The overwhelming majority of radiobiological experiments are, and will be, carried out on the ground and, of necessity, will use animals. The flux rate of HZE particles in space is low, and although the particles, if not appropriately shielded, could over a long period of time produce deleterious effects in humans, the use of such radiation for radiobiological experiments on small vertebrates in space is impractical because it is not possible to transport an HZE accelerator into space. Some radiation experiments in space, using sparsely ionizing radiation, have been carried out on human lymphocytes, microorganisms, and the small roundworm Caenorhabditis elegans. The radiation sources used were radioisotopes emitting ^-particles. None of these experiments have shown any significant synergistic or antisynergistic effect of radiation and microgravity. Similar experiments on mice on the ISS would require an x-ray or y-ray source to irradiate animals at 1 g and at microgravity. Since there is no compelling theoretical reason to expect that hypogravity will affect radiation end points in vertebrates, the committee's view is the same as that in the Strategy report—such experiments "with all their logistical difficulties, will not be rewarding and may not be worth the effort" (p. 190).
Hence, it is not clear how one might validate ground-based risk prediction in space by ISS utilization as suggested in NASA's Strategic Program Plan (NASA, 1998, pp. 17, 18, 20). The biological dosimetry behind shielding may be validated by ground-based experiments (see "Development and Validation of Countermeasures" below). On the other hand, the validation of biological risk estimates, behind shielding, of HZE nuclei and energetic protons cannot be done in space but must be done at a ground-based facility.
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