PAGEOS

PAGEOS (PAssive Geodetic Earth Orbiting Satellite) was a balloon satellite which was launched by NASA in June 1966. It was the first satellite specifically launched for use in geodetic surveying, or measuring the shape of the earth, by serving as a reflective and photographic tracking target. At the time, it improved on terrestrial triangulations of the globe by about an order of magnitude. The satellite, which carried no instrumentation, broke up between 1975 and 1976. One of the largest fragments of the satellite finally deorbited in 2016.

PAGEOS was part of a larger program of inflatable satellites that grew from the original concept by William J. O'Sullivan of a 30-inch diameter inflatable satellite in 1956 to measure air drag at high altitudes, called the Sub-Satellite. While the Sub-Satellite failed, the idea of a visible US satellite became very attractive after Sputnik launched in the Cold War, resulting in a program of similar, larger satellites. These included satellites Echo 1 and Echo 2 under Project Echo, which were also used for experiments in geodetic surveying; several air-density-focused Explorer satellites; and finally PAGEOS.