"For NASA, space is still a high priority. ...If we don't succeed we run the risk of failure."
Dan Quayle, U.S. vice-president (1989-1993)
X-rays are absorbed by Earth's atmosphere so one has to climb above the atmosphere to see them. NASA's Uhuru (1970) and the United Kingdom's Ariel V (1974) were spin-stabilized satellites that discovered around 400 bright X-ray sources. Astronomers realized that X-rays provide vital clues to the death-throws of stars, specifically supernova explosions and the final transitions to white dwarf, neutron star, and black hole states. X-rays are a vital-component of the radiation coming from energetic events such as solar flares.
Two technical advances helped the development of space telescopes. One was the construction of advanced confocal mirror systems and the other was the development of two-dimensional X-ray imaging gas scintillation proportional counters. Using these, the United States launched the first orbiting satellite containing a fully imaging X-ray telescope, the High Energy Astrophysical Observatory-2 (HEAO-2), in November 1978.
HEAO-2 was renamed "Einstein" when it was in orbit and operating correctly. Einstein discovered that nearly all astronomical bodies emit X-rays. Also the angular resolution (a few seconds of arc) and the sensitivity was such that accurate maps could be made of objects such as the Cygnus loop supernova remnant. The Einstein instruments were a thousand times more sensitive than those on Uhuru.
Einstein remained operational until April 1981. Other space telescopes have followed, such as Exosat, Rosat, Chandra, and XXM Newton, and all have continued the quest for ever higher detail and sensitivity. In addition 10 new .sources being discovered, it has been found that many X-rays are being emitted as material falls into the black holes at the center of active galactic nuclei.
