The Prospero satellite, also known as the X-3, was launched by the United Kingdom in 1971. It was designed to undertake a series of experiments to study the effects of space environment on communications satellites and remained operational until 1973, after which it was contacted annually for over 25 years. Although Prospero was the first British satellite to have been launched successfully by a British rocket, the first British satellite placed in orbit was Ariel 1, launched in April 1962 on a U.S. rocket.

Prospero was built by the Royal Aircraft Establishment in Farnborough. Initially called Puck, it was designed to conduct experiments to test the technologies necessary for communication satellites. Two experimental solar cells setups were tested. One was a test of a lightweight cell and mounting. The other was an attempt to replace the standard fused silica cover of solar cells with a Cerium oxide-based cover. Designs for telemetry and power systems were also tested. It also carried a micrometeoroid detector, to measure the presence of very small particles. The detector worked on the principle of impact ionisation. When the Ministry of Defence cancelled the Black Arrow programme, the development team decided to continue with the project but renamed the satellite Prospero when it was announced it would be the last launch attempt using a British rocket.