Van Allen Belts

Radiation belts are enormous populations of energetic, electrically charged particles—principally protons and electrons—trapped in the external magnetic field of a planet. Durable radiation belts exist at the planets Earth, Jupiter, Saturn, Uranus, and Neptune but not at Mercury, Venus, or Mars.

Nine artificial radiation belts of Earth were produced during the period 1958-62 by the injection of electrons from radioactive nuclei produced by United States and USSR nuclear bomb bursts at high altitudes. These experiments made important contributions to understanding the natural belts. Since 1962 such high altitude bursts have been prohibited by international treaty.

The inner radiation belt imposes an altitude ceiling on the region around Earth within which orbital flights of humans and animals can be conducted without exposing the occupants to excessive or fatal radiation exposures. Prolonged flights of human crews above an altitude of about 250 mi (402 km) are unsafe though rapid traversals of the radiation belts requiring only a few hours (as in the Apollo missions to the Moon) result in moderate exposures.

Contrary to some common statements, trapped particles are not radioactive. Rather they are mainly ordinary electrons and protons such as those accelerated in high energy physics laboratories. Radiation belts do not shield Earth's surface, though the magnetic field deflects some cosmic rays away from Earth. The atmosphere acts as an effective shield against many solar and other radiations that impinge on it.