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The health threat from cosmic rays is the danger posed by cosmic rays generated by the Sun and other stars to astronauts on interplanetary missions. Cosmic rays consists of high energy protons and other nuclei. They are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.
Contents
1 The deep-space radiation environment
2 Effects
3 Mitigation
3.1 Shielding
3.2 Drugs
3.3 Timing of missions
4 See also
5 Notes
6 References
7 External links
//
The deep-space radiation environment
The radiation environment of deep space is very 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 and the radiation belts.
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 GeV, so only secondary radiation can reach the surface. The secondary radiation is also attentuated by absorption in the atmosphere, as well as by radioactive decay in flight of some particles, such as muons.
Shielding by the bulk of the planet itself cuts the flux by a factor of two.
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 ("geomagnetic shielding");
The sun's magnetic field has a similar effect, tending to exclude galactic cosmic rays from the plane of the ecliptic in the inner solar system.
As a result the energy input of GCRs to the atmosphere is negligible about 10?9 of solar radiation - roughly the same as starlight.
Of the above four factors, all but the first one apply to low earth orbit craft, such as the International Space Station. Therefore, the only astronauts who have ever been exposed to a significant radiation flux from galactic cosmic rays are those in the Apollo program. Since the durations of the Apollo missions were days rather than years, the doses involved were small compared to what would occur, for example, on a crewed mission to Mars.
Effects
Like other ionizing radiation, high-energy cosmic rays can damage DNA, increasing the risk of cancer, cataracts, neurological disorders, and non-cancer mortality risks.
The Apollo astronauts reported seeing flashes in their eyeballs, which may have been galactic cosmic rays, and there is some speculation that they may have experienced a higher incidence of cancer. However, the duration of the longest Apollo flights was less than two weeks, limiting the maximum exposure. There were only 24 such astronauts, making statistical analysis of the effects nearly impossible.
The health threat depends on the flux, energy spectrum, and nuclear composition of the rays. The flux and energy spectrum depend on a variety of factors: short-term solar weather, long-term trends (such as an apparent increase since the 1950s), and position in the sun's magnetic field. These factors are incompletely understood. The Mars Radiation Environment Experiment (MARIE) was launched in 2001 in order to collect more data. Estimates are that humans unshielded in interplanetary space would receive annually roughly 400 to 900 mSv (compared to 2.4 mSv on Earth) and that a Mars mission (12 months in flight and 18 months on Mars) might expose shielded astronauts to ~500 to 1000 mSv. These doses approach the 1 to 4 Sv career limits advised by the National Council on Radiation Protection and Measurements for Low Earth orbit activities.
The quantitative biological effects of cosmic rays are poorly known, and are the subject of ongoing research. Several experiments, both in space and on Earth, are being carried out to evaluate the exact degree of danger. Experiments at Brookhaven National Laboratory's Booster accelerator revealed that the biological damage due to a given exposure is actually about half what was previously estimated: specifically, it turns out that low energy protons cause more damage than high energy ones. This is explained by the fact that slower particles have more time to interact with molecules in the body.
Mitigation
Shielding
Material shielding may be partially effective against galactic cosmic rays in certain energy ranges, but may actually make the problem worse for some of the higher energy rays, because more shielding causes an increased amount of secondary radiation. The aluminum walls of the ISS, for example, are believed to have a net beneficial effect. In interplanetary space, however, it is believed that aluminum shielding would have a negative net effect.
Several strategies are being studied for ameliorating the effects of this radiation hazard for planned human...(and so on)

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