The first artificial satellite, Sputnik 1, head-on. Sputnik was launched on 4th October 1957 by the former Soviet Union. The name Sputnik is Russian for 'travel companion' or satellite. The body was a highly polished aluminum alloy sphere which measured 58 centimeters in diameter and had four long antennas attached to it. Sputnik 1 was located in Earth orbit and transmitted radio signals back to Earth for a period of 23 days. Although the mission ended on 27th October 1957, it was not until 4th January 1958 that Sputnik returned to Earth. The success of Sputnik 1 prompted the US government to enter the Space Race with the launch of Explorer-1 four months later.