Explorer 1 Satellite Launched

On January 31, 1958, the United States placed Explorer 1 into orbit atop a Jupiter-C rocket from Cape Canaveral, making it the first successful American satellite. Designed and built by the Jet Propulsion Laboratory, it carried a cosmic ray detector designed by James Van Allen of the University of Iowa. The instrument recorded unexpectedly low cosmic ray counts at certain altitudes, leading Van Allen to theorize the existence of charged particle belts trapped by Earth's magnetic field. The zones became known as the Van Allen radiation belts.