A continuing series of US scientific SATELLITES covering a wide range of applications. The first of the series, Explorer 1, was the first US SATELLITE to be placed in orbit (on 31 January 1958). Its major achievement was the discovery of the inner VAN ALLEN BELT. By 1976, the number of successful Explorer launchings was well over fifty, and the scientific objectives of these SATELLITES have included studies of the upper ATMOSPHERE, interplanetary medium, micro-meteorites, and X-RAY, GAMMA-RAY and RADIO-ASTRONOMY, etc.
