These satellites carry into orbit telescopes and other astronomical instruments. The great advantage of doing this is that it takes the instruments high above the Earth's atmosphere, which, because of dust, water vapour and air currents, dims and distorts our view of the heavenly bodies. Above the atmosphere, the skies are crystal clear. The atmosphere not only dims and distorts the light from the stars. It also blocks many of the other radiations the stars give out, such as much infrared and ultraviolet, together with gamma rays and X-rays. Instruments lofted above the atmosphere can detect all of these radiations. In so doing, they give us a more complete picture of what the universe is really like. Among the most successful astronomy satellites have been Einstein and Exosat (X-rays), IUE (ultraviolet), IRAS (infrared), COBE (microwave) and Compton Observatory (gamma rays). In visible light, the Hubble Space Telescope has proved remarkably effective, despite having a serious design flaw.
