n.
Any of three unmanned U.S. experimental satellites.
Vanguard I (1958), the second U.S. satellite placed in orbit around Earth (after Explorer 1 ), was a tiny 3.25-lb (1.47-kg) sphere with two radio transmitters. Its flight path revealed that Earth is almost imperceptibly pear-shaped, confirming earlier theories. Vanguard II (1959) carried light-sensitive photocell s to provide information about Earth's cloud cover, but its tumbling motion rendered the data unreadable. Vanguard III (1959) was used to map Earth's magnetic field .