Explorer 4 launch. Explorer 4 being launched by a Jupiter-C rocket from Cape Canaveral (now Kennedy Space Centre), Florida, USA, on 26 July 1958. The rocket-shaped satellite, which weighed 25.5 kilograms, was housed in the nose cone of the rocket. It orbited Earth at heights of between 260 kilometres and 2200 kilometres until 23 October 1959. It stopped transmitting on 5 October 1958. It carried instruments, including a geiger counter, to make detailed measurements of charged particles trapped in the Van Allen radiation belt, a torus of charged particles held in place by the Earth's magnetic field. The Van Allen radiation belt was discovered by the Explorer 1 satellite launched in January 1958.