The Radiation Environment in Free Space and Inside Low-Earth Spacecraft's

There are three main sources of radiation in free space. These are the trapped protons and electrons, solar energetic particles, and the galactic cosmic rays. For human mission to Mars or moon, the trapped protons and electrons are of a minor concern. The flux of trapped particles at a fixed spacecraft altitude, and of galactic cosmic rays, decreases as the solar activity increases; however, the probability of a solar particle event increases as the solar activity increases. As these particles interact with the shielding material of the spacecraft and human body, they produce secondary particles, such as lighter fragments, target fragments, and neutrons. These particles can have far more damaging effect on cells than the primary particles. Thus, the radiation environment is very complex and varies with time. In this paper, we review the sources of charged-particles that contribute significantly to radiation risk to the astronauts, and estimates of exposure for some typical missions, such as Space Shuttle, International Space Station, and a human mission to Mars.