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 micrometeorite impacts. Its major accomplishment was the discovery of the Van Allen radiation belt, in a region around the Earth called the magnetosphere.

Between 1958 and 1981 there were 56 more Explorer spacecrafts to reach space, though the missions had little in common other than their small size and relatively low cost. They performed a variety of scientific missions, including thorough studies of Earth's atmosphere and ionosphere.