Explorer-I, officially known as Satellite 1958 Alpha, was the first United States earth satellite and was sent aloft as part of the United States program for the International Geophysical Year 1957-1958. It was designed and built by the Jet Propulsion Laboratory (JPL) of the California Institute of Technology under the direction of Dr. William H. Pickering. The satellite instrumentation of Explorer-I was designed and built by Dr. James Van Allen of the State University of Iowa.
The satellite was launched from Cape Canaveral (now Cape Kennedy) in Florida at 10:48 P.M. EST on 31 January 1958 by the Jupiter-C vehicle--a special modification of the Redstone ballistic missile--that was designed, built, and launched by the Army Ballistic Missile Agency (ABMA) under the direction of Dr. Wernher Von Braun. Jupiter-C, a direct descendant of the German A-4 (V-2) rocket, was originally developed in 1955-1956 as a high-performance rocket for testing purposes.
Once in orbit, the cosmic ray equipment of Explorer-I indicated a much lower cosmic ray count than had been anticipated. Dr. Van Allen theorized that the equipment may have been saturated by very strong caused by the existence of a belt of charged particles trapped in space by the earth's magnetic field. The existence of these Van Allen Belts, discovered by Explorer-I, was confirmed by Explorer-III, which was launched by a Jupiter-C on 26 March 1958.
The discovery of the Van Allen Belts by the Explorer satellites was considered to be one of the outstanding discoveries of the International Geophysical Year.
