Terrestrial Tail

As it whirled in orbit around the moon, the instrument-crammed Soviet spaceship Luna 10 was busily recording and reporting man's first continuous supply of data about the lunar environment. Though the Russians did not tell all they learned, the information they did release confirmed that their distant capsule was carrying out its fact-finding mission with singular success.

Luna's gamma-ray measurements indicated that the moon has a crust some what similar to the earth's. The satellite also established for the first time that both the number of meteorite particles and the strength of the magnetic field in the vicinity of the moon are higher than in interplanetary space. It also discovered 70 to 100 times as many energetic electrons as are expected in outer space. Russian scientists attributed the electrons to the "earth's magnetic tail."

Shaped like a Comet. This enigmatic Russian reference to the terrestrial tail was the first apparent confirmation that the earth's magnetic field extends as far as the moon, 239,000 miles away. Earlier observations by U.S. satellites and space probes established that the magnetic field is shaped like a comet with a tail that stretches at least 120,000 miles into space. Now it seems that the electrons Luna encountered near the moon are temporarily confined there by the earth's magnetic tail in much the same way that the electrons and protons of the Van Allen radiation belts are trapped in areas of the magnetic field closer to the earth.

As the moon revolves around the earth, it passes through the magnetic tail once a month. The passage, which takes two or three days, occurs around the time when the moon is full, on the opposite side of the earth from the sun. Because the moon was full on April 5, only two days after Luna 10 went into lunar orbit, the Russians presumably detected an almost immediate rise in the number of electrons, then a sharp drop-off a few days later as the moon passed out of the tail.

Waving in the Breeze. The earth's magnetic field is formed into its comet-like shape by the solar wind, a stream of charged particles continuously emitted from the sun at velocities that vary from 670,000 m.p.h. to about 1,600,000 m.p.h. On the side of the earth that faces the sun, the wind compresses the field into a rounded shell that extends only about 40,000 miles into space. On the dark (or antisolar) side, the field is pushed into a tail that is hundreds of thousands of miles long and waves in the solar breeze. Gaseous comet tails are similarly affected by the solar wind, which always blows them in an antisolar direction--no matter which way the comet is moving.

The solar wind may also be responsible for the moon's magnetic field reported by Luna. Scientists believe that charged particles from the sun induce tiny electric currents in the moon. These, in turn, generate a weak magnetic field which--like the earth's--is probably distorted into a cometlike shape and may even have its own collection of energetic electrons for Luna to detect. The presence of these electrons would be characterized by a peak of radiation every three hours--each time Luna passed through the lunar tail on the antisolar side of the moon.

Luna's discovery of high concentrations of electrons near the lunar surface caused an immediate flurry of press reports about possible danger to future manned moon missions. These were quickly brushed aside as "unfounded speculation" by University of Iowa Physicist James Van Allen, discoverer of the earth's radiation belts. Electrons with the energies reported by Luna were so "soft," he said, that they "could not even penetrate a thick piece of tissue paper."

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