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Managing Space Radiation Risk in the New Era of Space Exploration Summary Space exploration is a risky enterprise. Rockets launch astronauts at tremendous speeds into a harsh, unforgiving environment. Spacecraft must withstand the bitter cold of space and the blistering heat of reentry. The skin of these vehicles must be strong enough to keep the inside comfortably pressurized and tough enough to resist damage from micrometeoroids. Spacecraft meant for lunar or planetary landings must survive the jar of landing, tolerate dust, and be able to take off again. For astronauts, however, there is one danger in space that does not end when they step out of their spacecraft. The radiation that permeates space—unattenuated by Earth’s atmosphere and magnetosphere—can damage or kill cells within astronauts’ bodies, resulting in cancer or other health consequences years after a mission ends. The National Aeronautics and Space Administration (NASA) recently embarked on Project Constellation to implement the Vision for Space Exploration—a program announced by President George W. Bush in 2004 with the goal of returning humans to the Moon and eventually transporting them to Mars.1 To prepare adequately for the safety of these future space explorers, NASA’s Exploration Systems Mission Directorate requested that the Aeronautics and Space Engineering Board of the National Research Council establish a committee to evaluate the radiation shielding requirements for lunar missions and to recommend a strategic plan for developing the radiation mitigation capabilities needed to enable the planned lunar mission architecture. Specifically, the Committee on the Evaluation of Radiation Shielding for Space Exploration was asked to do the following (see Appendix A): Review current knowledge of radiation environments on the lunar and Mars surfaces, including radiation types, sources, levels, periodicities, and factors that enhance or mitigate levels. Critical knowledge gaps, if any, will be identified. Assess and identify critical knowledge gaps in the current understanding of the level and type of radiation health risks posed to astronauts during various surface activities—ranging from habitation in the CEV to extended exploration sorties and longer stays in exploration outposts—expected for the lunar and martian environments. Review current and projected radiation shielding approaches and capabilities, as well as other exposure mitigation strategies feasible in the lunar and Mars surface environments. Recommend a comprehensive strategy for mitigating the radiation risks to astronauts during lunar surface missions to levels consistent with NASA’s radiation exposure guidelines. The strategy will: Be consistent with NASA’s current timeline for lunar sortie and outpost habitation plans, 1 National Aeronautics and Space Administration, The Vision for Space Exploration, NP-2004-01-334-HQ, NASA, Washington, D.C., 2004.
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Managing Space Radiation Risk in the New Era of Space Exploration Recommend research to resolve critical knowledge gaps regarding the lunar radiation environment and risks, Recommend a research and technology investment strategy that enables development of the necessary shielding capabilities. The study will provide recommendations on what technology investments (e.g., multifunctional materials, localized shielding, and in situ materials) NASA should be making in preparation for lunar missions, and recommend development timelines to ensure NASA has the appropriate level of shielding in place to meet the planned schedules. The committee was also asked to consider the likely radiation mitigation needs of future Mars missions and to put emphasis on research and development alternatives that would enhance NASA’s ability to eventually meet those future needs. The complete statement of task appears in Appendix A. The term “shielding” in the charge to the committee reflects a central tradeoff that spacecraft designers and mission planners must take into account. On the one hand, increased mass is required to reduce exposure to radiation; on the other, costs limit the power and energy available for propulsion to put that mass into space and to operate it. At any accepted level of exposure to radiation, the requirement for additional mass may exceed the project costs; and at a given level of cost, exposure to radiation may exceed the acceptable level of risk. In the former case, the dollar, mass, and volume budgets soon come into conflict with the shielding requirements. In the latter case, practical, legal, moral, and political imperatives determine whether such levels of risk may be incurred. The committee agrees that current permissible exposure limits, as specified in NASA radiation protection standards, are appropriate. The committee strongly recommends that the permissible exposure limits specified in current NASA radiation protection standards not be violated in order to meet engineering resources available at a particular level of funding. Although the concept of shielding might be taken to mean no more than the use of materials interposed between a source of radiation—in this case, the space environment—and the individuals who are to be protected, such a definition is exceedingly simplistic. Materials used as shielding serve no purpose except to provide their atomic and nuclear constituents as targets to interact with the incident radiation projectiles, and so either remove them from the radiation stream to which individuals are exposed or change the particles’ characteristics—energy, charge, and mass—in ways that reduce their damaging effects. Spacecraft, structures, and containers in space, and the equipment and instruments that they hold, are made of materials possessing certain mechanical, chemical, electrical, and thermodynamic properties. Whenever designers and engineers can substitute other, multifunctional materials that serve the same purpose equally well but have nuclear and atomic characteristics better suited to attenuating radiation, a gain in shielding results. Similarly, a redistribution of equipment and components may be quite effective in reducing astronauts’ exposure to radiation. A well-known example is the modification of the aspect ratio of a structure to reduce the solid angle subtended by the parts with lowest mass. Similarly, concentrating mass around areas where crew members spend much of their time, such as sleeping quarters, can enable the temporal design of a volume more highly shielded from radiation. The effectiveness of shielding is extremely sensitive to an understanding of the biological mechanisms by which radiation affects human health and performance. At present, such understanding is very incomplete, leading to wide safety margins that dictate substantial shielding requirements and missions of limited duration. Furthermore, the effectiveness of properties of materials in reducing risk depends on the genetics, age, and gender of the exposed individuals and also varies for different components of space radiation. For these and similar reasons, the committee understood its task to extend beyond a focus on the addition and distribution of material, and to require instead a comprehensive consideration of all aspects of protection against radiation in space. The presentations made to the committee corroborated this understanding. Thus, this report addresses issues related to the composition and time-dependence of the space radiation environment, to nuclear propulsion and power, and to physical and biological interactions of radiation with matter, as well as to operational and construction-related aspects of space exploration. The committee finds that lack of knowledge about the biological effects of and responses to space radiation is the single most important factor limiting the prediction of radiation risk associated with human space exploration.
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Managing Space Radiation Risk in the New Era of Space Exploration At the present time, and assuming chemical propulsion, the permissible exposure levels would not allow a human crew to undertake a Mars mission and might also seriously limit long-term Moon activity. However, revolutionary progress in the biological sciences in recent years and ongoing breakthroughs could, if continued, substantially reduce the inherent uncertainties in and thus better quantify the risks for humans exposed to radiation in space. NASA’s current Space Radiation Biology Research program attempts to balance mission-oriented research with basic research and to leverage related research at other agencies, nationally and internationally. Such a program is possible only because the NASA Space Radiation Laboratory (NSRL), an essential facility that is unique in the world, can provide beams of particles of energies and charges found in space radiation for use in ground-based experiments in areas that could not be adequately investigated in space. Located within the Brookhaven National Laboratory (BNL), the NSRL is dependent on the existence of the Department of Energy’s (DOE’s) nuclear and high-energy research program. Accelerators can be and have been closed when the frontier of science moved elsewhere. If DOE determines that the research topics requiring BNL accelerators are no longer a priority, these accelerators will be shut down or reconfigured. It is impossible to predict if or when that might occur; however, a prudent strategy for NASA would be to assume that the BNL accelerators will not be available 15 to 20 years from now, and to plan accordingly. The committee strongly recommends that NASA’s Space Radiation Biology Research program be adequately funded. NASA should perform research at the NASA Space Radiation Laboratory aggressively to take advantage of the existing window of opportunity while this facility is still available. The results of the biological research will thus be able to have an impact on the Project Constellation missions in the short term, as well as provide knowledge essential for the management of space radiation risk in the long term. RADIATION ENVIRONMENT Data from many satellites have enabled the characterization of galactic cosmic radiation (GCR) and solar particle events (SPEs) near Earth, and these results serve to characterize the radiation incident on the surface of the Moon. Knowledge of the secondary radiation, which is produced by galactic cosmic rays and SPEs interacting with material on the lunar surface, is currently based on data from the Apollo, Lunar Prospector, and Clementine missions, and on calculations. The radial extrapolation of the GCR environment from Earth to Mars is well understood, based on measurements made by numerous scientific satellites as they traveled outward through the solar system. To within a few percent or so, the GCR environment at the top of the martian atmosphere is expected to be the same as that near Earth. There are very few simultaneous measurements of SPEs at Earth and at Mars, and current models are inadequate to extrapolate near-Earth measurements of SPEs to Mars. Knowledge of the secondary radiation environment on the surface of Mars is currently based on calculations and measurements taken by spacecraft in Mars orbit. Other sources of radiation relevant to Project Constellation include astronauts’ short trips through Earth’s trapped radiation belts, radiation generated by lunar- and martian-surface nuclear power systems, and eventually, perhaps, radiation generated by a spacecraft’s nuclear propulsion and power system. On the surface of Mars and, to a lesser extent, on the surface of the Moon, there will be a component of backscattered radiation, mainly neutrons. On Mars, there will also be a component of secondary radiation engendered by interactions with the martian atmosphere. GCR is a well-characterized background radiation whose level varies gradually over the 11-year solar cycle. SPEs are episodic emissions of high-intensity radiation from the Sun whose frequency also varies over the solar cycle. At solar maximum, there are a large number of SPEs and a relatively low level of GCR. At solar minimum, there are fewer SPEs but a higher level of GCR. GCR is less intense than the radiation associated with an SPE, but it contains heavy, energetic particles that a reasonable amount of spacecraft shielding cannot completely stop. In fact, as these particles travel through the shielding, they may fragment into secondary radiation. Furthermore, GCR is a constant presence. The committee found that current knowledge of the free-space GCR component of the radiation environment is sufficient to
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Managing Space Radiation Risk in the New Era of Space Exploration support human missions to the Moon. However, human missions to Mars have not yet been sufficiently defined to make a judgment about the effects of the GCR environment on astronauts. SPEs are very intense but occur over a relatively short period of time (hours to days). Because SPEs generally have energies much lower than those of GCR, their effects can be greatly mitigated with the proper amount of shielding. However, the timing of their occurrence is difficult to predict; an astronaut performing an extravehicular activity could receive an acute or fatal dose of radiation if shelter could not be reached in time. The committee identified the following requirements to improve the understanding of SPEs: Determination of factors that drive SPE variability at energies relevant to astronaut safety; Understanding of solar conditions that give rise to large, fast coronal mass ejections; Validation of models of the interplanetary medium–solar wind and interplanetary magnetic field; and Use of a design-standard SPE in evaluating the adequacy of shielding. RADIATION-RELATED RISKS As stated above, the lack of knowledge about biological effects of space radiation is the single most important factor limiting the prediction of radiation-related risk associated with human space exploration. The understanding and interpretation of biological end points such as increased cancer risk and other biological effects are central to designing appropriate and cost-effective shielding. Unfortunately, NASA’s space radiation biology research has been compromised by recent cuts in funding, particularly for research addressing noncancer effects. The major knowledge gaps in radiation-related health risks are in the following areas: Carcinogenesis, Neurological damage, Degenerative tissue diseases, Acute radiation syndromes, and Immune-system responses. In addition to health-related risks, radiation can pose additional risks. For example, astronauts may be unable to accomplish prime mission objectives if they are not permitted to leave an outpost because of a radiation storm. To be robust, a mission must include contingencies for emergencies, such as including excess consumables so that the crew can spend extra time on the Moon, if needed. SHIELDING APPROACHES NASA and Lockheed Martin presented the committee with an overview of the radiation protection work completed and in progress on Orion, the Project Constellation vehicle that will carry astronauts to the orbit of the Moon. The committee found that the methodology used—ray-tracing analysis combined with state-of-the-art radiation transport and dose prediction codes—is appropriate for estimating dose within the Orion vehicle and can help guide decisions about the amounts and types of spot or whole-body shielding that should be added to provide protection during solar particle events. As presented to the committee, the Orion Radiation Protection Plan appears to meet the minimum radiation protection requirements as specified in the NASA radiation protection standards. But any reduction in the requirements outlined in the Orion Radiation Protection Plan may pose potentially unacceptable health consequences. The committee recommends that all elements of Project Constellation employ the radiation protection and risk management limits necessary to meet the NASA radiation protection standards presented to the committee. Other elements of Project Constellation are still in early concept-development phases, and no detailed radiation analysis or radiation protection design has yet been done. It is expected, however, that design efforts for the other elements will ensure the same level of radiation protection as those for Orion. Ultimately, all designs should meet the NASA radiation protection standards presented to the committee.
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Managing Space Radiation Risk in the New Era of Space Exploration NASA is considering the following radiation protection strategies for the human exploration of the Moon: the use of surface habitat and spacecraft structure and components, provisions for emergency radiation shelters, implementation of active and passive dosimetry, scheduling of extravehicular operation to avoid excessive radiation exposure, and proper consideration of the ALARA (As Low As Reasonably Achievable) principle. These strategies, if properly implemented, are adequate to meet the radiation protection requirements for short-term lunar missions. In addition to the above strategies, longer-duration lunar and Mars missions will require a reduction of the uncertainty in current predictions of radiological risk, plus the possible development of medical countermeasures. TECHNOLOGY INVESTMENTS To enable lunar missions with astronauts, the committee recommends the following technology investments, listed in priority order: Radiation biology research. NASA’s Space Radiation Biology Research program should be adequately funded. NASA should perform research at the NASA Space Radiation Laboratory aggressively to take advantage of the existing window of opportunity while this facility is still available. The results of the biological research will thus be able to have an impact on the Project Constellation missions in the short term, as well as provide knowledge essential for the management of space radiation risk in the long term. Testing transport code predictions. The predictions derived from calculations of radiation transport need to be tested using a common code for laboratory and space measurements that have been validated with accelerator results, existing atmospheric measurements, and lunar and planetary surface measurements as they become available. Research on solar particle events. NASA should maintain a vigorous basic science program that can clarify the mechanisms that produce SPEs and lead to accurate, quantitative predictions of SPE behavior and identification of observables critical in forecasting SPEs or all-clear periods. Empirical data for shielding design. NASA should ensure that necessary experimental data in sufficient quantities are collected, analyzed, and managed in a manner appropriate for their use in designing radiation shielding into spacecraft, habitats, surface vehicles, and other components of human space exploration. The data should include information on energy and angular dependence of cross sections for production of nuclear interaction products, and on their multiplicities. Forecasting of SPEs. Forecasting and warning of SPEs will be an essential part of a comprehensive radiation mitigation strategy. Timely collection of appropriate data and communication of resulting forecasts and warnings will be mission-critical. In situ monitoring and warning. Warning and monitoring dosimetry, active and passive, is required wherever there is a human presence beyond low Earth orbit. Multifunctional materials. Where appropriate, replacement of traditional materials with multifunctional materials should be encouraged, with the goal of improving radiation shielding. In situ shielding tradeoffs. NASA should conduct studies of tradeoffs to determine whether it is more cost-effective to transport prepared shielding materials from Earth or to construct shielding in situ with transported materials and equipment. Review of existing neutron albedo datasets. The predictions of computer codes developed by NASA Langley Research Center need to be compared with existing data, especially data for secondary radiation and neutron albedo. Existing datasets should also be reviewed to assess their value in determining the extent to which albedo neutrons on the lunar and martian surfaces may constitute a significant component of the radiation environment. Lunar and planetary surface measurements performed in the pursuit of other exploration objectives may become available; if so, the data should be used for statistically significant comparisons with theory whenever appropriate. Surface fission power demonstration—nuclear power for Mars. NASA could take advantage of the Moon as a testbed for human exploration of Mars by incorporating the development and testing of fission reactor technology into lunar plans.
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Managing Space Radiation Risk in the New Era of Space Exploration OTHER ELEMENTS OF A COMPREHENSIVE PLAN Exploration is not a one-mission objective; it is not even a few-missions activity. To be sustainable, exploration requires a long-term commitment to radiation risk management: long-term objectives and resources cannot be neglected in favor of short-term expediency. As humans voyage beyond Earth orbit, mission duration will increase, making radiation risk management even more critical. Managing radiation risk is and will continue to be an integral part of exploration mission design and execution. Radiation experts have been involved in all aspects of the design and development of Orion so far, from hardware to mission operations protocols. However, the committee judges that responsibilities need to be better defined to ensure a continuation of this positive trend. The committee recommends that an independent radiation safety assessment continue to be an integral part of mission design and operations. On exploration missions, a member of the crew should be designated as the flight crew radiation safety officer. This person would be the point of contact with mission control, monitor on-site dosimetry, and ensure communication and coordination with ground control for the response to radiation warning levels. Operational flight rules for human exploration missions are not yet drafted but should be integrated with the lunar architecture planning process. To provide operational space weather support, monitoring and forecasting capabilities should have clear requirements, coordinated not only between the design and operational portions of NASA, but also between NASA and its interagency partners. The committee recommends that the nation’s space weather enterprise integrate its scientific expertise with operational capability through coordinated efforts on the part of NASA, the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), and the Department of Defense (DOD). Where multiple end users benefit, NOAA is appropriate as the lead organization in charge of operational forecasts. However, for NASA-unique lunar support requirements, NASA’s Exploration Systems Mission Directorate should take a leadership role in defining and providing resources. It is often difficult to apply the ALARA principle to radiation risk in a setting in which multiple risks exist, such as a space mission. The committee endorses the application of the ALARA principle to radiation risk management, an approach that further emphasizes the need for radiation safety advocacy as a component of the development team for Project Constellation. At this time, operational plans are not sufficiently advanced and well defined to provide evidence that the ALARA principle has or has not been properly implemented. The design of spacecraft, habitats, and missions should incorporate NASA radiation standards, and an established limit for radiation risk, as incorporated in NASA radiation standards, has to be included in “Go–No-go” decisions for every mission. Now and in the future, the capability to protect astronauts from the harmful effects of radiation will depend on a continuing supply of world-class knowledge and talent. Due to reductions in the scope of NASA’s radiation protection plan, the current pool of intellectual capital will shrink as researchers retire and are not replaced. NASA should try, perhaps as part of an interagency effort, to attract and engage young researchers and the broader radiation community at a level sufficient to meet the demands for radiation protection of astronauts in lunar mission operations and martian mission planning. This effort should encourage cross-pollination of ideas together with preservation of institutional knowledge by promoting interactions between researchers of different backgrounds and experience levels and by addressing issues that are relevant to, but broader than, space radiation. Thus far, NASA has demonstrated a good effort to protect the next generation of space explorers from radiation. It is vital to maintain this effort throughout the design of all Project Constellation vehicles and smoothly transition that expertise as planning and execution of operations proceeds.