Space radiation has a major impact on all NASA activities. The NASA Space Radiation Health Program has as its goal to enable a permanent human presence in space without exceeding acceptable risk from exposure to ionising radiation. It has been devised to develop the database and the knowledge required by NASA to accurately predict and to efficiently manage radiation risk. There are three main sources of exposure to radiation. In low-Earth orbit (LEO), for crews on the Shuttle and the International Space Station (ISS), protons and electrons trapped in the Earth's magnetic field are the main source of radiation exposure. Outside of the Earth's magnetic field, crews are exposed to the galactic cosmic-ray (GCR) background radiation that permeates interplanetary space, and to solar particles. Details of the space radiation environment are discussed in the other contributions to this symposium (Badhwar, following contribution). Solar disturbances, referred to as solar particle events (SPEs), occasionally cause much larger fluxes of particles, mainly protons with energies in the region of several hundred MeV. Peak flux during an SPE may be two to five orders of magnitude greater than background within hours of the onset of the event, an SPE may also alter the Earth's magnetic-field lines. Periods of enhanced flux may last for days, with successive peaks due to multiple events and enhancements during shock passage. The GCR consists of approximately 85% protons, 14% helium, and 1% heavier particles. The heavy particles are called HZE particles. They are nuclei of elements with high-energy, E (ranging from several tens to several thousand MeV/nucleon), and atomic number Z > 2. They have been fully stripped of electrons in their passage through interstellar space and therefore have a charge that is also equal to Z. The most energetic GCR particles are also able to penetrate into the Earth's magnetic field and are the source of cosmogenic radionuclides in the atmosphere.
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