Space radiation

The health threat from cosmic rays is the danger posed by galactic cosmic rays (GCR) and solar energetic particles to astronauts on interplanetary missions or any missions that venture through the Van-Allen Belts or outside the Earth's magnetosphere. Space radiation health risks also occur for mission in low Earth orbit such as the International Space Station (ISS) where GCR dominate organ dose equivalents, while radiation exposure limits can be reached for cumulative times on ISS of about 2 years (with variable maximum durations dependent on crew age at mission and sex).

Galactic cosmic rays (GCRs) consist of high energy protons (85%), helium (14%) and other high energy nuclei (HZE ions). Solar energetic particles consist primarily of protons accelerated by the Sun to high energies via proximity to solar flares and coronal mass ejections. They are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.

Heavy ions and low energy protons and helium particles are highly ionizing forms of radiation, which produce distinct biological damage compared to X-rays and gamma-rays. Microscopic energy deposition from highly ionizing particles consists of a core radiation track due to direct ionization's by the particle and low energy electrons produced in ionization, and a penumbra of higher energy electrons that may extend 100's of microns from the particles path in tissue. The core track produces extremely large clusters of ionization's within a few nano-meters, which is qualitatively distinct from energy deposition by X-rays and gamma-rays; hence human epidemiology data which only exists for these latter forms of radiation is limited in predicting the health risks from space radiation to astronauts.

In October 2015, the NASA Office of Inspector General issued a health hazards report related to space exploration, including a human mission to Mars.

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