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Galactic cosmic rays are high-energy charged particles that enter the solar system from the outside. They are composed of protons, electrons, and fully ionized nuclei of light elements.

The magnetic fields of the Earth, the sun, and the galaxy itself tend to scramble the paths of the galactic cosmic rays, so that when one is detected, nothing can be inferred from the direction of its origin. Because many galactic cosmic rays are of extremely high energy, they must have originated in very energetic processes. Some are believed to have been accelerated by the shockwaves of supernovae. In the high-energy tail of the distribution, some galactic cosmic rays have energies so high that no known physical process could have created them.

Some of the isotopes observed in galactic cosmic rays have half-lives that are comparable to the time interval since their formation, and isotopic ratios can therefore carry some information about the amount of time that has passed since they were formed. In a few cases, there are isotopes in galactic cosmic rays which are unstable with respect to electron capture, but because they are fully stripped, they have not decayed in flight.

Most galactic cosmic rays have energies too low to penetrate the earth's atmosphere, and the radii of their helical trajectories in the earth's magnetic field tends to channel them to the poles; in this respect, these galactic cosmic rays are exactly like the charged particles that make up the solar wind. When they strike the atmosphere, they can create large showers of secondary particles, including exotic ones such as muons, and these secondary particles are what can be detected at the earth's surface.

Very high-energy cosmic rays can penetrate the earth's atmosphere, and the radii of their helical trajectories are thousands of kilometers, so they are not as effectively channeled by the earth's magnetic field.

Galactic cosmic rays are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.^[1]

Life on the earth's surface is protected from galactic cosmic rays by a number of factors:

1. 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.
2. Shielding by the bulk of the planet itself cuts the flux by a factor of two.
3. Except for the very highest energy galactic cosmic rays, the radius of gyration in the earth's magnetic field is tight enough to confine them along lines of magnetic flux, and channel them into the Van Allen Belt.
4. 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.

Of these four factors, all but the first one apply to low earth orbit craft, such as the International Space Station (although the ISS crew gets most of its dose while passing through the Van Allen Belt). Therefore, the only astronauts who have ever been exposed to a significant radiation flux from galactic cosmic rays are those in the Apollo program. 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.

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.^[2]

Several strategies are being studied for ameliorating the effects of this radiation hazard for planned human interplanetary spaceflight:

• Spacecraft can be constructed out of hydrogen-rich plastics, rather than aluminum.^[3]
• Liquid hydrogen, which would be brought along as fuel in any case, tends to give relatively good shielding, while producing relatively low levels of secondary radiation. Therefore, the fuel could be placed so as to act as a form of shielding around the crew.
• Water, which is necessary to sustain life, is very dense in hydrogen and could be used to form a shield similar to liquid hydrogen.^[4]
• Electromagnetic fields may also be a possibility.^[5] However, this raises a number of problems: (1) the fields act in opposite directions on positively and negatively charged particles, so shielding that excludes positively charged galactic cosmic rays will tend to attract negative ions; (2) a very large power supply would be required in order to run the electrostatic and magnetostatic generators, and superconducting materials might have to be used for magnetic coils; (3) the possible field patterns might tend to dump charged particles into one area of the spacecraft.

None of these strategies currently provides a method of protection that would be known to be sufficient, while using known engineering principles and conforming to likely limitations on the mass of the payload. Part of the uncertainty is that the effect of human exposure to galactic cosmic rays is poorly known in quantitative terms.

• Cosmic ray

• What Are Cosmic Rays?
• Cosmic Rays at the Open Directory Project
• 23 April, 2002, BBC News, Cosmic ray mystery 'solved'
• 4 November, 2004, BBC News, Supernova produces cosmic rays
• Clock Isotopes in Cosmic Rays
• Cosmic Rays - Samples of matter