Discovery of Van Allen belts proves to be a lucky discovery

Ken Tapping's weekly astrophysics column from the DRAO research station near Penticton


On October 4, 1957, the space age officially began with the launching by the Soviet Union of Sputnik 1, the first manmade object to orbit the Earth. When the USSR “got there first”, it threw the West into a panic to catch up. On January 31, 1958 the U.S.A. launched its own satellite, Explorer 1. This was a rush project, and the intention was to get something into orbit, any sort of orbit. A pencil-shaped satellite was quickly put together, carrying fairly rudimentary instruments, which included, luckily, a simple Geiger counter, a device that counts high-energy particles and gamma rays.

Sputnik’s orbit took it between 215 and 939 kilometres from the Earth’s surface. Explorer’s orbit took it between 358 and 2,550 kilometres out into space. Moving through this large height range every time it orbited the Earth resulted in a dramatic discovery.

When at its closest to the Earth’s surface (called the perigee of the orbit), the Geiger counter recorded the expected number of “clicks” per minute, due to high-energy cosmic rays. However, as the satellite moved upwards towards it highest point (called the apogee), the count rate rose, and then, surprisingly, above about 2,000 kilometres out, the count rate fell to zero. On the way back down to the perigee, the counts suddenly started again and fell to their lowest point at the perigee, after which the entire cycle repeated, for orbit after orbit.

Was this zero-count issue a hardware error or some other experimental problem? That is when James Van Allen of the University of Iowa, came up with a possible explanation. He suggested the reason the count rate fell to zero is that the radiation above about 2,000 kilometres was so high that the Geiger counter could no longer count them. Van Allen suggested that the Earth is surrounded by belts of solar and cosmic ray particles that are trapped in our planet’s magnetic field.  Subsequent investigations proved Van Allen to be right, and showed the Earth to be surrounded by two radiation belts, that are now known as the Inner and Outer Van Allen Radiation Belts.  On their way to the Moon, the Apollo astronauts had to pass through these belts. However, there is no serious hazard when one quickly passes through, although it would be unwise to spend a lot of time in them.  Most of our space missions, such as the International Space Station, orbit well below the belts.

We would expect any planet with a magnetic field to have radiation belts.  The most dramatic ones found so far are those around the planet Jupiter. Its radiation belts would endanger the lives of human astronauts and can damage the electronics of spacecraft going through them. The two Voyager spacecraft moved through at high speed. However, in the 1990’s the Galileo spacecraft spent years exploring the space around Jupiter. It suffered numerous radiation-induced failures but sent us back a lot of very useful and new information, not only science, but also how we can make more robust space technology. Space is a hostile place, but so is the Antarctic. We can live and work at the South Pole, and we are solving the problems of living and working in space.

Jupiter dominates the southern sky during the night. Saturn rises about 2 a.m. The Moon will reach first quarter on the 18th.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astro-physical Observatory, Penticton.