Radiation Belts

Radiation Belts, zones in space around the Earth, and the planets Jupiter and Saturn, containing high-energy protons and electrons.

A Geiger counter, a type of particle detector, was installed in the first American satellite, Explorer 1, launched on January 31, 1958. It gave the first evidence that the Earth is surrounded by regions containing charged particles of high energy. These regions were named the Van Allen radiation belts, after James A. Van Allen of the University of Iowa, who was head of the team of scientists that conducted the experiments.

Around the Earth, the radiation belts consist of electrons and protons that are trapped in a doughnut-shaped region centred around the magnetic equator. This region extends from a few hundred kilometres above the Earth to 48,000 to 64,000 km (30,000 to 40,000 mi). The electrons and protons are supplied by cosmic rays and by the solar wind, which is a continuous stream of charged particles—mainly electrons and protons—emanating from the Sun. The particles are forced into spiralling paths about the magnetic field lines of the geomagnetic field (see Geophysics) by the Lorentz force (the force exerted on moving charged particles, perpendicular both to the direction of motion and to the magnetic field lines). Because the magnetic field increases near the poles of the Earth—that is, the field lines become denser—the particles are reflected and travel back and forth on spiralling paths between the north and south poles of the Earth. Most of the high-energy protons (those with energies greater than 10 MeV) are found in an inner belt at an altitude of 3,200 km (2,000 mi); electrons are more concentrated in an outer belt that extends many Earth radii into space.

Electronic circuits and solar cells of spacecraft deteriorate when exposed for long periods to fast protons, which can penetrate several millimetres of metal. The very intense radiation belts around Jupiter, especially, required effective protection of the electronic circuitry of the Pioneer and Voyager spacecraft. For living organisms, the radiation has the general detrimental effects of ionizing radiation (see Radiation Effects, Biological), so space missions are generally planned in such a way as to minimize exposure of the astronauts to the most intense regions of the radiation belts.