any of the largest series of unmanned U.S. spacecraft, consisting of 55 scientific satellites launched between 1958 and 1975. Explorer 1 (launched Jan. 31, 1958), the first space satellite orbited by the United States, discovered the innermost of the Van Allen radiation belts (Van Allen radiation belt), two zones of charged particles that surround the Earth. Other notable craft in the series include Explorer 38 (July 4, 1968), which measured galactic radio sources and studied low frequencies in space, and Explorer 53 (May 7, 1975), which was sent out to explore X-ray sources both inside and outside the Milky Way galaxy.