satellite observatory, Earth-orbiting spacecraft that allows celestial objects and radiation to be studied from above the atmosphere. Astronomy from Earth’s surface is limited to observation in those parts of the electromagnetic spectrum (see electromagnetic radiation) that are not absorbed by the atmosphere. Those parts include visible light and some infrared radiation and radio waves. The ability to place instruments in space opens all regions of the spectrum to observation. Even when operating in those wavelengths that penetrate to Earth’s surface, an observatory in space avoids the problems of seeing caused by atmospheric turbulence and airglow.

Beginning in the 1960s, the space agencies of the United States and several other countries independently and in cooperation developed satellite observatories specifically instrumented to explore cosmic phenomena in the gamma-ray, X-ray, ultraviolet, visible, and infrared regions. Among early spacecraft of note were the International Ultraviolet Explorer (IUE; launched 1978), which studied faint objects in the ultraviolet region, and the Infrared Astronomical Satellite (IRAS; 1983), which mapped the sky in the infrared region, finding hundreds of thousands of new stars and galaxies. The Hubble Space Telescope (HST; 1990) provided images of unprecedented resolution in visible and ultraviolet light, while the Compton Gamma Ray Observatory (CGRO; 1991) and the Chandra X-Ray Observatory (1999) permitted the investigation of gamma-ray and X-ray sources, respectively. Other spacecraft, such as Yohkoh (1991) and Hinode (2006), were specifically designed for studying various aspects of the Sun.

Although most observatories in space orbit Earth, a few have exploited orbits around the Sun. For example, the Solar and Heliospheric Observatory (SOHO; 1995) was maneuvered to the vicinity of a gravitational balance point (L1, one of the Sun-Earth Lagrangian points) located about 1.5 million km (0.9 million miles) sunward of Earth. In that location it observed the Sun uninterruptedly, without having to pass through Earth’s shadow. The Spitzer Space Telescope (2003), an infrared satellite observatory, was placed into a solar orbit with a period of revolution that causes it to drift away from Earth at a rate of 15 million km (10 million miles) per year. This keeps the telescope away from the thermal radiation of Earth.

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