any of a series of unmanned U.S. experimental test satellites. Vanguard I, launched March 17, 1958, consisted of a tiny 3.25-pound (1.47-kilogram) sphere equipped with two radio transmitters. It was the second artificial satellite placed in orbit around the Earth by the United States, the first being Explorer 1 (Jan. 31, 1958). By monitoring Vanguard's flight path, scientists found that the Earth was almost imperceptibly pear-shaped, in confirmation of earlier theories. Vanguard II, orbited on Feb. 17, 1959, carried light-sensitive photocells that were designed to provide information about the Earth's cloud cover, but the tumbling motion of the satellite rendered the data unreadable. Vanguard III, the last in the series, was launched several months later. It was used to map the Earth's magnetic field.