The first successful satellite launched by the United States. It was launched by the Army Ballistic Missile Agency on a Jupiter-C rocket from Cape Canaveral on January 31, 1958 about four months after the Soviet Union launched Sputnik 1. It weighed only 14 kg, compared to the hefty 84 kg Sputnik.

The satellite was built by the Jet Propulsion Laboratory and carried an instrument package which detected an intense band of radiation trapped in the magentosphere around the Earth. The package was developed by a team led by James Van Allen of the University of Iowa, and the bands were later named the Van Allen Belts in his honor. This was the first major discovery of the Space Age.

The satellite transmitted data until May 23, 1958 when its batteries gave out. It continued to orbit for 12 more years until it burned up on re-entry over the Pacific Ocean on March 31, 1970.