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Explorer 1 50th Anniversary
This page contains archived content and is no longer being updated. At the time of publication, it represented the best available science. However, more recent observations and studies may have rendered some content obsolete.
At 10:48 p.m. local time on January 31, 1958, the Explorer 1 satellite launched from Cape Canaveral, Florida, after launches scheduled the previous two days had been scrubbed due to high-altitude winds. A joint project between the U.S. Army and the California Institute of Technology/Jet Propulsion Laboratory (NASA was not yet established), the launch was a significant achievement for the United States. The previous fall, the Soviet Union had launched two Sputnik satellites, the second one carrying live cargo: a dog. The United States was anxious to catch up in the space race.
According to JPL historians, “Legend tells how JPL and the Army hustled into space, designing, building and launching Explorer 1 in less than 90 days after being granted permission to proceed. But the full story was neither that fast, nor that simple.” The technological foundations for Explorer were actually laid years before, relying in part on rocket technology developed during World War II. The scientific foundation had been brewing for nearly a decade, as well. In 1950, physicist James Van Allen hosted a party for several colleagues, and from their “pedigreed bull session,” as Van Allen reportedly called it, a proposal emerged to use rocket and missile technology to conduct scientific studies in space.
Just several feet long by several inches wide and weighing 14 kilograms (30.8 pounds), Explorer 1 took a looping orbit that ranged from 354 kilometers (220 miles) away from Earth at its closest range, to 2,515 kilometers (1,563 miles) at it farthest distance. The satellite made 12.54 orbits per day, and carried three science experiments. One experiment was designed to detect micrometeor impacts on the satellite and relied on a microphone and a grid of fine wires to detect objects as small as 10 microns (less than 0.0004 inches) in diameter. Another experiment measured temperature, which would inform the development of later spacecraft carrying temperature-sensitive payloads. The most important experiment, however, was designed to measure cosmic rays.
Explorer 1 was outfitted with a Geiger counter that could detect cosmic radiation, and this experiment yielded surprising results. Van Allen had anticipated that cosmic ray hits would increase with altitude, and they did—up to a height of roughly 1,100 kilometers (684 miles). Farther away from Earth than that, however, the Geiger counter fell silent. Based on data from Explorer 1 and Explorer 3 launched months later, Van Allen eventually surmised that at higher altitudes, cosmic radiation became intense enough to overwhelm the Geiger counter, rendering it mute. These findings led to the discovery of doughnut-shaped belts of intense cosmic radiation wrapped around our planet and held in place by Earth’s magnetic field. The innermost of these radiation belts are known as the Van Allen Radiation Belts in Van Allen’s honor.
After the late-night launch, scientists working with Explorer 1 endured an agonizing wait of nearly two hours before receiving transmissions from the satellite, proof that the mission was a success. Explorer 1 relayed its final transmission nearly four months after its launch, on May 23, 1958. It remained in orbit years afterwards, and finally plunged into Earth’s atmosphere and burned up on March 31, 1970. In total, the satellite made more than 58,000 orbits.
Image courtesy Jet Propulsion Laboratory. Caption by Michon Scott and Rebecca Lindsey.