The X-Ray Telescope (XRT) onboard the Space Shuttle Challenger on mission 51F. Mission 51F, also designated Spacelab 2, formed one of the missions developed for Nasa by the European Space Agency (ESA). The joint programme NASA-ESA started in 1983 with the mission Spacelab 1 but it was severely affected following the Challenger's destruction in 1986. The main goal of the X-Ray Telescope was to image the X-ray emission from clusters of galaxies in order to determine their weight. XRT is designed to be refurbished and reflown many times in order to achieve a large data base. Mission 51F was launched on July 29th 1985 and stayed in orbit for 10 days.