Radiation in the space environment comes from the trapped particle belts, solar particle events and cosmic rays.
Trapped Particle Belts
The radiation belts consist principally of electrons of up to a few MeV energy and protons of up to several hundred MeV energy. These are trapped in the Earth's magnetic field; their motions in the field consist of a gyration about field lines, a bouncing motion between the magnetic mirrors found near the Earth's poles, and a drift motion around Earth.
Radiation is an obvious concern for manned missions. In the near-term, manned activities are limited to low altitude, and mainly low-inclination missions. The International Space Station (ISS), Space Shuttle, EnviSat and other low altitude missions will therefore encounter the inner edge of the radiation belt. This region is dominated by the "South Atlantic Anomaly (SAA)" - an area of enhanced radiation caused by the offset and tilt of the geomagnetic axis with respect to the earth's rotation axis.
Basic motion of trapped particles in the earth magnetic field
Besides the SAA, the polar horns also play a role for radiation analysis at low altitudes (e.g. ISS type orbit). Polar horns are parts of the outer radiation belts, which are close to Earth. As can be seen in the simulation below, increased radiation flux due to the polar horns can be expected between 60 and 90 degrees lattitude. The SAA is clearly visible at the South Antlantic region around 30 up to 50 degrees lattitude.
Earth radiation belts with the South Atlantic Anomaly indicated
At Low Earth Orbit (around 400 - 500 km) altitudes, there are important interactions between the trapped radiation belts and the atmosphere, giving rise to a strong asymmetry in fluxes from East and West. This is important for oriented spacecraft such as the International Space Station.
Trapped electron flux at 380 km altitude; the polar horns and South Atlantic Anomaly are evident
During solar events, large fluxes of energetic protons are produced which reach Earth. The August 1972 event produced a peak flux in excess of 1E+06 protons/cm²/sec above 10 MeV energy. Such events are unpredictable in their time of occurrence, magnitude, duration or composition. The Earth's magnetic field shields a region of near-Earth space from these particles (geomagnetic shielding) but they easily reach polar regions and high altitudes such as the geostationary orbit.
The active Sun as seen by the Yohkoh Soft X-Ray telescope
Cosmic Rays originate outside the solar system. Fluxes of these particles are low but, because they include heavy, energetic ("HZE") ions of elements such as iron, they cause intense ionisation as they pass through matter. It is difficult to shield against these type of ions, and therefore they constitute a significant hazard. They give rise to single-event processes (SEU, latch-up) in large-scale integrated electronic components, as well as interference and an uncertain radiobiological effect.
Other Radiation environment issues
Other aspects of the radiation environment include induced radioactivity, planetary environments (Jupiter and Saturn have particularly hostile radiation environments), trapped ions and spacecraft nuclear generator systems.
Last update: 5 April 2007
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