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Radiation Risks and the Vision for Space Exploration
On April 27, 1972, the crew of Apollo 16 returned to Earth from a lunar exploration mission that lasted 11 days. Slightly more than 3 months later, on August 4, 1972, the largest solar energetic particle (SEP) event of the 22nd solar cycle commenced. Significant fluxes of high-energy protons began arriving at 1 AU less than 40 minutes after a major optical solar flare was observed. This event was also one of the largest and most dangerous of the space era. Four months after the event, on December 7, 1972, Apollo 17 was launched and began the final lunar exploration mission of the Apollo era.
In the ensuing three decades, various studies of the possible absorbed doses from this August 1972 event and their potential biological effects on human crews have been carried out (e.g., Wilson and Denn, 1976; Townsend et al., 1991, 1992; Wilson et al., 1997; Parsons and Townsend, 2000). In these studies, skin doses as large as 15 to 20 Gy1 were estimated behind shielding comparable in thickness to that provided by a spacesuit. Skin doses of this magnitude, delivered in less than a day, with dose rates as high as 1.5 to 2.0 Gy h−1, could have resulted in severe skin damage, including skin blistering and peeling. Even inside a spacecraft, skin doses as large as 2 Gy would have been possible. In addition, bone marrow doses at ~1 Gy could have been received by the crew, resulting in some hematological responses, including blood count changes and possibly nausea or vomiting inside a spacesuit or typical spacecraft. Clearly this event could have had severe consequences for either Apollo 16 or 17 if it had occurred during either of these missions.
Recent studies of historical data from polar ice core samples suggest that events much larger than that of August 1972 have occurred during the past several hundred years (see Box 1.1, “Long-Term Radiation Studies: The Ice Core Evidence”). The largest of these events appears to be the Carrington event of 1859. Estimates of possible organ doses from an event of this magnitude (~4 times larger than that of August 1972) indicate that substantial shielding would be needed to protect human crews (Townsend et al., 2005). Crew members performing extravehicular activities in space or surface exploration activities on the Moon during an event of this magnitude could receive potentially lethal exposures. It is also problematic that,
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BOX 1.1 LONG-TERM RADIATION STUDIES: THE ICE CORE EVIDENCE The variability of both galactic cosmic radiation (GCR) and solar energetic particles (SEPs) with the Sun’s ~11 year activity cycle has been well established on the basis of direct measurements made over the past 70 years, first from the ground and later with ground-based and space-based instruments. It has been known since the early 1960s that GCR fluxes are modulated by the interplanetary magnetic field (IMF) and are anticorrelated with solar activity, with the lowest fluxes occurring when solar activity is highest and vice versa (Figure 1.1.1).  FIGURE 1.1.1 Representative galactic cosmic ray ion spectra depicting the intensity variations between solar maximum and solar minimum conditions. The upper curve for each species is for solar minimum, when cosmic rays can penetrate into the inner heliosphere more easily. SOURCE: Courtesy of R.A. Mewaldt, California Institute of Technology. |
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In contrast, SEP events occur roughly in phase with the solar cycle, with proportionally more events late in the cycle and very significant variations from cycle to cycle. Within this overall statistical pattern, SEP events are unpredictable and can occur at any time during the solar cycle. Other patterns of short-term variability have been discovered as well: an approximately 22 year cycle in the shape of the GCR peak, for example, and a 27 day modulation of galactic cosmic radiation intensity related to the Sun’s rotational period. But while reliable, continuous direct measurements have made it possible to characterize the temporal behavior of GCR and SEPs within the modern era (as defined by the availability of such measurements), they cover too short a time period to capture longer-term, secular variations that are also important for an understanding of the space radiation environment and the solar processes that influence it. Fortunately, however, when galactic cosmic radiation and solar energetic particles interact with Earth’s atmosphere, they trigger nuclear and chemical reactions, the products of which, deposited and preserved in the polar ice, provide a record of cosmic radiation modulation and SEP activity that extends centuries, even millennia, into the past. One of the key products of this process is the 10Be isotope, which attaches itself to aerosols and, after a residence of several months in the atmosphere, precipitates onto Earth’s surface. In the polar regions, the precipitated 10Be is preserved in the successive layers of ice that build up over the centuries and that record the history of the local meteorology and of the global influences on it. By boring into the polar ice and extracting core samples from it, researchers can analyze the composition of the different ice layers and measure the amount of 10Be deposited in each. Because 10Be production is proportional to the cosmic radiation flux and because the isotope has a long half-life (1.5 × 106 years), the variations in the 10Be concentration measured in the ice layers can be used to reconstruct the changes in the GCR flux over periods of several millennia. Analysis of 10Be data has shown that GCR intensities were high though variable during extended periods of low solar activity in the past, such as the Spoerer (1420 to 1540 CE) minimum and the last part of the Maunder (1645 to 1715 CE) minimum. (The level of solar activity in the past is inferred from reports of auroral activity, which date back to the 11th century, and from the sunspot record, which has been kept since around 1600.) The variations in the 10Be concentrations for these periods indicate that—at times substantial—modulation of the GCR fluxes can continue even during periods of low solar activity and that, as in the case of the Maunder minimum, the modulation is not necessarily well correlated with the sunspot number. Further, comparison of GCR intensities in the modern era with those deduced from the 10Be data for earlier epochs indicates that during the past half century, the GCR intensity near Earth has been one of the lowest in the past 1150 years (Figure 1.1.2, McCracken et al., 2004). Nitrate (NO3) is a product of the chemistry initiated by the interaction of solar energetic particles with Earth’s upper atmosphere. Like 10Be, it is removed from the atmosphere by precipitation and accumulates in the polar ice, although on a much shorter timescale (<1.5 months versus ~1 year). It has recently been demonstrated (McCracken et al., 2001a) that spikes in the concentration of NO3 in polar ice cores provide a record of past large SEP events, just as 10Be data provide a means of deducing the GCR environment in earlier epochs for which direct measurements are not available. Reconstruction of past SEP events from ice core data has revealed that large events (those with fluences >2 × 109 cm−2 for particles with energies >30 MeV) occur |
 FIGURE 1.1.2 10Be concentrations measured in ice core samples from the South Pole provide a record of the variability in the GCR flux at 1 AU for the period 850 CE to 1958 CE. Intensities are highest during secular minima in solar activity, as denoted in the top panel by the letters and bold horizontal bars and indicated in the second and third panels by the sunspot number and auroral frequency, respectively. (O = Oort minimum; W = Wolf minimum; S = Spoerer minimum; M = Maunder minimum; D = Dalton minimum; and G = Gleissberg minimum.) SOURCE: McCracken et al., 2004. Copyright 2004, American Geophysical Union. Reproduced by permission of American Geophysical Union. with an approximately 80 year period (the Gleissberg cycle). The frequency with which large events occurred during earlier epochs was 6 to 8 times greater than the frequency of SEP events during the period from about 1960 to the present, which appears to be the minimum of the Gleissberg cycle that began circa 1910 (Figure 1.1.3, McCracken et al., 2001b). Knowledge of the space radiation environment of the past provides the historical context for an understanding of the space radiation environment of the present and urges caution in extrapolating from present conditions to those that might exist in the future. With respect both to GCR intensity and to the frequency |
because of the current limit of knowledge about radiation hazards, NASA’s current rules for acceptable risk in Earth-orbital operations for astronauts at the 95 percent confidence level would not allow a crew to stay in space for the length of time necessary to perform several space exploration missions, such as a human-to-Mars mission. NASA determines this level by using a statistical assessment of the uncertainties in the risk projection to limit the cumulative weighted dose (dose equivalent in units of sievert) received by an astronaut throughout his or her career.2
Following the presidential space policy announcement of the Vision for Space Exploration (VSE) in January 2004,3 NASA embarked on a long-term human and robotic space exploration effort that will include human missions to the Moon and Mars. Science to enable human exploration is inherently multidisciplinary, involving insights from many fields of science and technology. All of the past science strategy studies have been, by design, discipline-based. That is, they have provided scientific goals and priorities for a particular field or set of related disciplines. This approach to setting scientific goals for breakthroughs in individual fields is effective, and the current National Research Council (NRC) reports remain timely and relevant today in their respective areas.
However, NASA’s new Vision for Space Exploration opens up novel and previously unexplored issues whose nature can best be illustrated by the question, How, and by whom, is the decision to be made that the necessary medical, scientific, and technological knowledge has been acquired before the United States actually sends humans to Mars? No single NRC decadal survey or combination of surveys provides the type of advice needed for the new programs that are anticipated under the new VSE. Also, no single scientific or engineering discipline can provide the expertise and knowledge necessary to solve these problems optimally. Therefore, a reexamination of the decadal surveys has not provided ideal guidance for enabling science. Instead, crosscutting advice needs to come from cross-disciplinary groups of experts representing diverse scientific fields rather than from the traditional single-discipline advisory committees. The problem of understanding and mitigating the effects of space radiation is a prime example of such a crosscutting issue.
Understanding and mitigating the deleterious effects of space radiation on both astronauts and operational systems constitute a complex, multifaceted problem. Progress in countering the harmful effects of different space radiation environments has to draw on advances in solar and space physics, radiation monitoring, risk assessment, materials science, biomedical science, medical systems engineering, space systems design, and other areas. It also will be facilitated by the use of robotic “guinea pigs” rather than human subjects. A piecemeal approach to planning research and setting priorities under the guidance of individual scientific disciplines is unlikely to produce robust, reliable solutions. Therefore, there is a need, both internally in NASA and in the broader scientific and space operations communities, to foster a multidisciplinary approach.
THE SPACE RADIATION ENVIRONMENT
There are three main natural sources of radiation in space to which spacecraft and astronauts may be exposed: (1) galactic cosmic radiation (GCR), (2) solar energetic particles (SEPs), and (3) energetic particles trapped in a planetary magnetic field. Anomalous cosmic rays, accelerated at the solar wind termination, are judged not to pose a hazard. An additional important source of radiation are the secondary neutrons
produced by the interaction of energetic particles with a planetary atmosphere or surface (Clowdsley et al., 2001; Keating et al., 2005).
Galactic Cosmic Radiation
Galactic cosmic rays are highly energetic nuclei (mainly in the range 100 MeV per nucleon to 10 GeV per nucleon) believed to be accelerated at shocks produced by supernova explosions. GCR consists predominantly of protons, with alphas (He nuclei) as the next most abundant species. Trace numbers of heavier nuclei such as carbon, oxygen, and iron are also present. Although high Z energetic (HZE) particles are only a tiny fraction of the GCR population, they are of particular concern because they are highly ionizing and their biological effects are uncertain. (The rate of energy transfer from a GCR to the ionization of the background matter is proportional to Z2, where Z is the charge of the GCR particle.) Once GCR has entered the solar system, its fluxes are modulated by the solar wind and the heliospheric magnetic field, so that there is an 11 year periodicity in GCR intensity, with the most intense fluxes occurring in antiphase with solar activity. There are 27 day and 22 year periodicities in the GCR flux also observed. In addition, recent analyses of ice core data (see Box 1.1) have revealed longer-term variations in GCR intensity as well and suggest that the present GCR intensity may be anomalously low.
Solar Energetic Particles
Solar energetic particles are produced both by solar flares and by shocks driven by fast coronal mass ejections (CMEs). In large SEP events, both flare-accelerated and CME-accelerated particles are generally present, with the flare-accelerated populations characterized by 3He and heavy-ion abundances that are enhanced relative to coronal values. While smaller, impulsive, flare-associated events can occur at any time during the solar cycle, larger SEP events occur most frequently during periods of increased solar activity. Unusually intense, “worst case” SEP events occurred in February 1956, August 1972, and September 1989. Such events appear to be relatively uncommon in the present era but may have occurred with appreciably greater frequency in the past (see Box 1.1). As previously noted, the most powerful SEP event known to date occurred in 1859, with an estimated >30 MeV proton fluence of 18.8 × 109 cm2 (McCracken et al., 2001a).4
Earth’s magnetic field shields against GCR and SEPs, although imperfectly. The field in the polar regions is open to both GCR and SEPs; during intense magnetic storms, the region of open flux expands, allowing access for precipitating SEPs to lower latitudes. Galactic cosmic rays with sufficient rigidity impinge on the atmosphere at middle and low latitudes as well as at the poles. It is outside the magnetosphere, however, in interplanetary flight and on the surfaces of the Moon or Mars, that both kinds of radiation will present the greatest risk to astronauts.5
While GCR is the dominant source of radiation to which astronauts on lunar or interplanetary missions will be exposed, the GCR background and its modulation by solar activity and the interplanetary magnetic field (IMF) are relatively well understood and predictable (Badhwar and O’Neill, 1992). The principal unknowns in this case are the effects of the interaction of HZE particles with shielding materials and human tissue. In contrast, SEP events are episodic; are highly variable in composition, intensity, spectra, and temporal profile; and thus are difficult to predict. Despite the advances in knowledge and understanding
of SEP events made possible by Advanced Composition Explorer (ACE) and Ulysses data, there remain a number of fundamental questions about SEP acceleration, propagation, and the physical conditions (e.g., seed populations, IMF configuration, shock speed, and geometry) that influence the properties of SEPs observed at 1 AU. Answering these questions is a necessary condition for the development of a predictive capability useful for operational purposes.
Trapped Radiation
All of the magnetized planets have populations of highly energetic particles that are trapped in the planetary magnetic fields. The most extensively studied of these trapped populations are Earth’s radiation belts (the Van Allen belts) and the radiation belts of Jupiter. The region of trapped radiation within Earth’s magnetosphere consists of energetic protons, electrons, and heavy ions organized in two belts, a relatively stable proton-dominated inner belt and a highly variable electron-dominated outer belt. Energies range from ~100 keV to >400 MeV for protons and from 10s of keV to >10 MeV for electrons. Manned missions in geospace are flown in low Earth orbit, at altitudes below the inner belt6; however, astronauts embarking on or returning from journeys to the Moon or Mars will have to pass through the Van Allen belts and will be exposed for brief periods to high levels of radiation.
Secondary Radiation
Galactic cosmic rays and SEPs impinging on the atmosphere or surface of a planet or satellite produce secondary radiation, including energetic neutrons, which may contribute significantly to the surface radiation environment to which astronauts would be exposed. Modeling studies of the radiation environment at the surface of Mars (e.g., Wilson et al., 1999; Clowdsley et al., 2001; Wilson et al., 2004) have shown that, in addition to the spectra and fluence of the primary particles, the factors that determine the intensity of the secondary radiation produced are the density of the atmosphere and the composition of the surface, with a higher neutron yield from the dry regolith than from regolith covered with frozen CO2 or water ice. Characterization of the martian surface radiation environment through in situ measurements is required in order to validate the transport codes used in such studies; it is one of the core science objectives of the Mars Science Laboratory mission.
RADIATION RISKS
NASA is required by law to limit radiation exposure to humans in space and to implement appropriate risk mitigation measures in order to ensure that humans can safely live and work in the space radiation environment, anywhere, anytime. In this context, “safely” means that acceptable risks are not exceeded during crew members’ lifetimes, where “acceptable risks” include limits on postmission and multimission consequences (e.g., excess lifetime fatal cancer risk).
The risks associated with exposure to radiation cannot be measured directly. What is measured, or calculated, is an ensemble of physical data characterizing the radiation field. There are significant uncertainties in the relationship between the physical quantities and the risk. The most commonly used physics information is the absorbed dose, D. The absorbed dose is defined as energy deposited per unit mass, in
human tissue, at a microscopic level small enough to neglect the distortion of the field by the surrounding material, but large enough to neglect the effect of statistical fluctuations in energy deposition. This differs conceptually from the more commonly considered product of the fluence and stopping power, because the energy lost by incident radiation may be distributed over a much larger volume; however, in many common irradiation situations, the difference between absorbed dose obtained using stopping power and absorbed dose corrected for local deposition and energy fluctuations is not significant. Analog instrumentation (“dosimeters”) with calculated corrections generally provides an adequate estimate of radiation dose.7
The dose can also be calculated using measured or calculated characteristics of the radiation field. In that sense, all physical measurements and models intended to provide input for risk estimates are broadly referred to as dosimetry. The properties of charged-particle radiation of greatest importance are the atomic number, Z; the atomic weight, A; the energy per nucleon, E/A (a measure of the nucleus velocity); the number of particles traversing a given surface, or fluence, 
the direction of incidence; and the flux, or fluence per unit of time. The quantities can be used to calculate the ionization properties of each particle, that is, stopping power or some version of linear energy transfer (LET). The heavy charged particles under consideration have a sharply defined trajectory in matter, proceeding mostly in a straight line until they have lost all their energy and come to a stop. The distance traversed is the range, which can also be calculated from their physical properties. When these particles traverse matter they undergo nuclear reactions. The resulting radiation fields inside spacecraft are substantially different from those in free space, and include neutrons as well as charged secondaries (Wilson et al., 1991).
The microscopic nature of energy deposition becomes important in the case of heavy charged particles, since the dose close to the particle track can be several orders of magnitude greater that the dose averaged over an entire cell or cell nucleus, and the same dose delivered by different types of radiation can lead to different biological effects. For such types of radiation, the dose is weighted by an appropriate factor reflecting the greater effectiveness of the type of radiation, for example, heavy charged particles or neutrons. The limiting risk is often cancer mortality; in that case, the weighting factor is prescribed by regulatory agencies and is referred to as the quality factor. Other weighting factors are used to convert dose from different types of radiation, risk to different organs, and corrections for various other factors involved in the calculation of risk into a common scale proportional to risk.
These data are then used to calculate the risk. Risk is a stochastic variable, corresponding to a probability distribution of observing all significant health effects arising out of exposure to radiation in space. Such risk estimates are based on models of biological responses at different levels of system organization. Current models are based on observed rates of cancer mortality in atomic bomb survivors, extrapolating from high dose and dose rate to occupational doses and dose rates by means of a dose and dose-rate effectiveness factor (DDREF). While emphasis in the past has been on the risk of cancer, Figure 1.1 summarizes the much larger collection of risks and potential outcomes bearing on the health and performance of astronauts that need to be taken into account.
Although a detailed analysis of specific risks was beyond the scope of the workshop, the radiation protection community has established that at low doses and low dose rates, radiation exposure limits that adequately protect individuals from excessive increases in cancer rates also protect them from acute risks.
FIGURE 1.1 Summary of radiation risks to humans on exploration missions.
Degenerative tissue effects and damage to the central nervous system have not been detected at doses of low LET radiation that are considered acceptable with respect to cancer risk. However, there is some basis for concern that the HZE component of the space radiation environment may produce unique damage leading to degenerative tissue effects and/or central nervous system damage. This question is the subject of ongoing research using simulated galactic cosmic ray irradiation (see Box 1.2, “The NASA Space Radiation Program”). Acute effects of radiation are a concern at relatively high doses delivered at high dose rate. Consequently, they are a potential risk of solar particle events. Acute effects are also of concern in some
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BOX 1.2 THE NASA SPACE RADIATION PROGRAM NASA is required by law to set limits on human radiation exposure in space and to implement appropriate risk mitigation measures. Currently, NASA has an ongoing, multidisciplinary radiation program involving research in radiation biology and physics to implement these obligations. In addition, there is an established operational program at NASA—the Space Radiation Analysis Group, or SRAG—that integrates information on the radiation environment, radiobiological assessments, mission radiation measurements, and flight rules. Engineering methods are also under development by NASA-funded researchers to evaluate the performance of materials and devices exposed to radiation. The NASA program is intended to achieve three main goals: (1) to predict all significant health effects arising out of exposure to space radiation, (2) to reduce the uncertainty in risk predictions by acquiring essential biological knowledge leading to accurate models of risk assessment, and (3) to develop risk mitigation technologies. The program is based on the fact that only ground-based simulation of space radiation can yield statistically significant results for realistic experiments in a timely manner and at a cost far below that associated with space-based experiments. The role of space-based measurements is reserved for experiments able to provide statistically significant validation of sensitive model predictions and for radiation measurements not accessible to Earth experimentation (e.g., the neutron albedo of Mars). Accordingly, NASA has signed agreements with the U.S. Department of Energy leading to the construction, commissioning, and ongoing operation of the NASA Space Radiation Laboratory at the Brookhaven National Laboratory in Upton (Long Island), New York. This facility provides beams of charged particles ranging from protons to gold, at energies between 0.1 and 3,000 GeV per nucleon. The beams used generally consist of a single particle at a single energy, but mixed beams and multi-energy beams can be delivered, for example, to simulate the solar particle event spectrum of protons or the distribution of galactic cosmic rays. Beam-time proposals are reviewed by a Brookhaven/NASA science advisory committee with physics, biology, and engineering members to ensure compatibility between experiments, proper infrastructure support, and appropriate experiment design. Beam-time use by peer-reviewed program investigators is paid for by NASA, as is occasional use to gather preliminary data, noninterfering “piggyback” experiments, and peer-reviewed research funded by other NASA programs or other government agencies, subject to Memoranda of Agreement. Separate funding is required for recurrent use, at a level depending on beam use and long-term requirements. NASA’s space radiation program has strong communication with other elements of the NASA radiation protection community. In particular, it provides risk estimates to SRAG and other mission planners. |
medical applications or radiation, particularly in cancer therapy, and have been extensively studied in that context. Generally, studies have been limited to low LET radiation, although some data are available from accidental exposures to neutrons, which result in higher LET ionizing particles.
Currently, career exposure to radiation is limited so as to lead to less than a 3 percent increase in lifetime fatal cancer risk (excess relative risk, or ERR) relative to the average cancer mortality risk of the entire population (approximately 20 percent). These allowable risks are determined using estimates made by the National Council on Radiation Protection and Measurements (NCRP) of age- and gender-dependent
risks as a function of radiation dose. NASA will ensure that this risk limit is not exceeded at a 95 percent confidence level using a statistical assessment of the uncertainties in the risk projection to limit the cumulative weighted dose (dose equivalent in units of sievert) received by an astronaut throughout his or her career.8 Based on the physical characteristics of space radiation, current practice uses the product of dose and a tabulated quality factor (Q) to calculate the dose equivalent (in units of sievert) as the quantity best related to risk. On this basis, radiation limits are established to ensure that the limiting health risks are not exceeded.
The values of Q in current use were selected to represent the risks of radiations, such as neutrons and alpha particles from radioisotopes, delivered at low dose rate, that may be encountered in terrestrial activities. One of the recognized sources of uncertainty in risk estimates is the applicability of these values of Q, based on the stopping power of the radiation, for HZE particles. High-velocity heavy particles and lower-velocity lighter particles can have the same stopping power (and therefore the same Q), but the energy deposited in individual biological cells and the number of cells affected by these particles can be quite different. Studies with cultured mammalian cells and particles from high-energy accelerators often show that the biological effects of different particles with the same stopping power are not exactly the same. The significance of these results in terms of health risks to organisms is still unknown. However, the relationship between health risk and stopping power or other properties of the directly ionizing particles is critical for predicting risk in space, because the interactions of GCR particles with spacecraft components, shielding, and tissue result in the production of nuclear fragmentation products that have charge, velocity, and stopping power different from those of the primary particles. The physical properties of these fragmentation products depend, in part, on the atomic structure of the materials of the spacecraft. If one is using an inaccurate relationship between the physical properties of the fragmentation products and the biological risk they produce, the effectiveness of different shielding materials in terms of risk reduction might be misjudged. Although this contribution to uncertainty in risk estimates was well known among workshop participants, it was beyond the scope of the workshop, and discussion was generally limited to issues with respect to an accurate determination of Q based on its current definition.
Another source of uncertainty is the effect of dose rate that depends both on the biological endpoint (the risks in Figure 1.1) and the physical properties of the radiation. For HZE radiation, carcinogenesis may be nearly independent of dose rate, while acute risks depend very strongly on dose rate. Consequently, the risk due to acute effects depends strongly on the magnitude of solar particle events, while the risk due to carcinogenesis depends on the total dose equivalent. The estimate of which risk dominates may depend on the timing of the mission relative to the solar cycle.
The legal and practical requirements of maintaining occupational radiation safety include the establishment of criteria to keep radiation exposure as low as reasonably achievable (also known as the ALARA principle). Good radiation protection practice thus involves setting up a margin of safety. At NASA, that margin of safety has been defined as the level of radiation exposure that will result in an estimated risk below the limit at the 95 percent confidence level. The overwhelming contribution to these uncertainties comes in the biological area, owing to the lack of knowledge regarding the effects of protracted exposure, the values of suitable quality factors as a function of the stopping power for the HZE particles and neutrons, and the suitability of extrapolation from the biological systems used in the laboratory to the human situation. This is true as well for risks other than cancer risks. For this reason, the radiation research requirements are preponderantly focused on radiobiological uncertainties. Physical sources of uncertainty contribute to
risk estimates but are mainly of importance in operational considerations. Radiation limits (i.e., standards) have not been established for missions planned in the VSE. The Earth-orbital standard (i.e., 3 percent probability of mortality) is often used as an example when determining whether estimated Mars mission risks fall above or below it for various mission scenarios.
The large uncertainties that exist at present increase the cost of missions owing to the large safety margins required as a consequence, and they also limit the ability to judge risk mitigation methods, such as improvements in shielding or biological countermeasure effectiveness. Operational measures and radiation shielding are currently the main means of reducing radiation risk; improved biological markers have the potential to enable improved early diagnostics; the discovery of means of biological prevention and intervention may lead to significantly more powerful methods to overcome the biological consequence of exposure to radiation, including better radioprotectants.
Ultimately, the establishment of limits is complicated by several factors. Among these is that scientists cannot predict all the significant risks: radiation as the cause of some health effects, especially at moderate doses, is only conjectured; the time-dependence of some clear risks is not well known—for example, the appearance of cataracts many years earlier than normally expected; and the correlation of radiation with genetic (i.e., hereditary) and environmental (e.g., microgravitational) factors is known poorly or not at all. In addition, there is substantial uncertainty in the risks that can be predicted, so that predictions for a Mars mission may be too small or too large by a factor of two to three. As a consequence, the number of safe days on mission (i.e., the mission duration for which a crew member will not exceed risk limits within a 95 percent confidence interval) is currently less than three 180 day International Space Station missions and less than a 1,000 day Mars mission.
Biological knowledge is insufficient for the design of practical prevention and intervention methods, and reliable biomarkers predicting individual radiation risk are not available. The existence of only limited data on nuclear interactions of space radiation with matter and the limitations in models of radiation transport in matter contribute to these uncertainties and restrict the development of methods to optimize the distribution of spacecraft materials for optimal shielding configurations (the so-called multifunctional use of materials for designated spacecraft functions that simultaneously have optimal shielding properties, e.g., certain plastics relative to standard metal components).
As a consequence of these limitations, the number of days that a crew member can spend in space (including on multiple short missions) without exceeding the radiation standard established for Earth-orbital missions at the 95 percent confidence level is considerably less than the number of days required for missions within the Vision for Space Exploration (see Table 1.1). Note from Table 1.1 that 10 g/cm2 aluminum shielding is clearly inadequate for a Mars mission.
TABLE 1.1 Projections of Age- and Gender-Dependent Maximum Mission Days in Deep Space for a 95 Percent Confidence Level to Stay Below a 3 Percent Excess Fatal Cancer Probability
HARDWARE RISKS
Solar activity can affect instrumentation, spacecraft subsystems, and communications in several ways. Among the space environment effects that are of concern for instruments, spacecraft, and communications are the following: single-event effects in electronics and sensors, the total radiation dose to components, radiation damage to sensors and solar cells, and electrostatic charging.
Solar energetic particle radiation degrades the performance of solar cells. This radiation may also affect electronics in all types of instrumentation and can also interfere with all kinds of sensors, both by direct ionization and by the activation of the sensor or surrounding materials. Direct ionization can interfere with the imagery obtained using charge-coupled-device (CCD) cameras and may degrade optical and thermal control surfaces. Activation can interfere with gamma ray spectrometers used for scientific investigations. All of these effects are of concern for missions to the Moon and Mars.
As shown in Figure 1.2, single-event upsets occur in microelectronics when an individual charged particle, usually a heavy ion, deposits enough charge at a sensitive portion of the circuit to cause that circuit to change state. The physical size of the electronics element tends to determine the sensitivity as well as the probability that a single-event upset will occur.
Not only are galactic cosmic rays (specifically the heavy-ion component) important in causing space environment effects on hardware, but protons and heavy ions from solar particle events or in the trapped radiation belts (especially in Earth’s South Atlantic Anomaly region) can cause significant problems during critical phases of space missions. The heavy ions in Earth’s radiation belts can often be handled by mass shielding. However, many times it is nearly impossible to shield against very energetic cosmic heavy ions. The complexity of the space environment makes solutions to some solar event upset problems difficult. However, there often are workable and effective hardware and software solutions.
Large disturbances on the Sun’s surface have long been known to accelerate very energetic particles and also often give rise to strong traveling shock waves in the interplanetary medium. Given a proper IMF connection between the disturbance site on the Sun and a spacecraft, very energetic solar protons can begin reaching satellite environs within tens of minutes and peak in a matter of hours. These very energetic protons can cause very prompt effects on hardware. A more delayed effect results from the shock waves often produced in the solar wind by coronal mass ejections. Since radial propagation speeds are normally ≤1,000 km/s for these disturbances, it takes 1.5 to 2 days for a shock wave to reach 1 AU.
It has also been demonstrated (e.g., Reagan et al., 1983; Vampola, 1987) that irradiance of space systems by very energetic electrons can cause deep-dielectric charging. In this process, very high energy (i.e., very penetrating) electrons bury themselves in dielectric materials (e.g., coaxial cables and other insulators). These electrons then give rise to high electric fields (potential differences of several kilovolts) in their vicinities until eventually an intense breakdown occurs (see Figure 1.2). In many cases an irrefutable correlation of spacecraft anomalies with the high-energy electron environment exists, and the plausible physical charging relationship is well established (Baker, 2004).
A significant effect of lower-energy particle bombardment (from the standpoint of space operations) is the occurrence of spacecraft surface charging (see Baker, 2004). During a surface-charging event, insulated regions on a spacecraft may charge to several kilovolts potential (usually negative relative to the ambient potential). This charging occurs because of a lack of current balance between the local plasma medium and the spacecraft surface (as illustrated in Figure 1.2). When a spacecraft is immersed in a cool, dense plasma, the incident particles (electrons and ions), as well as secondary emitted particles, photoelectrons, and backscattered electrons, all balance. This gives a low net spacecraft potential. However, in a very hot, tenuous plasma, current balance can be difficult to achieve, and large potentials can build up.
FIGURE 1.2 Space weather and its effects. (Top) The connected Sun-Earth system. A large coronal mass ejection propagates from the Sun toward Earth and its magnetosphere. (Bottom) Space-weather-induced effects on an Earth-orbiting spacecraft: (A) single-event upsets due to energetic ions, (B) deep-dielectric charging due to relativistic electrons, and (C) surface charging due to moderate-energy electrons. SOURCE: Reprinted with permission from Baker, 2002. Illustration: Preston Morrighan. Copyright 2002, American Association for the Advancement of Science.
From an operational standpoint, differential charging of satellite surfaces can lead to significant discharges. Discharges introduce noise into subsystems and may interrupt normal spacecraft operations or represent a false command. In the process of discharge breakdown, physical damage may occur. This may change the physical characteristics (thermal properties, conductivity, optical parameters, and so on) of the spacecraft. Furthermore, the release of material from the discharge site has been suggested as a contamination source for the remainder of the vehicle (see Baker, 2004, and references therein).
Instruments and equipment that will be used on missions to the Moon and Mars need to be tested for their suitability and robustness in a variety of space environments. These environments include diverse regimes that range from conditions near Earth to interplanetary space, to the Moon and Mars. If commercial off-the-shelf parts and systems such as personal computers and videocameras are used heavily, they will need testing to ensure performance in the disparate environments. This will require access to adequate high-energy particle beams at accelerators for testing and related performance measurements to simulate the space radiation environment under controlled conditions.
Space environment modeling plays a vital enabling role for missions to the Moon and Mars. At Earth, static trapped radiation belt models such as AE8 and AP8 that predict electron and proton flux spectra in Earth’s radiation belts are inadequate and outdated. Even the more recent 1990s-era models based on data from the Combined Release and Radiation Effects Satellite are limited because they were based on a very brief interval (about 1 year of data). Updated models that are dynamic, taking into account current solar wind and magnetospheric conditions, are needed in order to provide the history of variations on timescales that range from solar cycle to minutes. If Mars mission architecture includes parking a transit vehicle at geosynchronous orbit, it will be necessary to understand the spacecraft charging environment better, including short-term variations at that location. More work is needed to gain an understanding of the most appropriate SEP models and methods that characterize these conditions, including extreme-event studies, risk-based models, and data-based analysis of long-term records. Improved models of proton and heavy-ion environments (flux, fluence, and energy spectra) in SEPs are needed because of their effects on systems.
Generally, the engineering approach is to harden systems against worst cases; however, the unexpected can always occur. In such circumstances, a number of actions can be taken in response to predictions of poor space weather. Sensors can be safed, noncritical systems can be shut down to prevent damage and latch-up, sensors can be oriented in a direction that is least susceptible to damage, increased attention can be given to monitoring operations and to the interpretation of sensor data, and mission activities can be limited during high-background events.
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