Van Allen Belts

In 1958-59 radio astronomers discovered that the planet Jupiter has an enormous radiation belt of high energy electrons. This discovery provided a powerful impetus The Van Allen belts. Illustration by Hans & Cassidy. Courtesy of Gale Group.

for the investigation of Jupiter and the other planets by scientifically instrumented spacecraft.

Beginning in 1962 in situ investigations of the planets have been conducted by American, Soviet, and European spacecraft. The radiation belt of Jupiter has been explored in detail. Enormous radiation belts of Saturn, Uranus, and Neptune also have been discovered and investigated. It has been found that Venus, Mars, and Mercury have no durable radiation belts, but there are significant magnetospheric effects at these planets because of the obstacles that they present to the flow of the solar wind. Such effects have also been observed at the Moon and at three comets.

A durable radiation belt at a planet can only exist if that planet is strongly magnetized so that energetic electrically charged particles can be trapped durably in its external magnetic field. Earth, Jupiter, Saturn, Uranus, and Neptune meet this condition by virtue of electrical currents circulating in their interiors to produce huge electromagnets.

Venus, Mars, Mercury, the Moon, comets, and asteroids are insufficiently magnetized to retain radiation belts. It is likely that the planet Pluto is also in this group.

Pulsars and other distant astrophysical objects have radiation belts.