Health Threat From Cosmic Rays - The Deep-space Radiation Environment

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 (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.

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 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 milli-Sieverts (mSv) of cosmic radiation annually (separate from other sources of radiation exposure like inhaled radon) due to atmospheric shielding. At 15 km altitude, above most of the atmosphere's protection, radiation dose 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
2. 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". See the implications of the Van Allen belts for space travel for more information.
3. 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.

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. 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).