Some of the major X-ray astronomy missions include:

Uhuru

In the early 1970's, NASA's Uhuru X-ray satellite, equipped with a relatively simple instrument - a sensitive X-ray detector similar to a Geiger counter attached to a viewing pipe to locate the source -- made some astounding discoveries. Uhuru detected evidence of black holes and superdense neutron stars pulling matter from companion stars, and vast expanses of hot gas in gigantic systems containing thousands of galaxies.

Skylab The first large focussing X-ray telescope was the Apollo Telescope Mount aboard Skylab in the early 1970's. This pioneering telescope used two pairs of concentric mirrors to make stunning X-ray images of the Sun. It set the stage for the development of the Einstein X-ray Observatory.

Einstein--the First Imaging X-Ray Observatory NASA's Einstein Observatory, launched in 1978, was the first large X-ray telescope with mirrors. It made the first X-ray images of shock waves from exploded stars, and images of hot gas in galaxies and clusters of galaxies. Einstein also located accurately over 7000 X-ray sources and made possible a new way to study the mysterious dark matter that surrounds many galaxies.

ROSAT The Roentgensatellite or ROSAT, a joint venture between Germany, the United Kingdom and the United States, carried an even larger X-ray telescope into orbit in 1990. It has expanded the number of known X-ray sources to more than 60,000 and has proved to be especially valuable for investigating the multi-million degree gas present in the upper atmospheres of many stars.