Radiation Belt
Space near the earth is not as beset with micrometeorites as some space pessimists have feared. During last week's Washington meeting of the American Physical Society, Drs. Edward Manring and Maurice Dubin of the Air Force Cambridge Research Center told about the experiences of the Army's satellite Explorer I, which carries two meteorite detectors. One of them, a microphone that picks up the slight vibrations in the satellite's shell that are caused by the smallest dust particles, registered only seven hits during the 120 minutes that the transmitter could be heard. The other detector, a set of delicate coils designed to be damaged electrically by meteorites at least 10 microns (1/2500 in.) in diameter, showed no more than one hit (possibly none) in 32 days. The satellite was not damaged, and Manring and Dubin conclude that only long exposure to this concentration of micrometeorites would do it any harm.
Choked Tubes. Less reassuring news came from a team of cosmic ray experts at the State University of Iowa headed by Dr. James A. Van Allen. Both Explorer I and Explorer III, said Van Allen, ran into a belt of intense radiation at about 600 miles elevation. Each of the satellites carries a single Geiger tube to count cosmic rays. The radio transmitter of Explorer I sends a signal whenever the tube has made 128 counts. Explorer III has a magnetic tape that records the tube's counts during each circuit of the earth and reports to a ground station.
When the Explorers' orbits were carrying them near the earth, they both reported reasonable numbers of cosmic rays, around 30 per second, but as they climbed up toward their apogees the count came faster. At 1,100 kilometers (684 miles) the tubes registered as high as 140 counts per second. Then a strange thing happened. As the satellites climbed even higher, the transmitters reported no rays at all. During orbit after orbit the counter of Explorer III was silent for 15 minutes. When the satellites swung down again to lower levels, they resumed reporting reasonable numbers of cosmic rays.
Van Allen was sure that no ray-free belt could exist between the earth and space. The only reasonable explanation, he decided, was that the silenced Geiger tubes had been knocked out temporarily by radiation too intense for them to handle. So he subjected a spare tube to X-ray bombardment in the laboratory. After studying its behavior, he decided that the tubes carried by the satellites must have passed through radiation equivalent to 35,000 counts per second, but were so choked up that they could not report their experience.
Plasma from the Sun. The radiation belt, Van Allen conjectured, is probably a "plasma" made of disassociated hydrogen atoms (protons and electrons) that came originally from the sun and are held high above the earth by the earth's magnetic field. The belt may extend outward for two earth radii (8,000) miles before it disappears. Van Allen suspects that the supply of plasma fluctuates a good deal; the particles tend to leak down to the earth's atmosphere and are replenished from time to time by fresh particles shot into space by disturbances on the sun.
The radiation zone is by no means a "death belt" that will keep humans from reaching space, but it might do some damage to men who live for a long time in a satellite. Van Allen figured that the radiation level inside the satellite might reach about 0.06 roentgens per hour. At this rate a man would receive in five hours his maximum weekly permissible dose of 0.3 roentgens. A small amount of lead shielding would reduce the dose to a supportable level. The crew of an outbound spaceship need not worry about the radiation belt. If moving fast enough to leave the earth, they would pass through it in about 20 minutes.
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