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.