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The health threat from cosmic rays is the danger posed by galactic cosmic rays and solar energetic particles to astronauts on interplanetary missions. Galactic cosmic rays (GCRs) consist of high energy protons (85%), helium (14%) and other high energy nuclei (HZE ions).〔 Solar energetic particles consist primarily of protons accelerated by the Sun to high energies via proximity to solar flares and coronal mass ejections. They are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.〔(Can People go to Mars? )〕〔 〕 In October 2015, the NASA Office of Inspector General issued a health hazards report related to space exploration, including a human mission to Mars. ==The deep-space radiation environment== The radiation environment of deep space is different from that on the Earth's surface or in low Earth orbit, due to the much larger flux of high-energy galactic cosmic rays (GCRs), along with radiation from solar proton events (SPEs) and the radiation belts. Galactic cosmic rays create a continuous radiation dose throughout the Solar System that increases during solar minimum and decreases during solar maximum (solar activity). The inner and outer radiation belts are two regions of trapped particles from the solar wind that are later accelerated by dynamic interaction with the Earth's magnetic field. While always high, the radiation dose in these belts can increase dramatically during geomagnetic storms and substorms. Solar proton events are bursts of energetic protons accelerated by the Sun. They occur relatively rarely and can produce extremely high radiation levels. Without thick shielding, SPEs are sufficiently strong to cause acute radiation poisoning and death.〔(Biomedical Results From Apollo - Radiation Protection and Instrumentation )〕 Life on the Earth's surface is protected from galactic cosmic rays by a number of factors: # The Earth's atmosphere is opaque to primary cosmic rays with energies below about 1 gigaelectron volt (GeV), so only secondary radiation can reach the surface. The secondary radiation is also attenuated by absorption in the atmosphere, as well as by radioactive decay in flight of some particles, such as muons. Particles entering from a direction close to the horizon are especially attenuated. The world's population receives an average of 0.4 millisieverts (mSv) of cosmic radiation annually (separate from other sources of radiation exposure like inhaled radon) due to atmospheric shielding. At 12 km altitude, above most of the atmosphere's protection, radiation as an annual rate rises to 20 mSv at the equator to 50–120 mSv at the poles, varying between solar maximum and minimum conditions.〔(Evaluation of the Cosmic Ray Exposure of Aircraft Crew )〕〔(Sources and Effects of Ionizing Radiation, UNSCEAR 2008 )〕 # Except for the very highest energy galactic cosmic rays, the radius of gyration in the Earth's magnetic field is small enough to ensure that they are deflected away from Earth. Missions beyond low Earth orbit leave the protection of the geomagnetic field, and transit the Van Allen radiation belts. Thus they may need to be shielded against exposure to cosmic rays, Van Allen radiation, or solar flares. The region between two to four Earth radii lies between the two radiation belts and is sometimes referred to as the "safe zone".〔(【引用サイトリンク】 publisher=Goddard Space Flight Center, NASA )〕 See the implications of the Van Allen belts for space travel for more information. # The interplanetary magnetic field, embedded in the solar wind, also deflects cosmic rays. As a result, cosmic ray fluxes within the heliopause are inversely correlated with the solar cycle. As a result, the energy input of GCRs to the atmosphere is negligible – about 10−9 of solar radiation – roughly the same as starlight.〔Jasper Kirkby; (Cosmic Rays And Climate ) CERN-PH-EP/2008-005 26 March 2008〕 Of the above factors, all but the first one apply to low Earth orbit craft, such as the Space Shuttle and the International Space Station. Exposures on the ISS average 150 mSv per year, although frequent crew rotations minimize individual risk.〔(Space Radiation Organ Doses for Astronauts on Past and Future Missions ) Table 4〕 Astronauts on Apollo and Skylab missions received on average 1.2 mSv/day and 1.4 mSv/day respectively.〔 Since the durations of the Apollo and Skylab missions were days and months, respectively, rather than years, the doses involved were smaller than would be expected on future long-term missions such as to a near-Earth asteroid or to Mars〔 (unless far more shielding could be provided). On 31 May 2013, NASA scientists reported that a possible manned mission to Mars〔 may involve a great radiation risk based on the amount of energetic particle radiation detected by the radiation assessment detector (RAD) on the Mars Science Laboratory while traveling from the Earth to Mars in 2011–2012.〔〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「health threat from cosmic rays」の詳細全文を読む スポンサード リンク
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