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Explorer I

the first United States scientific satellite

Officially known as Satellite 1958 Alpha, Explorer I was sent aloft on January 31, 1958 as part of the United States program for the International Geophysical Year 1957-1958. It was designed and built by the Jet Propulsion Laboratory of the California Institute of Technology under the direction of Dr. William H. Pickering. The satellite instrumentation it carried was designed and built by Dr. James Van Allen of the State University of Iowa.

Explorer I

Once in orbit, the cosmic ray equipment of the satellite 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 radiation from a belt of charged particles trapped in space by the earth's magnetic field. The existence of what we now know as the Van Allen Belt was confirmed by Explorer III, which was launched on March 26, 1958. The discovery of the Van Allen Belts was the most important discovery made by Explorer I, and probably one of the most outstanding discoveries of the International Geophysical Year.


Total Weight 30.8 pounds (14 kilograms)
Weight of Instrumentation 18.35 pounds
Length 80 inches (203 centimeters)
Diameter 6.25 inches (15.9 centimeters)

Launch Vehicle Jupiter-C rocket

Orbit Perigee 220 miles (354 kilometers)
Orbit Apogee 1,563 miles (2,515 kilometers)
Orbit Period 114.8 minutes

Instrumentation cosmic-ray detection package, internal temperature sensor, three external temperature sensors, nose-cone temperature sensor, micrometeorite impact microphone, micrometeorite erosion guages

Last Transmission May 23, 1958
Entered Earth's Atmosphere March 31, 1970

diagram of Explorer 1

International Geophysical Year

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This page was last updated on 03/06/2017.