Vanguard

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 photocells 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.

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