Explorer 1. Computer artwork of America's first successful artificial satellite, Explorer 1 showing part of its interior. Explorer 1 was launched on 31 January 1958, after the USSR took the USA by surprise by launching the world's first artificial satellite, Sputnik 1, on 4 October 1957. The satellite measured 2.05 metres in length and weighed 14 kilograms. It orbited Earth at heights of between 360 kilometres and 2520 kilometres until 31 March 1970. It carried, amongst other instruments, a geiger counter, used to detect cosmic rays. Data collected by the satellite led to the discovery of the Van Allen radiation belt, a torus of charged particles held in place by the Earth's magnetic field.