Doughnut Around the Earth

The first detailed description of the belt of lethal radiation that swathes the earth was given last week by Dr. James A. Van Allen of the State University of Iowa. Often called the "Van Allen radiation," the belt was discovered by the instruments that the Army's satellites carried into space.

The belt, Scientist Van Allen told the American Physical Society at Chicago, seems to be a great doughnut made chiefly of fast-moving electrons and protons circulating around the earth on both sides of its magnetic equator (see diagram). Only the lower parts have been observed with any accuracy. The upper limits are deduced from knowledge of the magnetic field. The Air Force's Pioneer, soaring far past the 1,400-mile level reached by the Explorers, confirmed "tentatively and partially" that the lethal radiation drops off sharply around 7,000 miles.

Dr. Van Allen thinks most of the particles come from the sun, shot out by eruptions and trapped by the earth's magnetic field. The strength of the radiation belt is probably variable, like the amount of water in a leaky bucket that is filled at irregular intervals. When the sun is quiet, the particles in the belt gradually leak down to the atmosphere and disappear perhaps causing the aurora. The belt grows weaker and weaker until a new transfusion of particles from the sun makes it strong again.

Van Allen did not say so, but his diagram points out clearly that a cone-shaped area over the magnetic poles is almost radiation-free. Obvious conclusion: the space ports of the future may have to be in far northern Canada or Antarctica, where men can soar into space through the escape zones over the magnetic poles, thus eluding the lethal hazards of the Van Allen belt.

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