Travel outside the Earth's protective atmosphere, magnetosphere, and gravitational field can harm human health, and understanding such harm is essential for successful manned spaceflight. Potential effects on the central nervous system (CNS) are particularly important. A vigorous ground-based cellular and animal model research program will help quantify the risk to the CNS from space radiation exposure on future long distance space missions and promote the development of optimized countermeasures. — Human Research Program Requirements Document, HRP-47052, Rev. C, dated Jan 2009.