Van Allen Belts

The first successfully launched American satellite of Earth was Explorer I. It was propelled into orbit from Cape Canaveral, Florida, on January 31, 1958, by a four-stage combination of rockets developed by the United States Army Ballistic Missile Agency and the Jet Propulsion Laboratory (JPL). The principal scientific instrument within the payload was a Geiger tube radiation detector developed by Professor James A. Van Allen and graduate student George H. Ludwig of the University of Iowa. The instrument's intended purpose was a comprehensive survey of cosmic ray intensity above Earth's atmosphere.

The launch of an improved version of the radiation instrument was attempted on March 5 on Explorer II, but an orbit was not achieved because the fourth stage rocket failed to ignite. On March 26, the launch of the improved instrument on Explorer III, including the first magnetic tape recorder ever flown in space, was successful.

The in-flight data from the radiation detectors on Explorer I and Explorer III revealed that there are enormous numbers of energetic, electrically charged particles trapped in the external magnetic field of Earth. This discovery was promptly confirmed and extended later in 1958 with additional space flights by the Iowa group and others, including Soviet investigators on Sputnik III. In subsequent years the continuing study of this phenomenon has included the efforts of over 1000 scientists in at least 20 different countries.