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Assessment of Programs in Space Biology and Medicine 1991 8 Radiation Biology The radiation environment of space is considerably less benign than on Earth. Planning for extended human sojourns in space mandates that we have quantitative knowledge about the dose rates and the types of radiation that will be encountered. Similarly, the effects of the different types of radiation encountered in space, especially deep space, must be defined quantitatively. Radiation environments will be defined as a result of dosimetric measurements made in space and models that include attention to factors that cause the marked temporal variations in radiation fluxes. Much of the necessary radiobiology research can be carried out on Earth with defined radiation sources. Basic experiments in space are required to investigate and understand the interactions if any between radiation and microgravity. STATUS OF DISCIPLINE The radiation environment within the magnetosphere is known with fair precision. Measurements on Shuttle missions at different altitudes and orbital inclinations have established the ability of models to predict dose rates encountered in low earth orbits. Still, the precision of the information about the radiation environment that will be experienced in geosynchronous orbit and within space vehicles in deep space is not yet adequate for the estimation of risks of radiation effects. The biological effects of low-energy transfer (LET) radiations, such as protons and electrons, are relatively well known because of the large body of data obtained from ground-based studies. This is not the case for high-energy (HZE) particles that are a small but important component of galactic cosmic rays. Current knowledge about radiation environments and radiobiology relevant to space has been summarized in a number of reports, including the Space Science Board's 1988 report Space Science in the Twenty-First
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Century—Life Sciences. SCIENTIFIC GOALS The Goldberg Strategy did not deal comprehensively or specifically with radiation studies. The report did, however, note some scientific objectives, for example, (1) to measure the specific effects of radiation versus gravity on genomic stability and the appearance of aberrant cell lineage over several generations of representative organisms; (2) to establish an extended program of research into the effects of HZE particles on developmental events; and (3) to investigate whether HZE particles enhance the probability of malignant transformation. PROGRESS Despite budgetary restrictions, NASA has managed to maintain a limited but ongoing research programs in the fields of both radiation dosimetry and radiobiology. Studies of radiation environments have been carried out at both the Agency's own centers and several universities. Measurements of radiation of various types have been made on all Shuttle missions as well as some Soviet missions in collaboration with USSR scientists. Instrument development has also been supported. Ground-based studies are currently under way on the effect of fragmentation of HZE particles and on the secondary particles, areas of importance for deep space missions. In the field of radiobiology, a modest ground-based program has been supported by NASA. The Agency has had to rely mainly on universities and the national laboratories for such research as it does not now have any radiobiological studies at its own centers. The radiobiological studies supported by NASA and other agencies have been concerned mainly with the effect of HZE particles on DNA, cell survival, mutation, malignant transformation of cells in vitro, carcinogenesis, and the induction of cataracts and life shortening in mice. These studies have provided some information about the relative biological effectiveness (RBE) of a number of different types of heavy ions. Certain generalizations about the relationship of LET and RBE for some specific acute and late effects can be made from these studies. For example, the values for RBE increase with increasing LET up to about 100-200 KeV/µm. Studies were carried out on the German D-1 Mission to determine whether there were interactions between radiation and microgravity on embryogenesis and organogenesis in C. morosus. Results indicate that (1) HZE particles caused anomalies when traversal of HZE particles occurred during organogenesis and (2) the combined exposure to HZE particles and microgravity acts synergistically on eggs at all developmental stages, resulting in high rates of
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developmental anomalies. Recently, NASA supported a study by the National Council on Radiation Protection (NCRP) of the available information in order to estimate the risks from radiation exposures in space. This study has resulted in the development of guidelines concerning radiation protection standards that can be applied to astronauts on missions in low earth orbit such as the Space Station. The radiation environment inside space vehicles in deep space must be defined more precisely. There are plans to make the measurements of radiation fluxes beyond the Van Allen radiation belts and to determine the energy and LET spectra of the various types of radiation. For all the radiation environments in space, there is a need to acquire real-time measurements of the energy, LET spectra, and LET fluxes of the different types of radiation involved. Continuing ground-based radiobiological studies should (1) determine the effects of protracted proton irradiation (protons predominate in low earth orbits and are the major component of galactic cosmic rays); (2) investigate the effects of HZE particles, including cancer induction; and (3) establish whether HZE particles cause damage that may only appear late in life and that cannot be predicted from knowledge about the effects of other radiation qualities. Basic studies on the relationship of energy deposition and biological effects with HZE particles can provide information important to the general understanding of radiation effects. There is a need for a number of studies using different biological systems to determine the nature and importance of interactions of radiation and microgravity. These studies should be broad in range including investigations of chromosome stability and chromosome aberrations, cell proliferation, and cellular functions important to immune competence. The studies of the effects of microgravity on embryogenesis and development and growth are covered elsewhere in this report, but these investigations will provide a further opportunity of studying radiation effects and the question of interaction between radiation and microgravity. To undertake studies of the effects both of radiation alone and of interactions between radiation and microgravity, "LifeSat" has been proposed as a candidate for a FY 1992 new start. This free-flying satellite could accommodate various experiments including both rodents and simpler organisms that have been suggested and are under consideration. It is important that the sample sizes in such experiments are sufficiently large to ensure unequivocal results. For most endpoints this will mean that simple organisms be chosen. With experiments that use test systems such as C. elegans, it will be possible to study a number of different endpoints, including aspects of development. As the duration of the missions is extended, it will become possible to study the important aspect of protracted irradiation, and such studies should be carried out.
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LACK OF PROGRESS Because of limited flight opportunities to date, it has not been possible to conduct proposed studies of interactions of radiation and microgravity. NOTE ABOUT FACILITIES Current U.S. plans call for the BEVALAC facility at Lawrence Berkeley Laboratory, (the only U.S. facility currently capable of providing beams appropriate for the study of very high-Z particles) to be phased out. It will probably not be available after 1993. A proposed replacement for the BEVALAC would provide heavy-ion beams but not the very high-Z particles, such as iron, which is the heavy ion of particular interest in space radiation studies. It has been suggested that an alternative high-energy radiation research facility is the synchrotron at Darnstadt. There are, however, a number of practical considerations as to why this would not be desirable: (1) no biological experiments have yet been performed at the synchrotron; (2) there are currently a large number of investigators waiting to use the facility; and (3) there are no existing animal holding facilities, and none are planned. The use of a large number of animals is fundamental to this type of research. Time is short for providing an alternative research source to study heavy ions in order to continue the HZE-particle studies. If plans for extended human space missions are to be seriously pursued, it is essential to have a facility readily available for the study of HZE particles.