Sputnik 1, often referred to as simply Sputnik, 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. It sent a radio signal back to Earth for three weeks before its three silver-zinc batteries became depleted. Aerodynamic drag caused it to fall back into the atmosphere on 4 January 1958.
It was a polished metal sphere 58 cm (23 in) in diameter with four external radio antennas to broadcast radio pulses. Its radio signal was easily detectable by amateur radio operators, and the 65° orbital inclination made its flight path cover virtually the entire inhabited Earth.
The satellite's success was unanticipated by the United States.