first of four X-3 satellites orbited by Great Britain. It was launched with a British Black Arrow missile on Oct. 28, 1971, from the rocket-testing facility at Woomera, Australia. Prospero weighed 145 pounds (66 kilograms) and was primarily designed to test the efficiency of various technical innovations, such as a new system of telemetry and solar cell assemblies. It also carried detectors to measure the density of high-speed micrometeoroid particles of space dust in the Earth's upper atmosphere.