Van Allen Belts - What are they?

The Van Allen belts are regions of charged particles that have become trapped in the Earth's Magnetic field and surround us in two distinct regions or "belts". They are named after James Van Allen who led the team of Physicists that discovered them.

A model of the Van Allen Belts
 

The Inner belt

The inner belt is a confined region of high-energy protons made when high energy cosmic rays from distance galaxies collide with our atmosphere. They are found between 600km and 1000km above the Earth. Prolonged exposure, say during a space flight, would be very hazardous to humans and equipment. This is why low orbit satellites follow paths between the poles of the Earth to minimize time spent in this region.

The outer belt

This is a much more diffuse (thinner) but larger region of particles that have become trapped from the Sun's solar wind.

Van Allen Belts - Discovery

The inner belt was first discover in January 1958 by the US satellite Explorer 1 (designed originally to detect and study the intensity of cosmic rays from greater distances than could be measured from earth) using a Geiger counter. The existence of this belt was later confirmed by similar instruments on board the US satellite explorer 3 and the Russian satellite Sputnik 3 later that year.

The outer belt was first discovered in December 1958, also using a Geiger counter. This was discovered by Pioneer 3 (although Russian scientists working on data collected by Sputniks 2 and 3 claimed to have detected evidence of this second radiation belt prior to this).

The Primary mission of the Pioneer 3 probe was a lunar fly-by, but due to a fuel cut-out in the spacecraft’s main engine, the probe only managed to reach an altitude of 102,360km, this caused the probe to fall back to earth and burn up. However before burning up the probe’s Geiger counters provided important data that allowed Van Allen’s team to detect the existence of this second, outer belt. This was then confirmed by instruments on board the US probe, Pioneer 4 the following year.

Explorer 3