[Illustration: 5. Trial firing of a full-size mockup of _Explorer 1_
on the third-stage assembly of the Jupiter-C launch vehicle.]
[Illustration: 6. On the launch pad prior to sending the first
American satellite into orbit. _Explorer 1_, launched January 31,
1958, discovered the first two circular radiation belts surrounding
the Earth.]
The International Geophysical Year (1957-58) provided the impetus for
the first official American satellite effort, designated Project
Vanguard in 1955. Vanguard was a civilian effort that relied on a launch
vehicle built especially for the project’s purposes. The launch by the
Soviet Union of _Sputnik 1_ on October 4, 1957, caused the work on
Project Vanguard to go forward under great pressure. When Vanguard Test
Vehicle 3, carrying the first American earth satellite, exploded on its
launch pad on December 6, 1957, United States prestige reached a low
point.
On January 31, 1958, _Explorer 1_ became the first successful American
satellite. It originated in Project Orbiter, a joint study program of
the U.S. Army and the Office of Naval Research—a project that lapsed
after the 1955 decision to designate Vanguard as the official American
satellite effort. Following the _Sputnik_ success, the U.S. Army
Ballistic Missile Agency was instructed to proceed with its satellite
plans.
_Explorer 1_’s launch vehicle was a four-stage Jupiter-C rocket
designed, built, and launched by the Army Ballistic Missile Agency team
headed by Wernher von Braun. The satellite’s instrumentation was
prepared by James Van Allen and George Ludwig of the State University of
Iowa under project direction of the Jet Propulsion Laboratory,
California Institute of Technology.
_Explorer 1_ measured three phenomena—cosmic ray and radiation levels
(data that led to the discovery of the earth’s radiation belts), the
temperature in the vehicle (important in the design of future
spacecraft), and the frequency of collisions with micrometeorites. There
was no provision for data storage, and therefore the satellite
transmitted its information continually.
_Explorer 1_ was not the only orbiting American satellite for long. In
spite of the early problems, Project Vanguard succeeded in launching the
second American earth satellite on March 17, 1958.
The back-up _Explorer 1_ on exhibit is from the National Aeronautics and
Space Administration, Jet Propulsion Laboratory. California Institute of
Technology.