Sputnik 1 (Russian: Спутник-1, "Satellite 1") was the first artificial Earth satellite. The Soviet Union launched it into an elliptical low Earth orbit. It orbited for three weeks before its batteries died and then orbited silently for two months before it fell back into the atmosphere. It was 58 cm in diameter and a polished metal sphere with four external radio antennas to broadcast radio pulses.

Its radio signal was easily detectable by radio amateurs, and the 65° inclination and duration of its orbit made its flight path cover virtually the entire inhabited Earth. The satellite's unanticipated success precipitated the American Sputnik crisis and triggered the Space Race, part of the Cold War. The launch was the beginning of a new era of political, military, technological and scientific developments. The name "Sputnik" is Russian for spouse/traveling companion or satellite when interpreted in an astronomical context.

Tracking and studying Sputnik 1 from Earth provided scientists with valuable information. The density of the upper atmosphere could be deduced from its drag on the orbit, and the propagation of its radio signals gave data about the ionosphere.

Sputnik 1 was launched during the International Geophysical Year. The satellite travelled at about 29,000 kilometres per hour, taking 96,2 minutes to complete each orbit. It transmitted on 20.005 and 40.002 MHz, which were monitored by radio operators throughout the world. The signals continued for 21 days until the transmitter batteries ran out on 26 October 1957. Sputnik burned up on 4 January 1958 while reentering Earth's atmosphere, after three months, 1440 completed orbits of the Earth, and a distance travelled of about 70 million km.