orbiting observatory, research satellite designed to study solar radiation, electromagnetic radiation from distant stars, the earth's atmosphere, or the like. Because the atmosphere and other aspects of the earth's environment interfere with astronomical observations from the ground, especially in the ultraviolet and infrared portions of the spectrum, the decades since the 1960s have seen increasing emphasis on space-based observatories.

The U.S. Orbiting Solar Observatory (OSO) program, comprising eight satellites launched between 1962 and 1971, was one of the earliest series of orbiting observatories; it studied the sun's atmosphere and the sunspot cycle. Also beginning in 1962 and extending through 1979 were the launches of the six satellites in Great Britain's Ariel program, which concentrated on solar ultraviolet and X radiation. The Orbiting Geophysical Observatory (OGO) program consisted of six satellites, launched between 1964 and 1969, that provided data on the earth's atmosphere, ionosphere, and magnetosphere and on the solar wind. The Orbiting Astronomical Observatory (OAO) program comprised four satellites, launched between 1966 and 1972, that studied astronomical phenomena at ultraviolet and X-ray wavelengths inaccessible to earthbound equipment.

In the following years, a large number of satellites were launched to study solar and galactic radio waves, X rays, gamma rays, and ultraviolet rays. In addition to the United States a number of countries participated, among them the Netherlands with ANS-1 (1974–76), which studied soft and hard X radiation; India with Aryabhata (1975), which returned atmospheric data for only four days before being silenced by a power failure; Japan with Hakucho (1979–85) and Tenma (1981–84), both of which studied X radiation; and the European Space Agency (ESA) with Exosat (1983–86), an X-ray observatory. This period also saw the first cooperative efforts, such as the International Ultraviolet Explorer (IUE), a joint effort of the United States, ESA, and Great Britain (1978–96), which returned data on ultraviolet radiation for 18 years.

ROSAT [Roentgen Satellite] (1990–99), a joint German-U.S.-British project, studied both X-ray and ultraviolet wavelengths never before imaged from space. It detected a new class of bright stars that shine only in the ultraviolet part of the spectrum and X-ray emissions from comets. The Cosmic Background Explorer (1989–93) studied microwave background radiation that no star or other known object could emit—it is believed to have come from the creation of the universe (see cosmology). The Infrared Space Observatory (ISO; 1995–98), launched by ESA, found water in the Orion nebula and in the atmospheres of the giant planets and Titan, found fluoride molecules in interstellar space, and studied the "cool" galaxies first seen by the Infrared Astronomy Satellite (IRAS) in 1983. Another European-built orbiting observatory, the Solar and Heliospheric Observatory (SOHO), was launched by NASA in 1995. After reaching a position about 950,000 mi (1.5 million km) from the earth, where the gravitational attraction of the earth and the sun are in balance (called a Lagrangian point), SOHO initiated a program of solar physics studies, such as the solar wind and solar plumes.

To fully explore the cosmos it is necessary to collect and analyze radiation emitted by phenomena throughout the entire electromagnetic spectrum. Toward that end, NASA proposed the concept of great observatories, a series of four orbiting observatories designed to conduct astronomical studies over many different wavelengths. An important aspect of the program was to overlap the operations phases of the missions to enable astronomers to make concurrent observations of an object at different spectral wavelengths. The first member of the program and the largest orbiting observatory is the Hubble Space Telescope (HST), which was deployed by a space shuttle in 1990 and repaired in orbit in 1993. Subsequent servicing missions added capabilities to the HST, which observes the universe at ultraviolet, visual, and near-infrared wavelengths. The second great observatory, the Compton Gamma-Ray Observatory, was launched and deployed by a shuttle in 1991; it collected data on gamma-ray bursts, which are some of the most violent physical processes in the universe. The third great observatory, the Chandra X-ray Observatory, formerly called the Advanced X-ray Astrophysics Facility, was deployed from a shuttle and boosted into a high earth orbit in 1999; it focuses on such objects as black holes, quasars, and high-temperature gases throughout the X-ray portion of the electromagnetic spectrum. The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility, was the fourth and final element in the great observatory program; launched in Aug., 2003, and operating (except for a few instruments) until 2009, it filled an important gap in wavelength coverage not available from earthbound telescopes.

The CoRoT space telescope, launched in 2006 and operated by the French space agency in conjunction with ESA and other international partners, has searched for and discovered rocky exoplanets somewhat larger than the earth and studied stellar surface acoustical waves in an attempt to improve the understanding of the physics of stars. In 2009 ESA launched the Herschel Space Telescope, with a 138-in. (3.5-m) mirror. Positioned some 930,000 mi (1.5 million km) from earth on a mission that lasted into 2013, it observed wavelengths from the infrared to the submillimeter and studied the formation of galaxies in the early universe and the formation of stars as well as objects in the solar system. On the same Ariane rocket that carried Herschel, ESA also launched Planck, which observed (2009–2013) the cosmic microwave background radiation. Kepler, like CoRoT, was designed to search for rocky, earthlike exoplanets that might be habitable. It was launched by NASA in 2009 and uses a sensitive photometer to monitor the stars in its field of view for the transits of planets. The data from Kepler also has been used by other astronomers to make discoveries concerning the stars.

See also gamma-ray astronomy; infrared astronomy; space probe; ultraviolet astronomy; X-ray astronomy.