Whether astronauts on board the International Space Station (ISS), crews on spacecraft voyaging to other planetary surfaces, or colonists on other solar system bodies, human exposure to space radiation implies acceptance of associated radiation risks. According to the time span before their occurrence, the effects that may result from exposure to ionizing radiation may be divided into early (‘acute’) and delayed (‘late’) effects. Acute effects are deterministic and occur only if specific threshold doses are exceeded. During missions in low-Earth orbit (LEO), the dose rate or the projected dose of the radiation is too low to exceed the thresholds for these effects. This is generally true even in the case of a solar particle event (SPE). However, during a very large SPE, mission activities may need to be controlled and astronauts remain precautionary in seeking refuge in heavier shielded compartments of a space vehicle. Radiation protection for LEO is mostly concerned with late stochastic effects, comprising the cancer and hereditary effects. For the estimates of risk of induction of cancer by radiation, the major source of data is given by follow-up studies on the atomic-bomb survivors, while considerable uncertainty remains in the estimate of hereditary effects in humans.
Radiation protection limits are defined to prevent clinically significant deterministic effects and reduce the probability or risks of stochastic effects to the extent reasonably achievable (As Low As Reasonably Achievable, ALARA): the benefits from exploration must be balanced with the cost, safety, and ethical concerns when defining acceptable levels of risk for astronauts. Except for the principles of justification and optimization (ALARA), the concepts of terrestrial radiation protection are of limited applicability to human spaceflight, as until now only few experimentally verified data on the biological effectiveness of heavy ions and the dose distribution within the human body exist. Instead of applying the annual dose limits for workers on ground (20 mSv per year) also to astronauts, whose careers are of comparatively short duration, the overall lifetime risk is used as a measure. However, given the exceptional character of space travel, comparison of radiation risks with both the safest as well as the most hazardous terrestrial occupations seems unreasonable. The limits of effective dose recommended for operations in low-Earth orbits concern stochastic effects and are age and gender specific. For a ten-year career, the lifetime probability of cancer mortality must not exceed the average by more than 3%. For long-term missions outside Earth’s magnetic field, the acceptable level of risk has not yet been defined, since there is not enough information available to estimate the risk of effects to the central nervous system and of potential non-cancer radiation health hazards (e.g., cataracts, cardiovascular diseases …).
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