Explorer 1 was the first U.S. satellite successfully placed in orbit in 1958.
The satellite launched on January 31, 1958, from Cape Canaveral, Florida, aboard a Juno I rocket. The launch followed the Soviet Union's Sputnik 1 and Sputnik 2, which had begun the Space Race in 1957.
Explorer 1 carried instruments designed by a team led by James Van Allen. Its most important discovery was the existence of the Van Allen radiation belts, zones of energetic charged particles trapped by Earth's magnetic field.
The satellite was built by the Jet Propulsion Laboratory, while the Army Ballistic Missile Agency supplied the launch vehicle. Explorer 1 remained in orbit until March 31, 1970, making it a scientific milestone as well as a national achievement.