How Explorer 1 helped discover the Van Allen radiation belts

Explorer 1, the first U.S. satellite, launched from Cape Canaveral at 10:48 p.m. EST on January 31, 1958. Its Geiger-counter cosmic-ray detector, designed by James Van Allen of the University of Iowa, and a similar one on Explorer 3, revealed the Van Allen radiation belts, the first scientific discovery of the Space Age.

Some sources date the launch to February 1, including Van Allen’s own university. Both are right: by the time the rocket rose in Florida it was already the early hours of February 1 in Greenwich time. The success was announced at a 1 a.m. press conference in Washington on February 1, where JPL director William Pickering, Van Allen and Wernher von Braun held a full-size model above their heads.

The pressure was intense. The Soviet Union already had two satellites in orbit, and the first U.S. attempt, on a Vanguard rocket, had exploded on the pad in December 1957. The team would announce nothing until a station in California heard Explorer 1. “There was a period of eight minutes there [before contact], which is the longest eight minutes I’ve ever spent in my life,” Pickering recalled.

The Army Ballistic Missile Agency supplied the Jupiter-C rocket, with JPL providing the upper stages and packaging the payload; NASA did not yet exist. The satellite weighed about 31 pounds (14 kg), spun at 750 revolutions per minute, and was painted in white and dark green stripes to control its temperature.

The belts themselves took both satellites to find. Unlike Explorer 1, Explorer 3 carried a tape recorder that captured data for a whole orbit. Data from the two led to the discovery that Earth is circled by doughnut-shaped belts of energetic charged particles trapped by its magnetic field, now named after Van Allen. Explorer 1’s high-power transmitter lasted 31 days; its last transmission came on May 23, 1958.