Abstract
In order to prevent health hazards caused by long space flight, we have to establish necessary countermeasures for radiation protection and implement them to the operational rules. A primarily important task for this purpose is to quantify the health effects of long-term radiation exposure in space. According to the present system on radiological protection, it is predicted that astronauts will receive about 0.1Sv during a half-year stay in the ISS and about 1Sv in a future mission to Mars. JAXA has set a restricting exposure level as the dose increasing the cancer risk by 3 percent; astronauts can exceed this level in case of a long mission for years. However, these considerations are based on the information obtained on ground and the dosimetry system may be changed by coming medical data on astronauts' health. Particularly, biological effects of high-LET particles including the dose-rate effects and synergistic effects of other modifying factors are quite uncertain and to be studied. Keeping in mind living in space, we will be able to fundamentally discuss the definition of 'dose' (e.g. effective dose) with the advance of radiation research.
