Sputnik 1 was the first artificial satellite to transmit radio signals from orbit.
The Soviet Union launched Sputnik 1 on 4 October 1957, beginning the Space Age and the period known as the Space Race. The satellite carried two radio transmitters that sent regular beeps on different frequencies. Those signals could be detected by radio operators around the world as the satellite passed overhead.
Sputnik 1 was a polished metal sphere about 58 centimetres in diameter, with four external whip antennas. It orbited Earth at an altitude that varied roughly from 215 to 939 kilometres. The satellite’s radio transmissions helped demonstrate that an artificial object was orbiting Earth and allowed scientists to study the propagation of radio signals through the atmosphere.
The satellite reentered the atmosphere on 4 January 1958 after about three months in orbit. The United States launched Explorer 1 several months later, on 31 January 1958. Sputnik 1 is therefore distinct from later satellites such as Sputnik 2 and Vanguard 1.