Specs
Launch date: January 31, 1958 (Feb. 1, 1958 using Universal Time)
Launch vehicle: Jupiter C
Time to orbit Earth: 115 minutes
Furthest orbital distance from Earth (apogee): 2,515 kilometers (1,563 miles)
Closest orbital distance to Earth (perigee): 354 kilometers (220 miles)

History
After the Soviets launched the first satellite, Sputnik, in October 1957, the pressure was on the United States to build and launch a satellite of its own. Although the first satellite we tried (the Vanguard, in December 1957) didn't make it off the launch pad, the Explorer 1 was designed and successfully launched in just three months.

The Explorer 1 team, which included James Van Allen, equipped the satellite with instruments to measure micrometeorite impacts, the internal and external temperature of the satellite, and above all, the concentration of ions and electrons in space. Van Allen was searching for information on cosmic rays, which are fast ions coming from space. But when he found fewer rays than he expected, he guessed that the satellite's readings must have been thrown off by areas of high radiation in space, coming from highly charged particles caught in the Earth's magnetic field. These areas became known as Van Allen radiation belts when their existence was confirmed by another satellite a few months later.

Transmissions from Explorer 1 were sent back to Earth for about four months, but the satellite continued to orbit until March 31, 1970, when it burned up after entering the Earth's atmosphere.

The Science Center's Explorer 1
Our Explorer 1 is an engineering model of the satellite, on loan to us from the NASA Jet Propulsion Laboratory, located in Pasadena.