On October 4, 1957 the Soviet Union launched a small satellite into an elliptical low Earth orbit. The satellite was known as Sputnik. It consisted of a small polished silver sphere, 585mm in diameter, with four external radio antennae that transmitted pulses back to Earth. Direct data about the Earth's atmosphere could be determined from the drag on the satellite and the propagation of the radio waves through the ionosphere.

The launch of Sputnik heralded the start of the Space Race

Credit:NASA

Travelling at 29,000 kph Sputnik took just over 1.5 hours to complete each orbit. Sputnik transmitted radio signals fro 22 days until the batteries ran out. Sputnik continued in orbit until January 4, 1958, when it re-entered Earth's atmosphere and burnt up. In total Sputnik spent a total of around three months in orbit.