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An early series of US Earth-orbiting satellites and their associated rocket launcher. Vanguard 1 was the second US satellite, launched on 17 March 1958 by the three-stage Vanguard rocket. Tracking of its orbit revealed that the Earth is slightly pear-shaped. The series ended in September 1959 with Vanguard 3.

Subjects: Astronomy and Astrophysics.

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