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Explorer II splashed into the Atlantic early in March, but Explorer III was launched successfully on March 26. It contained a modified version of Ludwig's tape recorder—an amazing little instrument full of tiny, glittering parts that weighed only 8 oz. If it worked, it would gush out in five seconds all the cosmic-ray data from an entire orbit.
On March 28 Van Allen got the first tape and sat up all night poring over it. The cosmic-ray count seemed reasonable as long as the bird was at low altitude. When it climbed upward, the rate increased rapidly. Then, for some unaccountable reason, the count fell to nothing, stayed at nothing until the bird was back at lower altitude again.
Sulking Tube. Mystified, Van Allen hurried back to Iowa, where his assistants, Drs. Carl McIlwain and Ernest Ray, were puzzling over a copy of the same tape. The three almost simultaneously hit a solution. The high-flying Geiger tube was being swamped by too heavy a dose of some kind of radiation. This is a weakness of Geiger tubes. If required to count too many times a second, they sulk and do not count at all.
The amazing conclusion: the earth was surrounded by a belt of intense radiation, apparently trapped by earth's magnetic field. It might be deadly enough to interfere seriously with man's attempts to fly out into space.
The announcement caused an embarrassed flurry in high Washington circles. Van Allen learned that, at the suggestion of Physicist Nicholas Christofilos of Livermore Laboratory, the Department of Defense was planning to launch Project Argus, in which three atom bombs would be rocketed above the atmosphere and exploded (TIME, March 30). The high-speed electrons released were expected to be shunted around the earth by the earth's magnetic field. Van Allen's discovery that nature had already provided such electrons was a considerable shock.
With his fine-honed skill in maneuvering, Van Allen took advantage of Project Argus to advance his own studies. He proposed the launching of a new satellite in a more north-and-south orbit than any of its predecessors. Moving in this way, he explained, it would better observe the results of Project Argus. Incidentally, it would also give him coverage of the natural radiation belt in latitudes that earlier satellites had not reached.
Explorer IV went into a 51° orbit on July 26. It carried sophisticated instruments that Van Allen's laboratory had provided to distinguish between "hard" and "soft" radiation, and shielded Geiger counters designed to count radiation intensity at extremely high levels without blacking out. On Aug. 27 the first Argus shot sent man-made electrons zigzagging round the earth. The satellite cut through them back and forth, making about 250 passes before its batteries were exhausted on Sept. 20.
Behind the Northern Lights. When Van Allen made his first open report on Explorer IV, he had to avoid all mention of Argus because of military security. But he had plenty to tell about the natural radiation. He could say with assurance that a human satellite crew exposed to maximum Van Allen radiation for a few days would surely die. It looked as if the fierce particles, which slam close to the earth in the auroral regions, were the explanation of the ancient mystery of the northern lights.