point. Epoch 2000.0 means the date 2000, Jan 1.5 (12h on January 1). For a homogeneous dipole field, the iso-rigidity lines would be parallel to the (geomagnetic) equator.

The marked asymmetry with a peak above 17 GV of the cut-off rigidity at the Indian Ocean (long. 90 E, lat.

10 N) reflects the offset from the geographic centre of the magnetic centre by approximately 450 km in this direction.

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At the opposite side, in the South Atlantic, this offset results in the corresponding subsidence of the lower fringes of the inner proton belt, thereby creating the SAA. This explains why the bulk of radiation exposure in most LEOs is accumulated in this region.

For a given orbit, the shielding due to this effect is expressed by the geomagnetic transmission factor which specifies the fraction of the GCR or solar particle fluence rate of a given particle energy (or momentum given in units of GeV c−1 ) that has access to this orbit, or by the cut-off rigidity probability that specifies the probability that a particle with a given rigidity reaches that orbit.

Fig. 2.14(a) demonstrates the dependence of the geomagnetic shielding on the orbit inclination for a circular orbit at 223 km altitude. For an orbit of 28.5◦ inclination which evades the SAA for a large fraction, GCR with a momentum below approximately 4.2 GeV c−1 does not generally reach the flight route.

For a 45◦ inclination, this momentum threshold drops to approximately 1.1 GeV c−1 , and for polar orbits, at least 20% of particles with the lowest energies always have access to this altitude. On the other hand, the shielding effect vanishes for ions with a momentum above approximately 15 GeV c−1 , where at any inclination, all charged ions reach this orbit.

Fig. 2.14(b) shows the influence of the magnetic shielding on the particle spectra (e.g. for Fe), which varies strongly between differing inclinations. The functions in Fig. 2.14 do not, however, include the shadow effect of the Earth itself.

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Special Edition on Space Radiation The shadow effect of the Earth for the Hubble Space Telescope at a 28.5◦ inclination reduces the fluence rate of even the most energetic GCR by approximately 30%. An Earth observation satellite (e.g. TERRA), on the other hand, must use a near polar orbit and therefore can be accessed by charged particles of all energies.

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Its higher altitude also slightly reduces the shielding by the Earth’s shadow. The high inclination of the ISS of 51.6◦ makes this manned spacecraft accessible to SPE ions of 100 MeV/u or above. This is particularly important as, in the case of geomagnetic disturbances which often accompany solar events, this geomagnetic shielding is further reduced.

Fig. 2.15 demonstrates this loss of geomagnetic shielding for the ISS for storms, as characterised by the Kp index of global geomagnetic activity (http://isgi.cetp.ipsl.fr/des_kp_ind.html) which can vary between 0 and 9.

Under such storm conditions, a much larger fraction of SPE ions can reach the orbit of the ISS.

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Data acquired by the Battery-operated Independent Radiation Detector (BIRD) during Exploration Flight Test 1 (EFT-1)

The total absorbed dose for EFT-1 as measured by BIRD subunits and the associated RAMs are located in Table 1. The RAM absorbed doses represent the average for the individual TL/OSL dosimeters corresponding to each of the two RAM flight units, together with the standard error of the mean. The Left RAM and Right RAM TL/OSL dosimeter individual absorbed dose values had standard deviations of 5% and 4%, respectively.

The highest observed dose rates did not occur at the peak altitude, but instead occurred before and just after the peak altitude. This observation is explained by the changes in the spectral characteristics of the trapped radiation field along the trajectory; while still within the trapped belt region, the spectrum was comprised of lower-energy protons in the lower dose region between the two dose rate peaks.

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Dosimetric quantities as a function of the Orion MPCV trajectory are presented in this subsection.

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After analysis, the BIRD data were reformatted for use in Google Earth Pro. Figure 21 illustrates the absorbed dose rate along EFT-1’s highly eccentric orbit.

The proton flux encountered by a spacecraft as it traverses the SAA is anisotropic. Protons traveling toward the east are following geomagnetic field lines (or guiding centers) above the spacecraft’s orbit, while protons traveling toward the west are following geomagnetic field lines that lie below the spacecraft’s orbit. The radius of the cyclotron motion of energetic protons in the SAA is of the same order as the height of the atmosphere (atmospheric density scale height). Particles traveling toward the west traverse a significantly denser portion of the atmosphere than do particles traveling toward the east and are thereby more likely to undergo interactions with the atmosphere and be attenuated.

This phenomenon is referred to as the east/west trapped proton anisotropy. For spacecraft such as the Space Shuttle that typically have no fixed orientation relative to the geomagnetic field when they pass through the SAA, the effects of the trapped proton anisotropy tend to be averaged out over the duration of the mission. For spacecraft such as ISS that are in a fixed orientation relative to the geomagnetic field, the east/west trapped proton anisotropy can lead to differences of up to a factor of 3 in dose rate between the west and east sides of the spacecraft.

During EFT-1, the Orion MPCV flew through one low-altitude orbit and one highly eccentric orbit with an apogee of almost 6000 km. During this mission, the Orion MPCV did not pass through the trapped belts at altitudes similar to the ISS (the South Atlantic Anomaly, or SAA). The BIRD is unique to other space radiation instrumentation currently in use by NASA for crew health and safety, as it provides particle directionality by means of pixel detector technology. Theory predicts that for low altitudes, particle flux at lower altitudes in the trapped proton region should be anisotropic due to atmospheric shielding. For altitudes above about 1800 km, the environment should be more isotropic. The present analysis is complicated by the fact that the vehicle was rotating during certain portions of the flight. Despite this complication, the data appear to indicate directionality at lower altitudes and a more isotropic environment at higher altitudes within the trapped proton region.

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Radiation was not an operational problem during the Apollo Program

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Radiation was not an operational problem during the Apollo Program. Doses received by the crewmen of Apollo missions 7 through 17 were small because no major solar-particle events occurred during those missions. One small event was detected by a radiation sensor outside the Apollo 12 spacecraft, but no increase in radiation dose to the crewmen inside the spacecraft was detected. Solar-particle releases are random events, and it is possible that flares, with the accompanying energetic nuclear particles, might hinder future flights beyond the magnetosphere of the Earth.

It was recognized early in the Apollo program that high-energy particles from solar flares could pose a radiation hazard to the astronauts. They were especially vulnerable while they were in the thinly shielded lunar excursion module (LEM) or on the lunar surface. However, the command and service module provided enough protection to reduce exposures from solar particle events to acceptable levels. The Apollo missions were scheduled to take place during solar maximum years, when large solar particle events are more apt to occur. Research had established that virtually all particle events during solar cycle 19 were preceded by type IV solar radio bursts. However, not all type IV bursts were followed by particle events. (The same is true for solar flares observed in the hydrogen-alpha line, but there are many more flares than type IV radio bursts.) Studies carried out at MSC established a correlation between large type IV solar radio bursts and SPE size (time-integrated proton flux > 30 MeV). The radio flux was integrated over time to obtain a measure of the energy of the burst. The hypothesis was that the radio burst was produced by synchrotron radiation from electrons that are accelerated at the same time as the protons. Data from radio observatories at Ottawa, Canada (which operated at 2800 MHz) and Nagoya University, in Japan (which operated at 3000 MHz) were used for the study. Particle event data were taken from the Solar Proton Manual and a Boeing Company report.

While some solar flares produce relativistic-energy protons that can arrive in the Earth-Moon region within 30 minutes, the arrival times for most events are 4 to 6 hours after the flare and radio burst. Peak particle intensities do not occur until another 4 to 6 hours after the arrival of particles. The strategy was to use this time to move the Apollo astronauts off the lunar surface and have them return to the more heavily shielded command and service module. Information on the occurrence of a solar flare (observed by the hydrogen-alpha telescopes) and data from a large radio-frequency (10 cm wavelength) burst were transmitted back to MCC in Houston. Radiation specialists working on the radiation console, located in one of the MCC "back rooms," analyzed these data. If an event of a certain (estimated) size was believed to produce a substantial radiation dose to the astronauts, the flight director would be advised so that action could be taken to minimize their exposure.

Particle spectrometers and dosimeters onboard the Apollo spacecraft then detected the increase in the radiation environment, to verify that the particle event had arrived in cislunar space. This reduced the impact of a false alarm when a flare and type IV burst did not produce particles that propagated to cislunar space. Flight rules precluded launching into an SPE or landing on the Moon during such an event and terminated a lunar excursion if exposures were estimated to be above an acceptable level.

Fortunately, no large SPEs occurred during any Apollo mission. The large solar particle event of August 1972 occurred between the Apollo 16 and Apollo 17 missions and did not, therefore, affect them. A solar flare and radio burst occurred during the Apollo 12 mission, which had exercised the operational procedures.

In terms of hazard to crewmen in the heavy, well shielded Command Module, even one of the largest solarparticle event series on record (August 4-9, 1972) would not have caused any impairment of crewmember functions or ability of the crewmen to complete their mission safely. It is estimated that within the Command Module during this event, the crewmen would have received a dose of 360 rads to their skin and 35 rads to their blood-forming organs (bone and spleen). Radiation doses to crewmen while inside the thinly shielded Lunar Module or during an extravehicular activity (EVA) would be extremely serious for such a particle event.

Average radiation doses were computed for each mission (Table 2). Individual readings varied approximately 20 percent from the average because of differences in the shielding effectiveness of various parts of the Apollo spacecraft as well as differences in duties, movements, and locations of crewmen. Doses to blood-forming organs were approximately 40 percent lower than the values measured at the body surface. In comparison with the doses actually received, the maximum operational dose (MOD) limit for each of the Apollo missions was set at 400 rads (X-ray equivalent) to skin and 50 rads to the blood-forming organs.

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Special Edition on Space Radiation Radiation doses measured during Apollo were significantly lower than the yearly average of 5 rem set by the U.S. Atomic Energy Commission for workers who use radioactive materials in factories and institutions across the United States. Thus, radiation was not an operational problem during the Apollo Program. Doses received by the crewmen of Apollo missions 7 through 17 were small because no major solarparticle events occurred during those missions.

One small event was detected by a radiation sensor outside the Apollo 12 spacecraft, but no increase in radiation dose to the crewmen inside the spacecraft was detected.

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Apollo Light Flash Investigations

Crewmembers of the Apollo 11 mission were the first astronauts to describe an unusual visual phenomenon associated with space flight. During transearth coast, both the Commander and the Lunar Module Pilot reported seeing faint spots or flashes of light when the cabin was dark and they had become dark-adapted. It is believed that these light flashes result from high energy, heavy cosmic rays penetrating the Command Module structure and the crewmembers’ eyes. These particles are thought to be capable of producing visual sensations through interaction with the retina, either by direct deposition of ionization energy in the retina or through creation of visible light via the Cerenkov effect.

Crewmembers of Apollo 12 and 13 were questioned concerning this phenomenon during postmission debriefings. All reported the ability to "see" the flashes with relative ease when the spacecraft was dark with their eyes either open or shut. The Apollo 12 Commander stated that "There were big bright ones all over," and added that he had not seen anything similar during his two Earth-orbital Gemini missions. The Commander of the Apollo 13 mission also observed these flashes but could not remember seeing them during his earlier Apollo 8 mission.

The fact that the light flashes could be seen with eyes either open or closed suggests that the flash effect is produced by cosmic radiation penetrating the optical nervous system at some point. The fact that dark adaptation is necessary reinforces the view that the phenomenon is connected with the retina rather than with a direct stimulation of the optic nerve, since the biochemical changes associated with dark adaptation are localized in the retinal tissue.

The debriefing reports of crewmembers on the Apollo 11,12, and 13 missions led to the establishment of dedicated observing sessions on all subsequent Apollo flights. Three separate one-hour sessions were programmed for Apollo 15 and two one-hour sessions for Apollo 16 and 17. Simple blindfolds, designed to avoid corneal pressure, were used to obtain and maintain a state of complete dark adaptation during the observing session.

Crewmembers’ comments and descriptions of each event were radioed to tracking stations and simultaneously recorded on tape in the spacecraft.

The flashes were generally described as white or colorless. The only exception was the report by the Apollo 14 Lunar Module Pilot who described a flash as "blue with a white cast, like a blue diamond." Three basic types of flashes were reported. The most prevalent was the "spot" or "starlike" flash, which also has been referred to as a "super nova." Sixty six percent of the flashes were of this variety, described by the Apollo 15 Commander as resembling a photographic flashbulb that has been flashed across a dark arena, several hundred feet from the observer. The Apollo 14 LMP described the flash as being less clear than he had anticipated. "There still seemed to be at least two flashes, maybe a bright flash followed an instant later by a more subdued flash, or perhaps a halo-like effect - there does not seem to be a set pattern in each case. Sometimes it is a very clear single flash; at other times it seems to be followed by a halo. Sometimes it seems followed by an adjacent flash." On occasion, stars were reported in pairs, either both in the same eye or one star in each eye.

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The type of flash described as a "streak" was the second most abundant, occurring about 25 percent of the time. Some streaks were described as sharp lines, while others appeared to be diffuse. Still others were reported as dashed lines, with the most common version consisting of two principal segments with a gap in the middle.

All streaks had a sense of movement, appearing to be "going from left to right" or "coming straight at me." It has been hypothesized that these streaks were caused by particles with trajectories approximately tangent to the retina, and their apparent motion was due to either eye movement or the shape of the streak.

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The final type of flash was referred to as a "cloud" and occurred in eight percent of the cases. Clouds were flashes with no discernible shape and always appeared in the peripheral visual field. The Apollo 14 Command Module Pilot described the clouds as resembling a lightning discharge when viewed from behind terrestrial clouds in the distance. Some of the cloud flashes were so large as to appear to fill the entire periphery, while leaving the central visual field dark.

* A high phosphene level was reported during the first half of the session. **The crew were already dark adapted and seeing flashes when the time session began.

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Special Edition on Space Radiation ***The first seven flashes were not reported in real time the elapsed time to the first event is not available but is probably about 15 minutes. ****The total includes those not reported in real time. *****Complete event descriptions were not available.

LO = Lunar orbit CMP = Command Module Pilot LMP = Lunar Module Pilot CDR = Commander TEC = Transearth coast TLC = Translunar coast

The number of events of each type seen by each observer in individual one-hour sessions is shown in Table 1. This table also presents the elapsed time in minutes from the start of dark adaptation to the observation of the first event for sessions where that time is known. This elapsed time, that is until the first flash was seen, averages to 19.3 minutes, compared with an average event rate after dark adaptation of one event every 2.9 minutes.

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Analyses of the elapsed time between events for a particular observer, and between events for any observer, both indicate that the events seen during each one-hour session were randomly distributed in time. Further, there does not appear to be a significant preference for one eye or the other, either for a single event or for all events taken together.

It can be noted in Table I that no results are presented for the Command Module Pilot of the Apollo 16 mission.

He was the only Apollo crewmember briefed to look for the phenomenon who failed to see it. He volunteered the information that he considers his night vision to be poor.

* Dark adaptation time available for the CMP only. ** Averaged over four observers. ***Averaged over seven observers.

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Special Edition on Space Radiation An interesting feature of the light flash phenomenon is shown in Table 2. The data presented indicates the mean time between events after dark adaptation for each observer, and the average value for all observers for each session. Session averages were computed by weighting the individual values according to the corresponding dark-adapted observing times. It can be seen from the table that the average time between events was longer during transearth coast (TEC) (returning from the moon) observation periods than during translunar coast (TLC) sessions. TEC dark adaptation times (time to witness the first flash) also were considerably longer than those found during TLC sessions. In addition, most crewmembers commented that the flashes seemed not only less frequent during the TEC sessions but also much less brilliant. The most dramatic example of this difference occurred on Apollo 17, when all crewmen reported that no events were seen during the entire one-hour transearth coast session. During a similar translunar coast session, the two observing crewmen reported a total of 28 events.

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A system was developed for the Apollo 16 and 17 missions to obtain, for the first time, a direct physical record of incident cosmic ray particles which would allow correlation with crewmembers’ reports of light flashes. The measurement system is known as the Apollo Light Flash Moving Emulsion Detector (ALFMED).

The ALFMED was an electromechanical helmet-like device that supported cosmic radiation-sensitive emulsions around the head of the test subject. A direct physical record was provided of cosmic ray particles that passed through the emulsion plates and, in turn, through the head of the subject. The ALFMED contained two sets of glass plates coated on both sides with nuclear emulsion and supported in a protective framework. One set of plates was fixed in position within the headset and surrounded the front and sides of the head. A second similar set of plates was located exterior and parallel to the inner fixed plates and could be translated at a constant rate (10µ/sec) with respect to the fixed plates, thereby providing a time resolution for events to within one second.

The total translation time available was 60 minutes, after which the moving plates could be returned to the original or reference orientation.

The ALFMED film plates for the Apollo 16 mission were processed immediately following the flight and examined extensively at that time. The ALFMED fixed plates used for the flight had 200 µm-thick emulsion on both surfaces while the moving plates had 50 µm-thick emulsions. Thus the total emulsion thickness was approximately 500 µm. This, coupled with the extremely high particle flux experienced during the Apollo 16 mission (the highest for any of the Apollo missions), made it quite difficult to scan the plates as originally planned. It was therefore decided that, due to the delays involved, it would be advantageous to proceed with the Apollo 17 analysis first. Experience gained during Apollo 17 analysis procedures then might be used to improve Apollo 16 analysis techniques.

As a result of the difficulties in scanning the Apollo 16 plates, the Apollo 17 plates were flown with 50 µmthick layers on both sides, giving a total emulsion thickness of 200 µm. This greatly improved track detectability.

Analysis of the Apollo 17 plates yielded a total of 2360 individual tracks with directions that appeared approximately correct for passage through the eye of the astronaut. Of these tracks, 483 did not initially appear to have positional counterparts in the moving plates. These particles were all considered candidates for events which occurred during the period of observation and while the moving plates were displaced from their reference orientation.

Of the 483 tracks, 229 were in the front plate. Detailed trajectory measurements on the 229 front plate candidates revealed that 65 of that number passed through one eye or the other (or both). (Since the front plate was scanned first in each analysis step, the efficiencies for the various steps were probably somewhat less than those for the side plates.) Upon careful inspection, 50 of the 65 eye-directed tracks were found to have alined counterparts in the moving plate for the reference orientation, thereby reducing the front-plate sample to 15 genuine candidates.

The Monte Carlo calculations predict that one should expect approximately 30 Z ≥ 12 candidate tracks in the front plate which originated during the translation period. The current number of 15 candidates indicates that most probably the first scan efficiency for such tracks is roughly 50 percent. Researchers consider it unlikely that any Z ≤ 8 events are included in this first-scan sample, but experience leads us to believe that a rescan will result in a considerably improved efficiency, especially for the smaller charges.

Two of the 15 genuine candidates were found to coincide, to within five seconds, with reported flash observations. It is anticipated that after final measurements are made, the coincidences can be determined to accuracies of one or two seconds. The first coincidence is with the fifth light flash reported, and occurred some 1465 seconds after the plate translation began. It was described as "just a spot" in the left eye. The candidate particle

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Special Edition on Space Radiation traversed the left side of the left eye, moving upward and slightly to the right, and passed almost tangent to the retina. Detailed charge and energy measurements have not yet been completed; however, the particle was almost certainly heavier than oxygen.

The second coincidence is with the eleventh event reported, and it occurred after 2368 seconds of plate translation. The light flash was described as a glow, "about one eighth of an inch in diameter", and appeared to be three-fourths of the way out from the center to the edge of the visual field at about 10:00 o’clock in the right eye.

The second candidate trajectory passed through the right side of the right eye, heading from the front left to the right rear, and slightly upward. A rough estimate of its location as it would appear to the observer places it in the periphery at approximately the proper distance, but at 8:30 or 9:00 o’clock, rather than at 10:00 o’clock as reported. Eye movement at the time of observation might account for this minor discrepancy. This particle is also most probably heavier than oxygen.

Interplanetary Crew Dose Rates for the August 1972 Solar Particle Event

STEM Today, January 2017, No.16

The large solar particle event of August 1972 occurred between the Apollo 16 and Apollo 17 missions and did not, therefore, affect them.

Calculated organ absorbed dose rates behind 1, 2 and 5 g/cm2 aluminum shielding are displayed in Fig. 1. The shielding thickness (areal density) of 1 g/cm2 is representative of the protection provided by a typical spacesuit and 5 g/cm2 is typical of shielding provided by a manned spacecraft. Values for 2 g/cm2 of aluminum shielding are estimated to facilitate comparison between researchers predicted dose rates and those estimated previously using these same fitting techniques for the October 1989 SPE. Table 2 presents the total time during which the organ dose rates exceed some specified value. Table 3 presents the total dose accumulated for each organ during the period that the dose rate exceeds these values.

For a spacesuit (1 g/cm2 ), the dose rates are quite high. The skin dose rate peaks at nearly 1.4 Gy/h. Note from Fig. 2 that this is nearly an order of magnitude greater than the peak dose rate for the October 1989 event. In fact, the skin dose rate exceeds the NCRP criterion for "low dose rate" (0.05 Gy year−1 or 0.00057 cGy h−1 ) for over 31 h during which the accumulated skin dose is estimated to be over 15 Gy.

The skin dose rate exceeds the UNSCEAR low-doserate criterion (0.1 mGy min−1 or 0.6 cGy h−1 ) for nearly 25 h with a cumulative skin dose of about 15 Gy. The ICRP low-dose-rate criterion of 0.1 Gy h−1 (10 cGy h−1 ) is exceeded for nearly 19 h with a total skin dose accumulation of 14.8 Gy. Note also that the skin dose rate exceeds 1 Gy h−1 for over 7 h with a total skin dose accumulation of over 9 Gy. Although the eye dose rates are somewhat lower than the skin dose rates, with a peak dose rate of 0.9 Gy h−1 , the "low dose rate" criteria of all three advisory bodies are exceeded for 17 h (ICRP), nearly 24 h (UNSCEAR), and 31 h (NCRP) with cumulative eye doses ranging from 9.4 to 9.6 Gy. For the bone marrow, the dose rates are much lower due to the additional shielding provided by the overlying body tissue. The peak dose rate is 9 cGy h−1 and exceeds the low-dose-rate criteria for 25 h (NCRP) and 16 h (UNSCEAR), but not at all

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Special Edition on Space Radiation for the ICRP criterion. The bone marrow dose accumulated while exceeding the NCRP or UNSCEAR dose-rate criteria is approximately 0.8 Gy. Obviously, spacesuits will not prevent deterministic effects for SPEs as large as the August 1972 event.

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Inside a typical aluminum spacecraft (5 g/cm2 ), the organ dose rates are significantly lower, but remain high enough to warrant concern about deterministic effects. Peak dose rates are 21 (skin), 20 (eye) and 6 cGy h−1 (bone marrow). The NCRP low-dose-rate criterion is exceeded for 27 h (skin), 28 h (eye), and 17.5 h (bone marrow) with cumulative organ doses of 2.3, 2.0 and 0.4 Gy, respectively, during the time that the dose rate exceeds 0.00057 cGy h−1 .

The UNSCEAR low-dose-rate criterion is exceeded for 19 h (skin and eye) and 11 h (bone marrow) with cumulative organ doses of 2.3, 2.0 and 0.4 Gy, respectively. The ICRP low-dose-rate criterion is not exceeded for

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Special Edition on Space Radiation the bone marrow, but is exceeded for the skin (10 h) and eye (9.6 h). The organ doses accumulated during this time are 1.9 (skin) and 1.6 Gy (eye). A comparison of skin dose rates for the August 1972 and October 1989 events, behind 2 g/cm2 aluminum shielding, is displayed in Fig. 2.

Clearly the dose rates for the 1972 SPE are much higher; however, the October 1989 SPE cumulative dose was comparable to the August 1972 dose because of the protracted nature of the October 1989 event. Low dose rates defined by advisory bodies are related to stochastic effects. Unfortunately, information about the dose rate at which the effect on specific tissues for deterministic effects is reduced is unsatisfactory. A dose rate of 1 Gy h−1 is a high dose for LD50/30 and bone marrow cell killing. However, the precise relevant dose rate for deterministic effects in other tissues is less clear.

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Although the Apollo missions have placed men outside the protective geomagnetic shielding and have subjected them to types of ionizing radiation seldom encountered in earth environment, radiation doses to Apollo crewmen have been minimal. Spacecraft transfer from low earth orbit to translunar coast necessitates traverse of the regions of geomagnetically trapped electrons and protons known as the Van Allen belts. When beyond these belts, the spacecraft and crewmen are continuously subjected to high-energy cosmic rays and to varying probabilities of particle bursts from the sun (Fig.1).

Details concerning effects of solar-particle events on various phases of an Apollo mission are shown in Table II. In terms of hazard to crewmen in the heavy, well-shielded command module, even the largest solar-particle event on record (November 12, 1960) would not have caused any impairment of crewmember functions or

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STEM Today, January 2017, No.16

ability of the crewmen to complete their mission safely. It is estimated that within the command module during this event the crewmen would have received a dose of 60 to 100 rads to their skin and 10 to 30 rads to their blood-forming organs (bone and spleen).

Other estimates have indicated that skin dose from this event could have been as high as 270 rads. Radiation doses to crewmen while inside the thinly shielded lunar module or during an extravehicular activity (EVA) would be significantly higher for such a particle event. The radiation specialists at the Mission Control Center Space Environment Console, with the assistance of SPAN and the other monitoring system described in the appendix, must advise the Flight Direct or and Flight Surgeon of the radiation risks that would be involved with the event. If doses are projected to be detrimentally high, it would be advised that the astronauts not stay in the

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Special Edition on Space Radiation lunar module or perform EVA during this type of particle event. Rules that apply to lunar module stay or EVA during such an event are indicated in Table II under the mission phases "Lunar orbit" and "Lunar stay."

Apollo missions have not undergone any major space radiation contingency. However, the development of spacecraft dosimetry systems, the use of a space radiation surveillance network, and the availability of individuals with a thorough knowledge of the space radiation environment have assured that any contingency would be recognized immediately and would be coped with in a manner most expedient for both crewmember safety and mission objectives.

Routine radiation-protection problems dealing with manmade radiation sources have been solved by using standard health-physics procedures. Spacecraft radioluminescent-light-source problems were solved by improvement in shielding and containment of the promethium-147 isotope. It has been shown on the Apollo missions that the spacecraft and its crewmen have successfully avoided the large radiation doses that, before the Apollo missions, had been cited as a possible deterrent to manned space flight. Radiation doses to Apollo crewmen have been significantly lower than the yearly average of 5 rem set by the U.S. Atomic Energy Commission for workers who use radioactive materials in factories and institutions across the United States.

STEM Today, January 2017, No.16

Apollo Command Module The external shape of the Apollo CM (Fig. 2), like the Mercury and Gemini spacecraft, consists of a blunt entry face with a conical afterbody that was designed to minimize convective heating during atmospheric entry.

The center of gravity of the CM is offset from the axis of symmetry to generate the necessary lift to satisfy entry corridor and range requirements.

The TPS comprises the entire outer shell of the CM and consists of an ablator bonded to a stainless-steel structure that is fabricated in three subassemblies (Fig. 3). In addition to protecting the CM from the thermal environment, the outer shell transmits the aerodynamic loads to the primary structure during boost and entry and transmits the hydrodynamic pressures to the primary structure during a water landing. Because of the uncertainties concerning the flow field during CM elitry into the atmosphere of the earth, the magnitude of radiation heating, and the analysis of ablator-tometal junctions, the decision was made to fabricate the entire CM TPS from ablative material. This decision was made, even though temperature predictions indicated that reradiative metal’ shingles would provide sufficient thermal protection for much of the CM conical afterbody.

The ablative material selected for the TPS is designated Avco 5026-39G and consists of an epoxy-novalac resin reinforced with quartz fibers and phenolic microballoons. The density of this .material is 31 lb/ft3 . The ablator is applied in a honeycomb matrix that is bonded to a stainless-steel substructure. The phenolic honeycomb is first bonded to the stainless-steel shell with HT-424 adhesive, and then the ablator is inserted into the individual honeycomb cells with a hypodermic device that is similar to a caulking gun.

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STEM Today, January 2017, No.16

The thickness of the ablator varies with the local thermal environment and corresponding temperature profile, as shown in figure 2. Two typical cross sections of the TPS and primary structure are shown in figure 4.

Section A-A represents the stagnation heating area where the total heat load is a maximum and requires an ablator thickness of 2.7 inches. Section B-B cuts through the leeward side where the heating rates are lowest and the ablator thickness is 0.7 inch. The space between the outer TPS shell and the cabin structure is filled with a low-density (3. 5 lb/ft3 ) fibrous insulation, TG15000. This insulation is used to reduce the heat transfer between the outer shell and the cabin wall during space flight and, in particular, during entry into the earth atmosphere.

To accommodate the heat-shield deformations that occur because of the thermal extremes in space and entry heating, the conical section of the heat shield is attached to the aluminum cabin structure by means of a system of fiber-glass slip stringers. This attachment system (fig. 4) provides strain isolation between the inner and outer structures and reduces heat conduction from the heat shield to the cabin. The thermal control requirements for the spacecraft in outer space necessitates a relatively low thermal absorptance-to-emittance ratio of 0.4 for the surface of the CM. This low ratio is achieved with a pressure-sensitive Kapton polyimide tape that is coated with aluminum and oxidized silicon monoxide and that is applied over the entire external surface of the ablator. The installation of a boost protective cover over the conical portion of the CM prevents contamination of the thermal-control coating and the CM windows by aerodynamic heating during boost and by the tower jettison engine plume. The boost protective cover, which is attached to the launch escape tower and is jettisoned with the tower before orbital insertion, consists of a layer of cork bonded to a fiber-glass cloth backing.

In addition to the basic thermal environment design considerations, the Apollo heat shield also has numerous penetrations and protuberances for the installation of components such as windows, reaction control engines, antennas, and vents, as shown in figure 6. Each of these discontinuities in the TPS required special design considerations such as the recessing of the components and the use of densified ablators in local adjacent areas.

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STEM Today, January 2017, No.16

Special Edition on Space Radiation

The Apollo TPS consists of an ablator in a honeycomb matrix bonded to a stainlesssteel substructure. The sub-

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Special Edition on Space Radiation structure is made up of three subassemblies (fig. 3), which are referred to, respectively, as the aft heat shield, the crew compartment heat shield, and the forward heat shield. Each subassembly contains several brazed sandwich panels that are welded together by NR using a tungsten inert gas process. A typical weldassembly sequence for the forward compartment heat shield is shown in figure 7. This subassembly consists of four large brazed panels and four tower-well fittings, which are first welded together and to which the forward ring and panel and the aft ring are added to complete the assembly.

STEM Today, January 2017, No.16

A total of 41 brazed sandwich panels constitutes a shipset for each CM. These panels were manufactured by the Aeronca Corporation under subcontract to the NR Corporation. The first several shipsets were made of PH15-7M0 stainless steel; however, the later shipsets were constructed of PH14-8MO stainless steel because of the better cryogenic toughness of the 14-8 material.

After the panels are welded by NR, the three subassemblies are sent to the Avco Corporation for application of the ablator. The structure is first cleaned by scrubbing with an abrasive detergent slurry, and a primer coating is applied before the bonding of the fiber-glass honeycomb with HT-424 tape adhesive. The fiber-glass honeycomb core sections are then fitted in place over the tape, and the edge members are positioned at the same time. The assembly is vacuum bagged, and the adhesive is oven cured at 325â—Ś F for 1 hour. Inspection of the bonding of the honeycomb to the structure is made by a nondestructive ultrasonic transmission evaluation. Any unbonded areas are repaired, and then the assembly is ready for application of the ablator into the honeycomb.

This operation, termed "gunning", is the injection of the Avcoat 5026-39G into each cell of the honeycomb by means of a special gun developed for that purpose. The cylindrical cartridges containing the ablator are dielectrically heated to 160â—Ś F and are inserted in the gun. When the nozzle is positioned over the honeycomb cell, a solenoid-controlled air valve injects a blast of air into the cartridge and this entrains the ablator and carries it into the cell, filling it from the bottom to the top. There are approximately 370 000 cells in the honeycomb.

A photograph of the gunning of the honeycomb cells on the forward heat shield is shown in figure 8. When all the cells are filled, the assembly is vacuum bagged and the ablator is oven cured for 16 hours at 200â—Ś F;

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Special Edition on Space Radiation then, it is postcured for an additional 16 hours at 250◦ F. Then, the entire surface is machined on a numerically controlled turret lathe to the design-thickness requirements. The thickness of the ablator is measured by an eddy-current technique at preselected points, during machining as a process control, and after machining as a final acceptance measurement. The machined TPS is radiographed to detect any defects in the ablator, and repairs are made if necessary. Then silicone rubber gaskets are inserted in all door openings, and various details (such as bolt plugs, molded ablator parts for the abort-tower wells, and fiber-glass shear and compression pads) are bonded in place. After completion of these operations, the main ablator is checked for moisture content. A layer of thin, epoxy-based pore sealer and a moisture-protective plastic coating then are applied to the surface to ensure sealing of the porous ablator.

After this operation, the final weight and center-of-gravity measurements are made, and the heat-shield subassemblies are returned to NR for installation on the spacecraft. Before the CM is shipped from the prime contractor site to the NASA John F. Kennedy Space Center (KSC), the plastic coating is stripped off and the thermal-control coating with an adhesive backing is attached to the CM.

STEM Today, January 2017, No.16

Thermal Protection Subsystem Fabrication Concepts - Tiles Compared With Monolithic Ablator The TPS concept submitted initially by North American Rockwell(NR) to design and manufacture the Apollo spacecraft consisted of ablative tiles made from phenolic-nylon material bonded to a honeycomb-sandwich substructure made of aluminum. The substructure was to be built by the prime contractor, and the design, fabrication, and installation of the ablative tiles were to be accomplished by a subcontractor. In April 1962, a subcontractor was chosen to supply an ablative system consisting of molded tiles (typically l-foot square) of Avcoat 5026-22 ablator bonded to a stainless-steel substructure.

At approximately the same time (April 1962), recovered heat shields from Project Mercury were found to have experienced debonding of the tiled ablative center plug. This fact, together with the uncertainty regarding the thermodynamics at the joints between the tiles, led to a general lack of confidence by both NASA and NR in the tile method of ablator application.

Consequently, NASA instructed NR to conduct an alternative fabrication study of the ablator installation method being demonstrated successfully at that time on the Gemini spacecraft. The Gemini heat shield consisted of a fiber-glass honeycomb core filled with an elastomeric ablator. As a result of this study, a lower viscosity Avco ablator (designated Avcoat 5026-39) was developed that could be applied in a monolithic fashion to a phenolic fiber-glass honeycomb having a cell size of 3/8 inch. The fiberglass honeycomb was first bonded to the stainless-steel substructure with HT-424 adhesive, and the indiviaual honeycomb cells were then filled with the ablator.

Initially, the cells were filled with the ablator by tamping the dry ablator into the open cells, then curing the entire TPS installed on the vehicle. The tamping operation, however, caused considerable coiicern with respect to quality assurance and the possibility of damaging the substructure. Finally, the ablative material composition was modified so that it could be gunned in a mastic form (fig. 8) into the honeycomb cells. Although the monolithic ablator in a honeycomb matrix did provide a desirable fail-safe feature, it also resulted in longer manufacturing schedules and required additional inspection procedures.

Material Selection for Heat-Shield Substructure Stainless steel was chosen in preference to aluminum for the TPS substructure because of the fail-safe characteristics provided by a higher-melting-point alloy in the event of a localized loss of the ablator. The PH15-7M0 alloy was the alloy originally proposed by NR because of its high tensile strength (Ft u ≈ 200 000 psi at room temperature) and brazing compatibility. The initial heat shields were fabricated from this alloy. However, further investigations revealed that the material became brittle at low temperatures and the fracture toughness was unacceptable. The temperature criterion at this time for spacecraft during space flight was ± 250◦ F. Because of this fact, another material with better fracture toughness at -250◦ F was sought and the alloy PH14-8M0 (vacuum melted) was selected to replace PH15-7MO. The PH14-8M0 exhibited outstanding fracture toughness throughout the temperature range of -250◦ to 600◦ F. However, it was a relatively new alloy and an extensive development period was required to define the optimum welding and brazing process specifications.

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STEM Today, January 2017, No.16

Thermal Protection Subsystem Weight History The ablative material initially selected in April 1962 for the Apollo TPS was Avcoat 5026-22, which had a density of 66 lb/ft3 . The predicted TPS weight with this material was 1684 pounds.

Shortly thereafter, improvements (which included the addition of microballoons) were made to the material so that, by the end of 1962, a low-density version of the material (designated Avcoat 502639, with a density of 35 lb/ft3 ) was incorporated in the TPS. This represented a density reduction of 47 percent, but the corresponding reduction in predicted system weight was only 20 percent.

The low rate of system weight reduction was caused by the inclusion of additional requirements (primarily the boost heating environment) which had been overlooked during the initial design phase. In the years that followed (after 1962), some further minor improvements were made in the ablative material, which culminated in a material density of 31 lb/ft3 ; but the predicted system weight for the TPS continued to have an upward trend (fig. 11). The upward weight trend and the causes (which have been true historically of all aircraft and spacecraft projects) can be attributed to the following factors.

• The continually increasing number of protuberances on the outside moldline of the vehicle and a resulting increase in the local heating environment.

• The more refined analytical techniques that replaced earlier gross predictions.

• The addition of more rigorous thermal-control criteria as the spacecraft program progressed .

Because of management concern about the increasing spacecraft weight, an attempt was made in 1964 to reduce the spacecraft weight. The Block I1 design, which resulted from these changes, showed a decrease in TPS weight of approximately 200 pounds (fig. 11). This was achieved by • The elimination of the effects of boost heating environment by the introduction of a boost protective cover that was jettisoned with the launch escape tower (fig. 5).

• The reduction in the down-range requirement from 5000 to 3500 nautical miles (which provided a more realistic operational requirement).

• A reduction in the maximum initial entry temperature from 250◦ to 150◦ F (by the use of an external thermal-control coating) .

• The removal of some protuberances.

Two othe TPS were incorporated in 1968 and were based on recommendations by NASA. The first modification was the removal of several layers of nylon from the soft insulation blanket installed between the TPS substructure and the aluminum cabin. The removal did not compromise the thermal insulation performance, was accomplished without causing any schedule delays, and resulted in a weight saving of 64 pounds. The second recommendation (a simple manufacturing change) eliminated the application of the protective enamel paint that acted as a moisture barrier. Because the thermal-control coating sub se quently applied to the ablator surface was also a good moisture sealer, an extra coating of thin epoxy-based pore sealer was applied instead of the enamel. This sealer, together with the thermal-control coating, was sufficient to keep the moisture content in the ablator below the specified 2 percent.

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Radiological Protection and Medical Dosimetry for the Skylab Crew (SKYRAD)

STEM Today, January 2017, No.16

Radiological protection planning for the Skylab missions encompassed two major areas; those radiation exposures that were "expected" whose components were known with relative certainty and those radiation exposures that were "unexpected" or completely indeterminant. The expected radiation components were the trapped protons and electrons of the Van Allen Belts (figure 9-1), galactic cosmic rays, and the emissions of onboard radiation sources (table 9-I). The possibilities of unexpected exposure included energetic solar particle events, high altitude nuclear tests, and potential problems with onboard sources.

Premission analyses indicated that dose equivalents from the nominal environment of trapped (Van Allen Belt) particles and galactic cosmic radiations would be well below the limits adopted by National Aeronautics and Space Administration from the National Academy of Sciences recommendations for manned space flight (table 9-II) . These analyses indicated that the Skylab 2 mission (28-day duration) would be within the 30-day limit category, while Skylab 3 and 4 (59 days and 84 days, respectively) would be within the 90-day category. Because the nominal environment would result in doses well below these limits, operational radiation support was geared toward rapid identification and reaction to any enhanced radiation situation.

Passive Dosimetry Each crewman was provided with a passive dosimeter packet to be worn continuously throughout the mission.

The packet weighed approximately 14 g (one-half oz), and was designed to be worn on a soft strap on the ankle or wrist. The packet contained the following dosimetry materials for postflight analysis: densitometric film, nuclear track emulsions, polycarbonate and cellulose nitrate track detectors, lithium fluoride (TLD-700) chips, and tantalum/iridium foils.

In addition to passive dosimeters worn by the crewmen, passive dosimeters were placed within the Orbital Workshop’s film storage vault for the intervals from the beginning of Skylab 2 to the end of Skylab 2 (28 days) and from the beginning of Skylab 2 to the end of Skylab 3 (123 days). The film vault dosimeters were placed in locations with aproximate 2π shielding values of 13 and 23 g/cm2 aluminum. Relative to proton range in tissue,

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Special Edition on Space Radiation these depths in aluminum correspond to soft tissue depths of approximately 10 and 19 cm, respectively.

STEM Today, January 2017, No.16

Ground Radiation Monitoring Radiation protection support was provided by specialists in communications, computational analysis, and radiological health. Spacecraft data, satellite information, and solar observatory reports were utilized in evaluating the space environment, especially relative to radiation enhancement. The crewmen reported their personal radiation dosimeter readings (as integrated dose) on a daily basis, plus additional readings before and after each extravehicular activity. These readouts confirmed a continuously nominal radiation environment throughout each of the three missions. Although there were no radiation enhancements, the mission was not totally uneventful from a radiation standpoint. A few highlights are as follows.

Solar Activity–The Skylab missions were flown during a period when solar activity was approaching a minimum in the Sun’s solar cycle. Nevertheless several events of scientific interest occurred during the Skylab missions, however, particle emissions from these events were of low energy and relatively low intensity. These characteristics, coupled with the shielding effect of the Earth’s magnetic field, reduced radiation doses from solar particles to below the limits of detectability for onboard dosimetry instrumentation ( <10 millirad per event).

Nuclear Events–A series of four nuclear devices were detonated by France at their Murora Test Site during Skylab 3. The tests produced no ionizing radiation problems for Skylab. However, the possibility of eye damage to the crew from accidental observation of a test was recognized. Therefore, visual observation of ground sites in the vicinity of the test area was completely avoided.

Onboard Radiation Source Problems–One of the larger onboard sources ( approximately 200 mCi of promethium -147) was radioluminescent markings on knobs and dials of an experimental device, the experiment S019 "Articulated Mirror System." Roughly half of the total activity was applied to digital readout belts and wheels within a readout subassembly. Two malfunctions occurred with the device in-flight. First, a number of radioluminescent numerals (about one mCi each) became detached from one of the dial wheels, and second (perhaps because of the first), a belt of numerals became jammed and failed to indicate instrument position in the 10’s and 100’s places of rotational attitude.

The possibility of numeral detachment had been recognized late in the preflight preparations for the missions and the dial subassembly had been gasket-sealed to preclude escape of promethium-147 into the spacecraft atmosphere. The problem during the flight became one of how to obtain valid experimental results, either by fixing the jammed belt (without release of promethium-147) or by finding an alternative alignment method

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Special Edition on Space Radiation for the experiment. Ground based testing with a training model of the experiment equipment determined that the numeral belt could not be freed without breaking into the sealed dial unit. In the meantime, an alternative alignment method was devised and tested. The alternative method was successful and was utilized for the remainder of the mission.

STEM Today, January 2017, No.16

Dosimetry Results Integrated radiation doses at a tissue depth equivalent to lens of the eye were obtained daily by crew readout of personal radiation dosimeters. These dosimeters were worn the first 4 days of each mission and on all extravehicular activities. During the duration of each mission, the instruments were placed in the designated assigned positions shown in table 9-III.

Mean dose rates for similar positions in consecutive missions show a trend toward increased values as use of food, water, propellants, and other expendables reduced the overall spacecraft shielding. Thermoluminescent dosimeter results of the crew-worn passive packets are shown in table 9-III for comparison with the rates found throughout the spacecraft.

An upper limit estimate of the hard galactic radiation contribution is approximately 18 millirad per day; the approximate lower limit is 12 millirad per day. Comparison of these rates with the overall mean dose rates shown in table 9-III indicates that the galactic component accounted for 30 to 50 percent of the observed film vault doses, and roughly 20 to 30 percent of the crew dose means.

The majority of the remaining dose originates from protons of the Van Allen Belts and softer secondary radiations generated by passage of the primary particles through spacecraft materials. The evaluation of dose equivalents for mixed radiations in space is a complex subject and it is recommended that the reader consult the literature for rigorous discussion on this subject. There are, however, some notable findings which should be covered.

Primary Electronsâ&#x20AC;&#x201C;Van Allen Belt electrons did not penetrate into the spacecraft, nor were they found to penetrate deeply enough (3 mm tissue equivalent) during extravehicular activities to register on either the passive dosimeters or personal radiation dosimeters. Consequently, electron doses to the skin (tissue depth: 0.1 mm below 0.2 g/cm2 of space suit shielding) were calculated from electron-proton spectrometer data.

Dose Versus Shield Depthâ&#x20AC;&#x201C; Doses to the blood forming organs (tissue depth: 5 cm) were found to average 0.66 of the doses observed to the skin. These dose averages were obtained by integration of outputs from the dual sensors of the Van Allen Belt Dosimeter. The value of 0.66 also is in good agreement with a value obtained by interpolation between crew-worn and film vault dosimeter results. The sole difference between skin and eye doses (0.1 mm and 3.0 mm tissue depth, respectively) is the added dose to skin from electrons during extravehicular activities.

Quality Factor Versus Shield Depthâ&#x20AC;&#x201C; Film vault shielding was found to be relatively ineffective from a simple dose reduction standpoint (table 9-III). Despite the small dose reduction, however, quality factor could have decreased substantially if the dose reduction was solely due to filtering of lower energy particles. On the other hand, secondary buildup processes tend to increase quality factor as a function of shield depth. These competing effects could not be calculated accurately prior to the mission.

Therefore, researchers have relied primarily on postmission nuclear emulsion analyses of the film vault dosimeters to determine space radiation quality as a function of shielding. Comparison of emulsion data from the dosimeters worn by the crew and film vault dosimeters indicates that the filtering mechanism (reduced quality

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Special Edition on Space Radiation factor) is slightly dominant at shield depths up to 23.3 g/cm2 aluminum. At blood forming organ depth (5 cm tissue), quality factor is estimated equal to 1.5. In comparison, a quality factor of 1.6 is found for the crew-worn dosimeters beneath 0.3 g/cm2 of tissue equivalent shielding.

Neutron Dosimetryâ&#x20AC;&#x201C; Details of the iridium/tantalum neutron dosimetry system have been published previously . Thermal (0.02 to 2.0 electronvolts) and intermediate (2.0 to 2 X 103 electronvolts) neutrons were found to contribute to crew dose equivalent at a combined rate of approximately 0.1 millirem/day.

Direct measurement of fast neutron fluence by suspended track analysis of crew-worn nuclear emulsions was not possible due to the high track densities obtained on the Skylab missions.

However, upper limit dose calculations have been made based on nuclear emulsion disintegration star analyses (to determine neutron production rates) and iridium/tantalum evaluation, assuming that all activation is due to tissue albedo. Both methods show excellent agreement with upper limit rates of approximately 12.5 millirem per day for fast neutrons with mean energy of approximately one megaelectronvolts.

STEM Today, January 2017, No.16

Table 9-IV summarizes the dosimetry results for each crewman of the Skylab missions. As indicated in this table, there were certain variations in passive dosimeter wearing habits which required adjustments for data comparison purposes. Dose equivalents received by the Skylab 4 crewmen were the highest received in any NASA mission to date, but remained well within the limits established for the Skylab missions.

Due to the low rates involved (for example, less than 100 millirem per day to blood forming organs), dose equivalents for each crewman were well below the threshold of significant clinical effect. These dose equivalents apply specifically to long-term effects such as generalized life shortening, increased neoplasm incidence, and cataract production.

To place the mission values in perspective, the NASA career limits were 400 rem blood forming organs, 1200 rem skin, and 600 rem eye lens and were established from ancillary radiation exposure constraints recommended by the National Academy of Science and based upon a reference risk of doubling the incidence of leukemia and other neoplastic disease.

This reference risk was taken to be a dose equivalent of 400 rem. These career limits also entail a statistical risk of nonspecific life shortening of from 0.5 to 3.0 years. The Skylab 4 crewman could fly a mission comparable to one 84-day Skylab 4 mission per year for 50 years before exceeding these career limits.

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Visual Light Flash Phenomena (M106)

The primary objective of the study was to investigate the frequency and character of visual light flashes in low-Earth orbit as the Skylab trajectory passed from northern to southern latitudes. Because the trajectory periodically passed through the South Atlantic Anomaly (SAA), another objective was to investigate possible visual flashes during passage through this region.

Two periods of observation by one observer were planned for the Skylab 4 mission. These observation sessions were accomplished on orbits selected to provide the best data on both latitude and SAA effects. The first observation session occurred on mission day 74 and was 70 minutes in duration. The second occurred on mission day 81 and was 15 minutes shorter because of very critical time limitations during the last few days of the mission.

At the start of each session, the observer got into his sleep restraint, set a timer for the prescribed period (either 70 or 55 minutes), donned a blindfold, and began observing for light flashes. The subject allocated the first 10 minutes of each session for dark adaptation. During the first session, no particular position in the sleep restraint was specified. The spacecraft was in Solar Inertial Mode during both periods and local noon occurred very close to the equator in both cases.

For the second session, directions were given for head positioning that placed the anterior-posterior axis of the head parallel to the Earthâ&#x20AC;&#x2122;s magnetic field lines in the SAA. The astronaut described each light flash event

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Special Edition on Space Radiation on a voice tape recorder and a transcript was obtained after flight for analysis.

Unfortunately, no single orbit possessed ideal geomagnetic latitude and SAA conditions. The first session provided the best latitude conditions, but Skylab passed through the edge of the SAA region. The second session passed through the center of the SAA but did not achieve geomagnetic latitudes as high as in the first session.

A total of 168 flashes were reported. During the second session, there were 144 reported flashes while the spacecraft was in the center of the SAA, greatly outnumbering the 24 flashes observed when Skylab passed through the edge of the SAA in the first session.

The relationship between flash occurrence and geomagnetic latitude of High Mass Energy (HZE) particle flux was difficult to determine because of the few flashes observed and the varying lengths of time spent at different latitudes. It was known, however, that the time from equator passage was directly related to latitude.

When the cosmic ray flux was plotted against time from equator passing, and the number of light flashes observed inside and outside the SAA was added, a correlation of flash occurrence with cosmic ray flux (or geomagnetic latitude) appeared. Dosimeter data from the Van Allen radiation belts indicated increased radiation levels in the SAA, and the flash rate coincided well with the increased radiation levels.

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Although light flashes were reported as casual observations by the crews of Skylab 2 and Skylab 3, the events reported during Skylab 4 represented the first dedicated observations made in Earth orbit.

No flashes were observed during the Mercury and Gemini flights or during Apollo missions prior to Apollo 11. One explanation as to why no flashes were observed prior to Apollo 11 was that the eye must be darkadapted.

Furthermore, it was thought that the observer must be reasonably relaxed and free from most distracting activities to observe light flashes. This was not the case on earlier flights.

Also, without a precedent for their observation, there would probably be the tendency to discount minor flashes as nothing unusual and simply a harmless event in an environment where there were more important observations to be made. Once the phenomenon was known to occur, astronauts could observe and record these events.

The following conclusions were drawn from the data: • Dark-Dark adaptation of at least 10 minutes was required to begin observing the flashes.

• There was a strong correlation of very high flash rates with passage through the SAA. From physical arguments and event descriptions, it appeared certain that these flashes were due to radiation trapped inside the Van Allen Radiation Belts.

• There was evidence for the predicted latitude effect, although existing data were insufficient for a thorough statistical evaluation.

• A greater particle flux in the trajectory through the SAA during the second observation period probably explained the increased number of flashes observed at that time. However, there were also more flashes observed outside the anomaly during the second period where the cosmic particle environment should have been comparable. Researchers were unable to explain the phenomena.

It was suggested from the event rates and description of flashes during the SAA passes that particles heavier than protons may have been in the inner belt of trapped radiation. Existing knowledge of the inner belt included an upper limit of approximately one heavy nucleus per 1000 protons.

The Skylab 4 light flash data were compatible with this limit, but suggested the existence of a significant flux of energy particles with atomic number (Z)≥2. This provided strong motivation for making detailed and accurate measurements of the SAA (inner belt) heavy component during the Apollo-Soyuz Test Project mission.

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D ID YOU KNOW ?

VAN A LLEN P ROBES R EVEAL L ONG -TERM B EHAVIOR OF E ARTH ’ S R ING C URRENT

STEM Today, January 2017, No.16

New findings based on a year’s worth of observations from NASA’s Van Allen Probes have revealed that the ring current - an electrical current carried by energetic ions that encircles our planet - behaves in a much different way than previously understood.

The ring current has long been thought to wax and wane over time, but the new observations show that this is true of only some of the particles, while other particles are present consistently. Using data gathered by the Radiation Belt Storm Probes Ion Composition Experiment, or RBSPICE, on one of the Van Allen Probes, researchers have determined that the high-energy protons in the ring current change in a completely different way from the current’s low-energy protons. Such information can help adjust our understanding and models of the ring current - which is a key part of the space environment around Earth that can affect our satellites.

Figure 1: During periods when there are no geomagnetic storms impacting the area around Earth, highenergy protons (with energy of hundreds of thousands of electronvolts, or keV; shown here in orange) carry a substantial electrical current that encircles the planet (also known as "the ring current"). Credit: Johns Hopkins APL

"We study the ring current because, for one thing, it drives a global system of electrical currents both in space and on Earth’s surface, which during intense geomagnetic storms can cause severe damages to our technological systems," said lead author of the study Matina Gkioulidou, a space physicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "It also modifies the magnetic field in the near-Earth space, which in turn controls the motion of the radiation belt particles that surround our planet. That means that understanding the dynamics of the ring current really matters in helping us understand how radiation belts evolve as well."

The ring current lies at a distance of approximately 6,200 to 37,000 miles (10,000 to 60,000 km) from Earth.

The ring current was hypothesized in the early 20th century to explain observed global decreases in the Earth’s surface magnetic field, which can be measured by ground magnetometers. Such changes of the ground magnetic field are described by what’s called the Sym-H index.

"Previously, the state of the ring current had been inferred from the variations of the Sym-H index, but as it turns out, those variations represent the dynamics of only the low-energy protons," said Gkioulidou. "When we looked at the high-energy proton data from the RBSPICE instrument, however, we saw that they were behaving in a very different way, and the two populations told very different stories about the ring current."

The Van Allen Probes, launched in 2012, offer scientists the first chance in recent history to continuously monitor the ring current with instruments that can observe ions with an extremely wide range of energies.

The RBSPICE instrument has captured detailed data of all types of these energetic ions for several years. "We needed to have an instrument that measures the broad energy range of the particles that carry the ring current,

within the ring current itself, for a long period of time," Gkioulidou said. A period of one year from one of the probes was used for the team’s research.

STEM Today, January 2017, No.16

"After looking at one year of continuous ion data it became clear to us that there is a substantial, persistent ring current around the Earth even during nonstorm times, which is carried by high-energy protons.

During geomagnetic storms, the enhancement of the ring current is due to new, low-energy protons entering the near-Earth region. So trying to predict the storm-time ring current enhancement while ignoring the substantial preexisting current is like trying to describe an ele- Figure 2: During periods when geomagnetic storms phant after seeing only its feet," Gkioulidou said. affect Earth, new low-energy protons (with energy of tens of thousands of electronvolts, or keV; shown here in magenta) enter the near-Earth region, enhancing the pre-existing ring current (orange). Credit: Johns Hopkins APL Reference: NASA’s Van Allen Probes Reveal Long-term Behavior of Earth’s Ring Current , Website .

NASA’s Van Allen Probes Catch Rare Glimpse of Supercharged Radiation Belt , Website .

NASA’s Van Allen Probes Spot Electron Rainfall in Atmosphere , Website .

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STEM Today, January 2017, No.16

Apollo Experience Report , Protection against Radiation , NASA.

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