1. The principal risks of suffering early effects as a result of exposure to radiation in space arise from solar particle events (SPEs). It is not too difficult a task to provide appropriate shielding or storm shelters to protect against exposure during SPEs, but surveillance methods to predict and detect solar particle events from both sides of the sun relative to a spacecraft must be improved.
  2. The kinds of biological effects resulting from exposure to the ionizing radiation encountered in deep space do not differ from those resulting from exposure to x rays. However, the quantitative difference between the risks posed by x rays (low-LET radiation) and by heavy high-energy nuclei (high-LET radiation) may be large, and the magnitude of the human biological effects is largely unknown. An understanding of these effects—including cancer induction, central nervous system changes, cataract formation, heritable effects, and early effects on body organs and function—as well as of the shielding necessary to mitigate these effects for crew members, is essential for the rational design of space vehicles built for interplanetary missions.
  3. The task group members generally agreed that the potential late effects of radiation are the major concern in estimating risks to crew members. Of the known late effects, cancer is currently considered to be the most important. However, experimental data suggest that exposure to high-atomic-number and high-energy (HZE) particles may also pose a risk of damage to the central nervous system (CNS). Since it is estimated that during a 1-year interplanetary flight each 100-µm 2 cell nucleus will be traversed by a primary energetic particle of atomic number greater than 4,1 further experimentation is essential to determine if CNS damage is a significant risk.
  4. To estimate the cancer risk posed by exposure of humans to radiation such as HZE particles, for which no human data are available, it is necessary to use data on the Japanese atomic bomb survivors exposed to acute low-LET radiation and then extrapolate, based on experimental data, to estimate the risks posed by high-LET radiation. At present, the only comparative data for cancer are for studies on the induction of Harderian gland tumors in mice. Additional research is required to reduce the uncertainties of the assumptions inherent in this approach. To calculate risks associated with exposure to low-fluence-rate HZE particles, it is assumed, based on cell and animal studies, that there is not a large dose-rate effect.
  5. Biophysical models and data for cell killing and mutagenesis indicate that as the LET increases, the biological effect of the radiation increases to a maximum near a LET of 100 keV/µm and then decreases at higher LET. (See, for example, NCRP Report No. 98.2) However, no such decrease was observed in the one animal tumor for which data were obtained using a number of heavy ions with increasing LET.3 This discrepancy creates uncertainties in estimates of risks associated with exposure to particles at these higher LETs. To resolve these uncertainties, additional systematic studies are needed on the induction in animals of other radiobiologically well characterized cancers, such as leukemia and breast cancer. From a practical point of view, sufficiently accurate data can only be obtained from ground-based experiments using acute doses.
  6. The background frequencies of the heritable changes in humans, which might be increased by exposure to radiation, range from ~ 10 -5 to 3 × 10-3 per genetic locus.4 The minimum chronic dose that would double these values is ~ 4 Sv,5 a value greater than that given in NASA's current lifetime exposure guidelines. Hence, the genetic risk—the absolute increase in the frequencies of heritable changes—to an astronaut will be low. The risk to the gene pool of the overall human population will of course be far lower due to the relatively small number of space-faring humans.
  7. The doses of radiation to which crews are exposed in space are not expected to induce early deterministic effects, with the possible exception of skin damage and a temporary reduction in fertility. Skin damage is likely only following exposure at high doses outside the spacecraft. Experimental studies in dogs indicate that any reduction in fertility per unit dose of radiation may be greater for low-dose-rate, protracted exposure than for acute exposure.6
  8. The space vehicles used for missions of short duration in low Earth orbit have required minimal optimization of radiation shielding for crew protection purposes. In contrast, optimization of shielding for prolonged interplanetary trips will be a major factor in the design and cost of space vehicles. It will be

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