Image of Explorer 1 spacecraft. Courtesy of NASA/JPL

Explorer 1, launched on Jan. 31, 1958, was the first U.S. satellite to orbit the Earth. Its successful flight made the United States the second nation in space, following the Soviets who had launched Sputnik 1 just four months earlier.

Explorer 1 carried several scientific instruments which measured space temperature and the dangers of small meteorites. Its major accomplishment was the discovery of an area of radiation which surrounds the Earth.

Between 1958 and 1981 there were 56 more Explorer spacecrafts to reach space. Each was small, relatively inexpensive, and carried out a variety of scientfific missions.