Explorer 10 satellite undergoing testing in a NASA laboratory prior to being launched in 1961.—NASA

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.

This entry comes from Encyclopædia Britannica Concise.
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