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Managing Space Radiation Risk in the New Era of Space Exploration 6 Findings and Recommendations A complete list of the committee’s findings and recommendations from Chapters 1 through 5 appears below, in the order in which they appear in the report. Finding 2-1. Current knowledge of the radiation environment on the Moon. Data from many satellites have enabled the characterization of GCR and 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 Apollo, Lunar Prospector, and Clementine and on calculations. Finding 2-2. Current knowledge of the radiation environment on Mars. 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. Finding 2-3. Lunar GCR environment. Given the far larger uncertainties in biological effects, the committee finds that knowledge of the composition, energy spectrum, and temporal variation of the “free space” GCR component of the interplanetary radiation environment is sufficient to support the needs of the Constellation lunar missions. Nevertheless, it will be useful to benchmark GCR models against measurements reported by ACE in the upcoming second half of the 22-year GCR modulation cycle. Finding 2-4. Space radiation climate. Ice-core studies indicate that the past ~50 years may have coincided with a comparatively benign space radiation climate, in terms of both GCR modulation levels and the frequency of very large SPE events. Of particular concern is the possibility of a six- to eightfold increase in the number of very large SPE events, perhaps starting within the next decade. If such an increase were to occur, it would have a major impact on the design and operation of Exploration systems.
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Managing Space Radiation Risk in the New Era of Space Exploration Recommendation 2-1. Planning for long-term changes in space climate. NASA must ultimately judge how much weight to assign to the cautionary findings from ice cores on a potentially more severe space radiation climate in the future. Given that the Exploration initiative envisions a commitment of the nation’s resources that spans decades, NASA should ensure that the mission architecture has sufficient flexibility and margin to cope with such changes, should they occur. Finding 2-5. The King spectrum as a design standard. Although the committee recognizes the advantages of adopting a specific solar proton spectrum as the design standard, NASA’s current strategy of evaluating the efficacy of an SPE shielding configuration using only the August 1972 King spectrum is not adequate. Under typical depths of shielding for Exploration vehicles, the level of radiation exposure produced by other large events in the historical record could exceed the exposure of August 1972. Finding 2-6. Spectra data fitting. There is no theoretical basis for any of the published spectral fits to large SPEs. The extrapolation to energies beyond 100 MeV must therefore be guided by data. Solar proton spectral forms based on data that do not extend to ~500 MeV may very well give misleading results in evaluations of the efficacy of radiation shielding for astronauts. Recommendation 2-2. SPE design standards. The dose levels made possible by a shielding design should also be calculated using the observed proton spectrum from other large events in the historical record, even if it is not feasible to modify the shielding design as a result. The October 1989 event is particularly important in this regard. Recommendation 2-3. Uncertainties in spectra data fitting. NASA should make use of existing data to reevaluate the spectra beyond 100 MeV in large events in the historical record and should assess the impact of uncertainties in the high-energy spectra on the adequacy of radiation shielding designs. Finding 2-7. Knowledge of radiation from nuclear ground power. Experience with nuclear power on Earth has provided sufficient knowledge to create this capability on the Moon. The remaining challenges are engineering problems, not scientific problems. Experiments to show the operational safety of space and planetary-surface fission power systems, including unique design features such as compactness, light weight, and heat transport and heat rejection in reduced gravity, will be important. Finding 3-1. Uncertainty in radiation biology. 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. Finding 3-2. Funding cuts to radiation biology research. NASA’s space radiation biology research has been compromised by the recent cuts in funding, particularly in research addressing noncancer effects. Finding 4-1. State of radiation protection plans for lunar missions. The use of surface habitat and spacecraft structure and components, provisions for emergency radiation shelters, implementation of active and passive dosimetry, the scheduling of EVA operations, and proper consideration of the ALARA principle are strategies that are currently being considered for the Constellation program. These strategies, if implemented, are adequate for meeting the radiation protection requirements for short-term lunar missions. Recommendation 4-1. Strategic design of Orion. As the design of Orion continues to evolve, designers should continue to consider and implement radiation protection strategies. Finding 4-2. State of radiation protection plans for Mars missions. For longer-duration lunar and Mars missions the currently large uncertainties in radiological risk predictions could be reduced by future research. Without such research, it may be necessary to baseline large shielding masses and reduced-length missions, and/or delay human
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Managing Space Radiation Risk in the New Era of Space Exploration exploration missions until uncertainties in risk prediction and radiobiological methods of risk management have advanced to the point that they can be conducted within the limits of acceptable risk. Finding 5-1. The NASA Space Radiation Laboratory. The entire Space Radiation Biology Research program is critically dependent on the availability of the NASA Space Radiation Laboratory. This facility is dependent on the DOE heavy-ion physics program and may not be available if the needs of this program change. There are no other facilities available that meet the requirements for high atomic number and energy (HZE) space radiation biology research, worldwide. Recommendation 5-1. 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. Finding 5-2. Dose estimation in the Orion crew module. The use of ray-tracing analysis combined with state-of-the-art radiation transport and dose codes is an appropriate method for estimating dose within the Orion crew module, and can be used to guide decisions on the amounts and types of spot or whole-body shielding that should be added to provide protection during solar particle events. Finding 5-3. Orion Radiation Protection Plan. The Orion Radiation Protection Plan, as presented to the committee, appears to meet the minimum radiation protection requirements as specified in NASA’s radiation protection standards. Any reduction in the Orion Radiation Protection Plan may pose potentially unacceptable health risks. Finding 5-4. Existing transport data. New measurements do not need to be taken solely for the purposes of code validation. In addition, structure in the energy dependence of relevant fragmentation yields for heavy charged particles is considered to be sufficiently small to have a negligible impact on interpolation schemes. One exception is that additional data on production of light ions (Z = 1, 2) and neutrons may be required. Recommendation 5-2. 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. Finding 5-5. SPE prediction. At present, the ability to predict an SPE and to project its evolution once underway does not exist. Such a capability will play an important role in managing the SPE radiation hazard. Recommendation 5-3. 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. Finding 5-6. Experimental data for designers. NASA has not made an adequate effort to collect, catalog, and categorize existing experimental data obtained by the worldwide heavy-ion research community and to make it available in appropriate form to the shielding engineering community. Recommendation 5-4. 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.
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Managing Space Radiation Risk in the New Era of Space Exploration Finding 5-7. 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. Recommendation 5-5. Flight crew radiation safety officer. On exploration missions, a member of the crew should be designated as 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. Recommendation 5-6. In situ monitoring and warning. Warning and monitoring dosimetry, active and passive, is required wherever there is a human presence beyond low Earth orbit. Finding 5-8. Multifunctionality. Multifunctionality presents a way to increase the ability to shield against radiation by taking advantage of the mass of materials already included in the spacecraft design. Low-Z spacecraft materials offer some shielding advantages over conventional, higher-Z metallic structures. Recommendation 5-7. Multifunctional materials. Where appropriate, replacement of traditional materials with multifunctional materials should be encouraged, with the goal of improving radiation shielding. Recommendation 5-8. 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. Recommendation 5-9. Review of existing neutron albedo datasets. The predictions of computer codes developed by the 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. Finding 5-9. Reactor shielding. Significant research is required before nuclear fission can be used for surface power on Mars or to support exploration at a nonpolar location on the Moon. Recommendation 5-10. 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. Recommendation 5-11. Agency partnerships in space weather. The nation’s space weather enterprise should 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. Finding 5-10. Intellectual capital. 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. Recommendation 5-12. Engaging young researchers. 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
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Managing Space Radiation Risk in the New Era of Space Exploration 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. Finding 5-11. Radiation-incorporated design. 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. Radiation experts have been involved in all aspects of the design and development of Orion so far, from hardware to mission operations protocols. Finding 5-12. Risk leveling. NASA uses “risk leveling” to optimize its developments, taking into account all risks, not just a single risk or group of risks. This is an inexact science, however, because estimates of virtually all risks involve some uncertainties, and most risks have different end points or outcomes, ranging from latent to acute effects and from trauma to the risk of death. A diversity of expert specialties is required as inputs to make these decisions. Recommendation 5-13. Risk management. NASA should continue its current approach to radiation protection and risk management on Orion. Radiation safety advocacy should be continued throughout the design process to ensure that this protection is available in the final embodiment of the Orion Block-2. Finding 5-13. Operational implementation of the ALARA principle. Operational plans for NASA are not sufficiently advanced and well defined to provide evidence that the ALARA principle has or has not been properly implemented. Finding 5-14. Constellation Radiation Protection Plan. There is not yet a radiation protection plan for elements of Project Constellation other than Orion. Recommendation 5-14. Radiation protection in other Constellation elements. All elements of Project Constellation should employ the radiation protection and risk management limits necessary to meet the NASA radiation protection standards presented to the committee. Recommendation 5-15. Role of standards in design. Permissible exposure limits specified in current NASA radiation protection standards should not be violated in order to meet engineering resources available at a particular level of funding. To ensure that the design of spacecraft habitats and missions implement NASA radiation protection standards, An independent radiation safety assessment should continue to be an integral part of mission design and operations, and An established limit for radiation risk, as incorporated in NASA radiation protection standards, needs to be included in “Go–No-go” decisions for every mission. Finding 5-15. Long-term commitment. Exploration continues beyond a single mission and requires a long-term commitment to radiation risk management.
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