Explorer 1 becomes the first U.S. satellite and discovers the Van Allen belts
On this day · 31 January 1958
America's answer to Sputnik did more than reach orbit; its lone instrument stumbled onto a hidden ring of radiation around Earth.
On 31 January 1958, the United States launched Explorer 1, its first satellite, from Cape Canaveral, Florida. The success came just four months after the Soviet Sputnik 1 had stunned the world and opened the Space Age.
Designed and built by NASA’s Jet Propulsion Laboratory, the slender satellite weighed only about 14 kilograms and circled Earth roughly every 115 minutes. Its main instrument, a cosmic ray detector provided by physicist James Van Allen of the University of Iowa, returned a puzzle: at times it counted far less radiation than expected.
The detector wasn’t broken. It had been blinded by radiation no one knew was there.
Van Allen reasoned the instrument had been saturated by intense bands of charged particles trapped by Earth’s magnetic field. The discovery revealed the Van Allen radiation belts, now named in his honor. America’s first satellite thus delivered not just a Cold War answer to Sputnik, but the first major scientific finding of the space era.
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