Any of the largest (55-member) series of unmanned U.S. spacecraft , launched between 1958 and 1975.
Explorer 1, the first satellite sent into orbit by the U.S., discovered the innermost Van Allen radiation belt . Other notable craft in this series include Explorer 38 (1968), which measured galactic radio sources and studied low radio frequencies in space, and Explorer 53 (SAS 3; 1975), which investigated X-ray and gamma-ray sources within and beyond the Milky Way Galaxy.