See B. B. Rossi, Cosmic Rays (1964); L. I. Dorman, Cosmic Rays (1974); M. W. Friedlander, Cosmic Rays (1989).
Life on the earth's surface is protected from galactic cosmic rays by a number of factors:
Of the above 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. 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.
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 1950's), 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.
Several strategies are being studied for ameliorating the effects of this radiation hazard for planned human interplanetary spaceflight:
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. The required amount of material shielding would be too heavy to be lifted into space. Electromagnetic shielding has 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. Part of the uncertainty is that the effect of human exposure to galactic cosmic rays is poorly known in quantitative terms. NASA has a Space Radiation Shielding Program to study the problem.