radiation was much higher during the Apollo flight, which were on a lunar orbit (580 mrems for a 10 days flight) [22]. The Skylab missions indicated an average exposure of 50 to 90 mrems/day [6]. In the case of major solar flare, the acute radiation damage could lead to death. The effects of lesser doses are mostly long-term effects, such as cataract, lesions of the central nervous system, and cancer. As we saw earlier, the risk of cancer might be higher than in the general population for a same dose of radiation due to the deactivation of the T-lymphocytes. The amplitude of the synergistic effect is not known [21]. There is also a genetic risk resulting from chromosome damage: although it is not a threat to the general population, it might be a very important hazard to the individual astronauts. While heritable genetic effects have been questionably shown among atomic bomb survivors, the available evidence comes from studies with Drosophilia flies and mice. The dose necessary to double the spontaneous mutation rate is about 20 rem for an acute exposure of about 5-60 times the terrestrial radiation dose-rate. It must be remembered that there is no apparent threshold dose for genetic effects, and that the mutation yield is linearly proportional to dose. The countermeasure used is shielding, and constant surveillance for solar flare, in order to be able to warn the astronauts if they happened to be out of the spacecraft. There is usually a lag time of one hour between the recognition of the solar flare and actual danger to the astronauts. The shielding used in the latest spacecraft is 4 g/sq cm of aluminum, but this is insufficient to protect against HZE particles [20, 22], 3. Human Error Another hazard of space travel which is neither related to weightlessness nor to radiation is the risk of human error [4]. A trip to Mars would need the permanent awareness of teams both on ground and aboard the spacecraft 24 hours a day for a period of three years. Studies have demonstrated a worrying propensity of professionals, even in the middle of a life-threatening occupation such as air pilots, to fall asleep around 4 am, even if this is their usual shift. The major catastrophes of the past years (Three-Mile Island, Chernobyl) occurred at this time in the morning. The possibility of huge variations in the performances of astronauts and ground controllers would probably have to be taken into account. 4. Conclusion The physical reactions of man to weightlessness have been extensively studied and are now largely known and understood. The design of spacecraft able to create an artificial gravity would solve the problems of well-being, cardiac deconditioning and musculoskeletal loss resulting from life under microgravity. Shielding from cosmic radiation can only be achieved imperfectly and at high cost, which is why the preventive measure of limiting the cumulative exposure by keeping astronauts from participating in several missions is usually recommended. The risk of human error can be reduced by the commitment of the institutions responsible for the space programmes to provide adequate staffing and support for space missions.
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