Sputnik 1
Sputnik 1 was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program. The polished metal sphere, 58 cm in diameter with four external radio antennas, sent a radio signal back to Earth for about three weeks before its batteries were depleted. Tracking it gave scientists valuable data on the upper atmosphere and ionosphere. Its success was unanticipated by the United States, precipitating the American Sputnik crisis and triggering the start of the Space Race. It burned up on reentry on 4 January 1958.
