This _Sputnik_ model is from the USSR Academy of Sciences. Explorer 1 [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. Mariner 2 [Illustration: 7. Artist’s conception of _Mariner 2_ as it flew by Venus.] The first successful interplanetary spacecraft probed the environment of Venus, Earth’s closest neighbor. _Mariner 2_, working flawlessly, swept by the hot and cloudy planet at a closest approach of 34,834 kilometers (21,645 miles) on December 14, 1962.