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Managing Space Radiation Risk in the New Era of Space Exploration 5 Strategy for Radiation Risk Mitigation TECHNOLOGY INVESTMENTS TO ENABLE LUNAR MISSIONS A comprehensive radiation risk management strategy is based on an understanding of the biological effects of radiation exposure and includes physical shielding, radiation monitoring and forecasting, and operational flight rules. Because this strategy is science-based, it will lead to risk mitigation methods, such as accurate space radiation environment forecasts, improved spacecraft design, and methods of biological intervention. The recommendations listed below are roughly in priority order, but they are considered essential elements of a single group. Together, they are necessary and sufficient to implement an effective strategy for risk reduction leading to operational management of radiation risk. The committee considered Mars application as a factor that gave additional weight to a given research topic, but it did not prioritize technologies that have application only to Mars. The lone exception is the tenth topic listed below, Surface Fission Power Demonstration—Nuclear Power for Mars. It should be considered separate from the priority list, because it is not essential to lunar exploration (although it may be useful). However, it was listed because the Moon provides a unique technology development opportunity; in order to take advantage of that opportunity, this research should be included in NASA’s research plan. The prioritization process considered impact (payoff), time-urgency, current understanding versus gaps to fill, the probability of successful outcomes, and an estimate of the time and resources necessary to substantially meet the recommendations. Those efforts that must be implemented immediately to have a substantial impact have the highest priority—for example, challenges that impact the Orion configuration or will be difficult and costly to “retrofit” later. Therefore, challenges in Orion development received a high priority, as did research objectives with high payoff but long-term commitment. 1. Radiation Biology Research The rapid progress in our understanding of cancer mechanisms at the molecular level and the impact of radiation on these mechanisms should allow for more precise estimates of human cancer risk. Biological understanding has also grown more complex with newer concepts of genomic instability, bystander effects, and genetic susceptibility. It is essential that these breakthroughs in molecular understanding be tested in and translated to a better understanding of cancer in the whole organism and the impact of radiation types and energies on these mechanisms. The radiation effects need to be understood quantitatively as well as qualitatively. Through the application of sys-
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Managing Space Radiation Risk in the New Era of Space Exploration tems biology and risk modeling, quantitative cancer risks to humans will therefore be more precisely estimated for various radiation exposure scenarios. From a better understanding of the radiation spectra that the astronauts are predicted to experience, cancer risk models will identify those radiation types and energies that are the most significant and least precisely understood from a human-risk standpoint. This should guide the experimental biology research work, including possible future whole-animal studies. In this way the overall uncertainty in the estimated cancer risk to the astronauts can be minimized. The knowledge with regard to latent cancer effects from radiation greatly exceeds that of other potential chronic effects, especially cardiovascular disease and central nervous system effects. Researchers are just beginning to realize the potential importance of these other effects in relation to radiation exposures. There is a great need to understand their biological effects so that risk estimation is possible with reasonable precision for both acute and chronic exposures. The committee is concerned that these emerging adverse health effects are not receiving appropriate attention by NASA. The NASA Bioastronautics Roadmap (NASA, 2005), as well as the recent National Council on Radiation Protection and Measurements report (NCRP, 2006), provides worthwhile guidance to the research issues. There is just sufficient time to reduce the radiation risk uncertainties before the lunar mission, but not enough time to impact design for the first missions. Because of the much greater radiation risks associated with a journey to Mars, it is also essential that research be increased now in order to make rational decisions on the feasibility of such an endeavor. In order to meet these goals, ongoing cell and animal studies need to be expanded and oriented toward an understanding of the mechanisms responsible for radiation risk, including cancer as well as the noncancer risks thought capable of having a significant impact on astronaut health. One of the key enablers to reducing this uncertainty is the NASA Space Radiation Laboratory (NSRL), located at the Department of Energy’s (DOE’s) Brookhaven National Laboratory (BNL). Its combination of atomic number, energy, and flux makes the NSRL unique because it provides nearly the full range of particles and energies that constitute GCR, at fluxes that can go from a few particles per square centimeter per second to 100 million particles per square centimeter per second. Furthermore, the NSRL facility is dedicated to providing several thousand hours of beam time exclusively for NASA. The National Research Council’s (NRC’s) Radiation Hazards to Crews of Interplanetary Missions (NRC, 1996) stated that 3 months of beam time per year would be required to make progress on high-priority research questions. There is one other heavy-ion facility in the world (the Schwerionen Synchrotron Accelerator in Darmstadt, Germany) that can deliver the same range of particles and energies as NSRL, but it is fully dedicated to nuclear physics and can provide only occasional beam for space radiation experiments. Another handful of facilities (such as the cyclotron at DOE’s Lawrence Berkeley National Laboratory, the Proton Therapy Facility at Loma Linda University, and the Heavy Ion Medical Accelerator in Chiba, Japan) deliver a partial range of particles and/or energies, but are also fully dedicated to physics or radiation therapy programs, with restricted availability for space science experiments. These other facilities may prove to be cost-effective in providing ions and beam time for certain classes of experiments, but they cannot be seen as a substitute for NSRL. Finally, because NSRL is almost fully dedicated to NASA service, space-specific modifications can be obtained that are not available elsewhere, such as broadband beams, rapid changes between beams (e.g., iron to protons), large beam spots, in-beam incubators, laboratory facilities for on-line biochemical analyses, animal holding facilities, and dosimetry and data-acquisition services. DOE’s Brookhaven National Laboratory has an annual budget of $500 million. NSRL is a relatively small part of a much larger accelerator complex that serves the high-energy, the nuclear physics, and the heavy-ion physics communities. That support can be expected to last as long as the research conducted by these communities continues to be cutting edge and vital. However, accelerators can be and have been closed when the frontier of science moved elsewhere. If DOE determines that the research topics requiring BNL accelerators no longer are a priority, they 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 plan accordingly.
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Managing Space Radiation Risk in the New Era of Space Exploration 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 aggressively at the NASA Space Radiation Laboratory 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. 2. Radiation Protection in Orion At the time of this study, the radiation protection planning for Orion and other elements of the Constellation program was in its beginning stages. As noted in the findings and recommendations below, much of what will be needed is currently being addressed. However, the development program for this system is proceeding at a rapid pace, and design changes occur frequently. Because Orion eventually must operate away from Earth’s protective geomagnetic field, it will be critically important for NASA to incorporate radiation protection at the systems engineering level from the earliest stages and to vigilantly continue throughout all phases of mission development and execution. Nominally, these design efforts will be needed out to 2013, when Orion is scheduled to be completed. 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. 3. Validation and Verification of Transport Calculations Code validation is a series of tests intended to provide evidence that a given transport calculation method correctly describes the radiation field as modified by material volumes in a given experimental arrangement. This is a falsifiable hypothesis that cannot be proven true for all circumstances with a finite set of tests. However, critical predictions can be tested by statistically significant comparisons with experimental data and with the results of other computer codes. The latter type of comparison allows for the evaluation of the precision, stability, and reliability of the computational methods; it also establishes the relative practical advantages of different methods, such as computational speed, ease of use, and flexibility. However, the validation and accuracy of the calculations can only be ascertained by comparison with independent experimental data. The experimental data may be taken from archives or obtained in experiments designed to test particular features of a specific code. Data may not be available for all interactions of interest. In this case, extrapolation schemes may be used, based on available experimental or theoretical results for particular interactions of radiation with matter (stopping power, range, nuclear cross sections). A variety of radiation transport codes are currently in use. The codes used to analyze laboratory data were developed for beams with a narrow distribution of energies. Conversely, the versions of the transport codes that are used for shielding in space were developed for incident galactic cosmic rays with a broad distribution of energies. This difference causes computational problems that prevent validation of galactic cosmic ray codes with laboratory data.
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Managing Space Radiation Risk in the New Era of Space Exploration 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. 4. Research on Solar Particle Events Operations planning support to lunar and future Mars missions will require forecast tools that estimate the probability of a solar particle event (SPE) within the next few hours to days. Real-time mission operations support will substantially benefit from predictions of the expected peak flux, time to peak flux, total fluence, and duration of ongoing events within the first hour of event onset. However, today these tools are still limited. SPE predictions 1 day to several days in advance first require the operational capability to predict the onset or character of the source coronal mass ejections (CMEs) or the near-Sun ambient plasma characteristics where the most significant particle acceleration occurs. Even after an SPE has been observed to be underway, forecasts of peak flux are only good to within an order of magnitude at best. The National Oceanic and Atmospheric Administration (NOAA) and the U.S. Air Force provide current operational space weather forecasts. However, their models are inherently limited by the fact that they are fundamentally based on x-ray flare proxies to the SPE. Significant advances in near-term forecast capability will require the development of a physics-based model that builds on current understanding of the relationship between CMEs and SPEs and incorporates new information expected from ongoing science missions over the next 10 years. Such models could reasonably be expected to exist in time for the human return to the Moon in 2020. In the nearer term, over the next few years, significant improvements could be made by incorporating elements of the cutting-edge SPE research models into more operations-oriented prediction codes. In addition, a better understanding of the precursor conditions necessary for a significant solar particle event could be applied to reliably predict periods when a severe solar storm is extremely unlikely. These “all-clear” forecasts would be valuable to mission operations and mission planning. 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. 5. Empirical Data for Shielding Design Although much of the experimental data collected by the NASA measurements consortium has been analyzed and is available online, some data remain to be analyzed. However, these data, collected over more than 10 years, may not be adequate because they do not include information on energy-, angle-, or multiplicity-dependence of the fragments produced from nuclear interactions. In addition, only a partial set of the experimental data obtained by heavy-ion investigators throughout the world over the past several decades has been collected, cataloged, and put into databases accessible to NASA. Validations of transport codes with empirical transport data have been limited. In particular, comparisons with laboratory measurements of fluence spectra (number of fragments of each produced species as a function of energy and angle) cannot be made using deterministic versions of NASA’s space radiation transport codes, since they were developed for incident galactic cosmic ray and SPE spectra, which have
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Managing Space Radiation Risk in the New Era of Space Exploration a broad distribution of energies. Hence, published comparisons have been between predictions of different codes, or between code and space-based dosimetry measurements. The use of accelerator measurements is critical if adequate statistics are to be obtained with a sufficient number of beam and target combinations. That being said, great value can be gained from a carefully chosen but limited set of new laboratory experiments. The uncertainties currently attributed to physics are minor compared with the biological uncertainties, and NSRL beam time should be prioritized accordingly. 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. 6. In Situ Monitoring and Warning A comprehensive strategy for radiation protection goes well beyond the selection and arrangement of material to be incorporated in Orion, Lunar Lander, and elements of the lunar outpost. NASA will ultimately establish a system, or architecture, that incorporates various components beyond the physical barriers to radiation. The broader architecture is discussed in the report Space Radiation Hazards and the Vision for Space Exploration (NRC, 2006) and is illustrated by Figure 5-1, reprinted from that report. The architecture will include the following: Solar monitoring: What is going on at the Sun to observe activity that may lead to an SPE? Heliospheric monitoring: What is the state of the solar wind, interplanetary magnetic field, and fluctuations in the nominal solar wind to be able to predict the propagation of accelerated protons from the source to the astronauts? Energetic-particle monitoring: What is the solar proton and ion flux in the region near the astronauts? Real-time astronaut dose and dose-rate monitoring: What will the dose be, as measured under any shielding available to the astronaut? Communications and data fusion: What are the issues that may affect the ability to get the right data to the right place in a useful format? Flight rules and procedures: How will the mission react to sudden changes in the radiation environment to avoid overexposure and follow the principle of As Low As Reasonably Achievable (ALARA)? Each element has contributing components: observations, models, procedures, and so on. For example, radiation exposure forecasts will rely on in situ dosimetry (which will also be available to the crew) as well as transport models that start with the forecast or observed radiation environment and convert the external radiation field into estimates of the astronaut’s exposure. Mission impact will consider the exposure forecast, the flight rules, the mission manifest, and crew exposure histories. Recommended actions to minimize radiation exposure will be considered in the context of mission objectives and competing risks. Often overlooked or incorporated late in the design, the communication infrastructure is an important factor to consider from the outset in the construction of a total radiation protection architecture. For example, real-time telemetry from radiation monitors on the International Space Station are transmitted to Earth through antennas that are subject to being shut down during high radiation events. For a lunar architecture, some of the elements of a radiation shielding strategy may be located far from Earth and may require multiple round trips between the Moon and Earth before actionable advice is delivered to the astronauts. Tradeoffs are needed to determine optimal distribution of measurements, models, and decision making, particularly for surface excursions when an SPE may
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Managing Space Radiation Risk in the New Era of Space Exploration FIGURE 5-1 Operational elements of a radiation risk management architecture. SOURCE: NRC, 2006. disrupt communications. There may be times when the crew will have to make decisions without input from ground control. On a Mars mission, the communication to the astronauts will take up to 20 minutes to arrive from Earth. Data collectors may also be up to 20 minutes away from Earth and Mars (see Figure 5-2). Since the highest-energy particles move with speeds close to the speed of light, techniques are needed to ensure that warnings and support are provided in a timely fashion. During every spaceflight, radiation dosimeters will be required to determine the exposure to astronauts and to confirm compliance with regulations, as well as to indicate if exposure rates require the postponement or termination of a particular mission. The response of space dosimeters must conform to the biological risk for the broad spectrum of incident radiation. This is no easy task and research in this field should be performed along with research into the biological effects of radiation. Incorporation of in situ warning and dosimetry into Orion must begin immediately. Provisions for active and passive dosimeters into the Lunar Lander and outpost components (including all surface transportation elements) should be included in every iteration of the design, from the very earliest stages. If NASA’s Exploration Systems Mission Directorate establishes a formal working relationship with NASA’s space science community and with the NOAA Space Environment Center over the next 3 years, the proper elements of a comprehensive operational warning system can be in place when humans return to the Moon. The Geostationary Operational Environmental Satellite (GOES)-R and -S spacecraft, which are expected to become operational in 2017 and 2020, respectively, will have an extensive complement of operational space environment monitors. These spacecraft are already well into development, with instruments to be delivered as early as 2011. However, there will be a need for additional
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Managing Space Radiation Risk in the New Era of Space Exploration FIGURE 5-2 One-way communication times between elements of Mars mission will be measured in the tens of minutes. SOURCE: Turner and Levine, 1998. solar, solar wind, and particle environment monitors and communications elements. If there is to be an acquisition plan for additional exploration-specific space weather elements in place by 2020, a comprehensive requirements analysis should be completed by 2012. 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. 7. Multifunctional Materials Materials development is not an overnight process. Developing a brand-new material typically takes 15 to 20 years of research; a research program started today would have results available to designers after Mars mission planning had already begun. There are a number of carbon and polymer composites, however, that have been under investigation for years. Some of these materials would have a dual advantage of reduced weight and increased
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Managing Space Radiation Risk in the New Era of Space Exploration radiation protection. While they are not yet ready to be used for flight hardware, a 5- to 10-year research program could bring them to maturity in time to influence lunar outpost design, as well as removable equipment carried aboard Orion. However, the longer such a research program is delayed, the less benefit the end product will be able to provide to the Exploration Vision. Radiation shielding does not need to come in the form of thick plates attached to the bulkhead of a spacecraft. Everything inside—food, water, furnishings, equipment, and even other astronauts—provides additional shielding. Clever material selection and placement of objects such as shelving, lockers, and electronics can give the astronauts increased protection, resulting in a reduced need for heavy external shields. This is not necessarily a dedicated project, but something that should be considered and incorporated into the ALARA process for all Constellation elements. 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. 8. In Situ Shielding Tradeoffs Using lunar or martian regolith to shield a habitat seems like a way to provide radiation shielding without adding launch weight. However, even lunar soil is not free. If astronauts spend the time to construct shields by hand, it could reduce the overall value of the mission. If robotic or remotely operated machines are used to construct the shields, they must be transported to the site. These machines may or may not be applicable to other tasks required by the outpost. Finally, the Orion crew module would still need shielding for the transit period, particularly on a Mars mission. In situ shielding is a viable idea that offers potential benefit to Project Constellation. However, its benefits will vary based on the mission architecture. It will not be obvious whether it is more practical to carry terrestrial shielding or to dig in upon arrival. Analysis of the tradeoffs, concurrent with the development of the outpost architecture, will be valuable in determining the correct path. 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. 9. Review of Existing Neutron Albedo Datasets One radiation source that may not have been sufficiently considered on the Moon and on Mars is the secondary radiation (primarily neutrons and gamma rays, often referred to as “albedo”), produced by the interactions of GCR and high-energy solar protons with matter in the martian atmosphere or the surface of Mars or the Moon. The gamma-ray albedo is not believed to be a significant radiation hazard. Existing estimates of the total effective dose from lunar albedo neutrons indicate that they contribute approximately 10 percent of the dose on the lunar surface. If a complete investigation of the available data confirm this conclusion, it is not necessary to make any further measurements of lunar albedo neutrons for the purposes of radiation protection. Of potentially greater importance on Mars is the neutron albedo because of the extra neutrons generated by atmospheric nuclear interactions. The NASA Langley Research Center has developed computer codes to model secondary neutron production and has carried out extensive calculations for the Moon and for Mars, using GCR and the August 1972 SPE as the progenitors. It may be possible to validate these albedo-production codes with data from Mars Odyssey (e.g., Evans et al., 2006) and future lunar and Mars missions. However, opportunities to validate these codes with data from closer to home can be exploited. In particular, terrestrial measurements of the
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Managing Space Radiation Risk in the New Era of Space Exploration neutron albedo are available from aircraft (Goldhagen et al., 2004), at sea level (Nakamura et al., 2005), and at various altitudes in between (Kowatari et al., 2005). Sato and Niita (2006) have recently reported on modeling these observations using Monte Carlo techniques and the CREME96 model (Tylka et al., 1997) of the galactic cosmic rays. They will not, of course, address the aspects of the code that are tied to assumptions about the composition and structure of the lunar and martian surface and the martian atmosphere. For more information, see Adams et al. (2007), Harris et al. (2003), Petry (2005), Share and Murphy (2001), Share et al. (2001, 2002), and Wilson et al. (1989, 2004). 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. 10. Surface Fission Power Demonstration—Nuclear Power for Mars As discussed in Chapter 4, long-term missions to Mars cannot depend on solar power alone. Nuclear reactors are the best candidate for these conditions and are an established, mature technology. Most of the necessary work will involve modifying current technology for space, particularly martian operation. Not only must the reactors have adequate shielding to protect astronauts during nominal operation, but they must have enhanced safety and reliability as well. Many of the constraints on Mars are also present, to a lesser degree, on the Moon. There is harmful dust on the Moon, but it exists in a vacuum; Mars has atmospheric gas and windblown dust, which has corrosive and electrostatic effects as well as affecting a susceptibility to high-voltage breakdown. Lunar reactors would have to be somewhat autonomous, since astronauts cannot constantly attend to them, although they could potentially be controlled from Earth. Martian reactors would have to be very autonomous, because the communications delay makes teleoperation infeasible. In preparation for future missions to Mars, NASA can leverage the unique environment of the lunar surface to move beyond conceptual studies into a technology demonstration of space nuclear systems. On the Moon, solar power is also feasible, meaning that astronauts’ lives would not be dependent on the success of a reactor design. Furthermore, fission surface power would be necessary for a lunar settlement at a location other than the poles, should one become desirable. One year before a Mars mission, the fission surface power systems should be well demonstrated. The terrestrial use of nuclear power systems has gained substantial experience that can be leveraged in developing, demonstrating, testing, and manufacturing the prototype space reactor and space fission surface power system. A demonstration would require national support, in the form of a joint program including national laboratories and universities helping the NASA specialists to design and demonstrate successful nuclear space technology. Commercial nuclear entities will also be needed for fuel production. The particular challenges are as follows: Nuclear reactor design: Develop the reactor design, build the full-scale end-to-end prototype, and develop and test the reactor performances in simulated environments of the Moon and Mars; perform irradiation materials tests on the nuclear fuel, primary loop materials, and shielding. Software and database review for radiation transport modeling: Review the cross-section database for materials of interest under lunar and martian environmental conditions; review the existing computational tools for radiation transport modeling. Materials testing: Evaluate radiation effects under different environmental conditions (temperature, pressure, gravity) and fill in the gaps in the radiation-thermal-mechanical materials property database. Power conversion and heat-rejection technology: Develop the technology to demonstrate the best power conversion system and the heat-rejection components and systems.
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Managing Space Radiation Risk in the New Era of Space Exploration Shielding: Develop the series of tests for the optimal shielding packing and weight versus radiation attenuation effectiveness under constraints on minimal weight for transit to the surface and maximized multipurpose use of the materials involved in the shielding design. Instrumentation and remote control systems: Demonstrate the technology to support potentially required innovations in instrumentation and remote control systems, both hardware and software. Launching nuclear reactor into space: Develop the technology for safe launching of the nuclear reactor. While lunar reactors should be designed with martian operation in mind, additional development and testing will still be required for martian reactors. The Moon is not a perfect analog for Mars, but can be a valuable stepping-stone. 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. STRATEGIES FOR KEEPING RADIATION RISK WITHIN NASA GUIDELINES Combined with the technology investments outlined above, the following recommendations constitute a comprehensive strategy for mitigating radiation risk to Constellation astronauts. Transition from Research to Operations There is a significant gap between operational SPE forecast tools and the improved understanding of these events that has developed over the past decade. The slow transition of research to operations is a well-documented issue in this field; it has been discussed in numerous recent studies (NRC, 2000, 2003, 2006; OFCM, 2006). Space weather research and forecasting is inherently interdisciplinary and increasingly interagency. Support to lunar missions will build on existing capabilities but will have unique requirements as well. Improved forecasting of SPEs in support of human activities at the Moon in turn will have a benefit to terrestrial users of space weather forecasts. There needs to be a strategy for integrating knowledge, needs, and satellite requirements so that the agencies involved (NOAA, the National Science Foundation, NASA, and the Department of Defense) can work together to improve monitoring and forecast capabilities. This recognition of the interagency flavor to space weather is not new to this committee. The decadal research strategy in solar and space physics (NRC, 2003, pp. 19-20) contained the following recommendations: The principal agencies involved in solar and space physics research—NASA, NSF, NOAA, and DOD—should devise and implement a management process that will ensure a high level of coordination in the field and that will disseminate the results of such a coordinated effort—including data, research opportunities, and related matters—widely and frequently to the research community. For space-weather applications, increased attention should be devoted to coordinated NASA, NOAA, NSF and DOD research findings, models, and instrumentation so that new developments can quickly be incorporated in the operational and applications programs of NOAA and DOD. 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
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Managing Space Radiation Risk in the New Era of Space Exploration 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. Human Capital Infrastructure Intellectual Capital A research program establishes the intellectual capital that medicine, engineering, and management apply to accomplish an agency mission. During the Apollo era, ample intellectual capital was available from research in metallurgy, semiconductors, computers, and aerodynamics, largely performed under the auspices of agencies and institutions other than NASA. The critical element in space exploration is the human. No comparable research legacy is available now to draw on in order to ensure the health and performance of humans in space and their subsequent quality of life. The overwhelming majority of current biomedical and radiation physics research is focused on issues that do not allow easy application to the NASA mission. One of the vital strategic roles of having a research program is the availability of a science community able to recognize, develop, and apply breakthroughs to the NASA mission. However, it takes 5 to 15 years before the state of the art in biology results in substantial breakthroughs with applications to the NASA missions. Nevertheless, incremental progress in radiation risk management is possible, and has been significant over the past 15 years. To be successful, a multidisciplinary, mission-oriented research program requires several components: Critical mass, Stability, Credibility, Accountability, and Integration. These components are not independent. A critical mass of investigators is required to maintain credibility, to provide mechanisms of accountability such as reviews, and to provide wide enough coverage of research areas to make integration possible. Stability of research support is required by the life cycle of scientific research, which incorporates the duration of graduate and postdoctoral study and research and generates the commitment of sufficient time and effort to make measurable progress. In biology, an additional constraint is posed by the life scales of experimentation. Cells in culture need to be established, maintained, and characterized before and after experiments. Animals need to be bred, housed, and selected before irradiation. Radiation effects may not become apparent for many cycles of cell division, whether in cell culture or in an animal tissue. Especially for validation of hypotheses, it may be necessary to wait for a significant fraction of an animal’s life before a statistically significant result can be ascertained. For this reason, the timescale of meaningful advances in biology can be estimated as being 5 to 15 years. Critical Mass To illustrate these concepts, in 1990, all of the radiation research at NASA consisted of about a dozen scientists, and the entire budget for radiation biology, physics, and transport code development was approximately three-quarters of a million dollars. An expansion of the budget was planned but, as a consequence of years of neglecting research, NASA found itself faced with the fact that it takes time to build up a science community. The current NASA radiation research community was built up slowly and painfully, one scientist at a time. It was somewhat easier to find individuals able and willing to review research proposals, but the scores given to most proposals were poor, reflecting the fact that, initially, many outstanding and very capable scientists were disinterested in responding to a NASA Research Announcement. Hence, it was only the occasional graduate student or bright postdoctoral student, attracted by a stipend, who improved the quality of the field. These younger people
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Managing Space Radiation Risk in the New Era of Space Exploration had the ability to attract brighter students in turn, and the development of interesting research areas brought in scientists from related fields, who had not theretofore taken an interest in radiation (much less in heavy charged particle radiation). The turning point in the radiation biology program came when NASA and the National Cancer Institute embarked on a joint, 5-year program to support research in genomic instability. This is a key concept in the understanding of cancer progression, and high-energy heavy ions turned out to be a very useful tool in its study. As a consequence, studies related to genomic instability became part of the mainstream of radiation research related to cancer, as well as in space radiation research. A wide variety of investigators in many related areas became interested in this type of problem and added to the ranks of the growing science community. A similar interagency collaboration, with the DOE’s Low Dose Program, continues to this day, leveraging the research of both agencies. Concerning the development of necessary nuclear models and databases and the completion of the radiation transport codes needed for shielding and risk assessment, the cadre of radiation physics expertise available within the larger space radiation protection community has been and continues to be very small and is shrinking. NASA needs experts in radiation transport who are also experts in the nuclear interactions of heavy charged particles. It is generally true that most of the developers of transport codes are within the reactor engineering community and have little or no understanding of charged-particle transport as applied to high-energy heavy ions. They are also generally unfamiliar with the nuclear models and databases needed to carry out HZE particle transport for space applications. Within the field of nuclear physics, the areas of current investigation, interest, and expertise are far removed from the needs of the space radiation transport community. Few nuclear physicists are knowledgeable about radiation transport methods and codes. Thus, there needs to be an effort by NASA to maintain an adequate level of available expertise and knowledge, both intramural and extramural, in space radiation transport and interactions. The intramural expertise is needed to ensure that in-house knowledge is available to design engineers, tool developers, and mission planners. Since space radiation physics and transport, as applied to space radiation protection, are not typically part of any university curriculum, extramural expertise and research personnel are needed, particularly at universities, to provide training of new, knowledgeable personnel to replace existing scientists and engineers as they retire. 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 supply 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. Integration of Radiation Protection into Design of Vehicles and Missions Managing radiation risk is an integral part of exploration mission design and execution. As humans leave Earth orbit and mission duration increases, it becomes even more critical. This is not something that can be added the day before liftoff. Truly effective radiation shielding requires the interaction of multiple disciplines—materials, biology, space physics, and communications. Furthermore, the concept of radiation shielding must then percolate into the design of Constellation vehicles and missions. A model of a successful radiation protection process is shown in Figure 5-3. At this time, the path from research to radiation protection standards seems to be well established, although the quality of the research is threatened by current budget cuts. Similarly, the process to provide dosimetry for monitoring, validating, and recording the radiation environment and the processes to engender forecasts and warnings of high radiation levels are both understood; the content of these activities is being developed in various ways.
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Managing Space Radiation Risk in the New Era of Space Exploration FIGURE 5-3 Suggested risk management process. NOTE: NCRP, National Council on Radiation Protection and Measurements; ALARA, As Low As Reasonably Achievable. However, space systems designers have many more concerns than radiation. If radiation safety is not embedded into the approval process, portable polyethylene shielding could be removed from the design at the last minute in order to meet weight requirements. Alternatively, if the designers realize at a late stage that they have not used enough protection, they may have to add a layer of parasitic shielding, causing a science experiment or a spare part to be removed. Clever shielding designs—particularly those that include multifunctional components—can provide adequate protection for a relatively low weight cost, but only if they are integrated early on. 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.
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Managing Space Radiation Risk in the New Era of Space Exploration 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. At the request of the National Aeronautics and Space Administration, the Institute of Medicine recently examined the process by which NASA establishes spaceflight health standards for human performance. The standards-setting process is currently designed to address acceptable levels of risk for three categories of in-flight health concerns, including space permissible exposure limits (e.g., radiation exposure standards). The committee found that the initial standards-setting process developed by NASA is a carefully designed evidence-based process that involves input from relevant stakeholders. The flight rules—or even the processes of developing flight rules—have yet to be started. Whether or not space radiation risk can be efficiently managed hinges on these rules. Imagine that a CME erupts from the Sun, preceding an SPE. A new monitoring satellite detects the CME and sends a message to Earth. NASA’s Space Radiation Analysis Group calculates a nowcast, predicting exactly when and for how long the astronauts need to stay inside. The lunar module provides sufficient radiation shielding to protect the crew. However, it is the flight rules and operational procedures that allow (and require) mission control to reconfigure the day’s tasks so that the astronauts are safe. Furthermore, it is not sufficient to create overly conservative procedures. Although there is no loss of life, a mission can still fail because an astronaut sits huddled in a shielded room when it is actually safe for him or her to be drilling for samples outside. Designing flight rules that maximize the safety of the crew and of the mission will require taking advantage of the expertise available in the space radiation research and design community. 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. Radiation risk management is one of the major challenges of solar system exploration. The Exploration Vision is NASA’s first mission that intends to truly face it head-on. It is not an impossible problem, but neither can it be taken lightly. Every vehicle, every mission must take radiation into account at some level—either it provides sufficient protection, or there is an operational procedure which ensures that the crew is able to get to a protected place. Reaching this goal will require research, technology development, and a great deal of cooperation. Finding 5-15. Long-term commitment. Exploration continues beyond a single mission and requires a long-term commitment to radiation risk management. REFERENCES Adams, J.H., M. Bhattacharya, Z.W. Lin, G. Pendleton, and J.W. Watts. 2007. The ionizing radiation environment on the Moon. Advances in Space Research 40:338. Evans, L.G., R.C. Reedy, R.D. Starr, K.E. Kerry, and W.V. Boynton. 2006. Analysis of gamma ray spectra measured by Mars Odyssey. Journal of Geophysical Research 111(March):E03S04, doi:10.1029/2005JE002657.
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