Shielding Strategies for Human Space Exploration

The shield effectiveness of lunar regolith is compared with possible prefabricated shield materials from Earth, including comn~ercially used shield nlaterials in nuclear facilities. Several of the fabricated materials categorized as neutron absorbers and moderators exhibit favorable characteristics for space radiation protection. Although this effort is not intended to be a definitive trade study for specific shielding recommendations, attention is given to several factors that warrant consideration in such trade studies. For example, the transporting of bulk prefabricated shield material as opposed to the transporting of regolith-moving and processing equipment to the lunar surface is assessed on the basis of space Exploration Initiative (SEI) scenario studies. Other shielding strategies such as the processing of regolith with a composite material are considered to reduce the amount of bulk regolith required, to enhance its shielding characteristics, and to form a more structurally sound shield. Nevertheless, launching all the shield material from Earth may still be a viable alternative to the use of regolith from the standpoints of costeffectiveness, EVA time required, and other risk factors. INTRODUCTION The protection of crewmenlbers from the harmful effects of ionizing radiation is an important issue that must be addressed during the development of lunar base mission scenarios. For the shorter duration missions, the possibility of an extremely large solar proton event occurring will drive the radiation protection requirements. As mission duration increases, the steady contributions to dose from galactic cosn~ic radiation will become more important. This study investigates the properties of various shielding materials for protection against both large solar proton events and galactic cosmic radiation (GCR). Various strategies exist for the protection of crewmenlbers at an established lunar base. Popular concepts employ using in-situ materials to reduce launch mass requirements from Earth [ 1 4 ] . However, this will not reduce shielding launch requirements to zero mass because of the heavy equipment required to cover the habitats, although much of the equipment may serve multiple purposes. Other considerations of this strategy include the protection of 15-300 SHIELDING STRATEGIES FOR HUMAN SPACE EXPLORATION thc crew from a potentially large solar proton event while they are in the process of covering their habitats and the logistics of such a covering operation on the lunar surface. In this study, regolith shielding is compared with shields of aluminum, lithium hydride, magnesium hydride, various polymers and borated polymers, regolith-epoxy mixtures, and water. This information provides a materials database which is not only applicable to lunar surface operations, but also to cis-lunar and interplanetary space transfer vehicles. The amount of shielding required will ultimately be based on radiation exposure limits set forth by regulatory agencies for exploratory class missions. RADlATlON EXPOSURE Currently, no limits have been recommended by the National Council on Radiation Protection and Measurement (NCRP) for exploration missions [5]. However, for planning purposes only, the NCRP suggests that the limits established for astronauts in low-Earth orbit (LEO) may be used as guidelines for other missions if the principle of ALARA (as low as reasonably achievable) is followed [5]. LEO limits for the skin, ocular lens, and vital organs are shown in table 1. The NCRP is cul-sently in the process of revising the LEO recommendations as a result of larger estimates of cancer risk coefficients [6] . For high-energy radiation from GCR and solar proton events, the dose delivered to the vital organs is the most important with regard to latent carcinogenic effects. This dose is often assumed equal to the blood-forming organ (BFO) dose. When detailed body geometry is not considered, the BFO dose is usually computed as the dose incurred at a 5-cm depth in tissue (simulated by water in these analyses). Likewise, the skin and ocular lens dose can be conservatively approximated by the 0-cm dose. Table 1. Ionizing radiation exposure limits for low-Earth orbit [5]