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Space Telescope
I INTRODUCTION
Space Telescope, telescope or other astronomical detector mounted on an artificial satellite that orbits Earth. Space telescopes have several advantages over Earth-based telescopes. They offer a much clearer view of astronomical objects because their instruments are far above Earth's turbulent, distorting atmosphere. Telescopes in space are free from light pollution, artificially generated light that makes it difficult to observe faint astronomical objects. Space telescopes also can cover the entire celestial sphere, whereas portions of the sky may not be accessible to stationary ground-based telescopes, depending on their location on Earth. Observations made by telescopes in orbit have revolutionized astronomers' view of the universe.
Space telescopes also are not limited to observing the narrow band of light that is visible to the eye. Instead they have access to the entire electromagnetic spectrum, including infrared light, ultraviolet light, X rays, and gamma rays. The atmosphere entirely blocks some portions of the spectrum from reaching the ground, so satellites that can detect radiation from those portions offer new windows onto the universe that carry a wealth of information about planets, stars, and galaxies, and also the processes that shape them. Phenomena such as active galaxies and black holes cannot be fully understood without comparing data from across the electromagnetic spectrum.
Space telescopes range in complexity from small satellites, which often survey the entire sky, to larger “observatory-class” satellites, which can target particular objects. These larger satellites generally require more intensive control from scientists on the ground, who choose objects to be studied and help point the satellites in the correct direction.
Nearly a century ago Russian theorist Konstantin Tsiolkovsky and German rocket scientist Hermann Oberth recognized the advantage of placing an astronomical telescope in space, where starlight would not be blurred by the turbulence of Earth's atmosphere. But it was not until after World War II (1939-1945), when rocket boosters were developed capable of hurtling satellites into orbit, that the dream of space-based astronomy became reality.
The British Ariel program launched the first astronomical satellites. Ariel 1, launched in April 1962, studied the Sun's ultraviolet and X-ray emissions. The next space telescopes were from the Orbiting Astronomical Observatory (OAO) program of the United States National Aeronautics and Space Administration (NASA). OAO 2, the first successful OAO, was launched in December 1968 and carried infrared, ultraviolet, X-ray, and gamma-ray detectors and telescopes. As the first of their kind, these satellites provided valuable background for later and more complex space telescopes.
II X-RAY OBSERVATORIES
Earth's atmosphere absorbs almost all the X rays that enter it from space. The first X rays found from sources other than the Sun or Earth were detected in 1962 by a U.S. rocket equipped with X-ray detectors. The source of these X rays was a binary star called Scorpius X-1.
X rays are often associated with high-energy events that are of interest to astronomers. Supernovas (stars that explode at the end of their lives) and the centers of active galaxies emit X rays. X rays are also emitted by binary star systems in which the gravitational pull of a small, dense star such as a white dwarf (a very compact, small star) or a neutron star (the collapsed remnant of a massive star) is pulling gas off of its normal companion star and heating it to millions of degrees.
The first space telescope dedicated solely to the study of X rays from astronomical objects was NASA's Explorer 42 (also called Uhuru), launched in December 1970. Uhuru found thousands of X-ray sources, from objects within the Milky Way's galactic neighborhood to sources at the edge of the observable universe. The High Energy Astronomical Observatory (HEAO) 2, also called the Einstein Observatory, was launched in November 1978 and carried the first focusing X-ray telescope, which was capable of making images of X-ray sources.
The German Roentgen Satellite (ROSAT) was launched in June 1990 and cataloged nearly 100,000 X-ray sources. NASA's Chandra Observatory (formerly the Advanced Astronomy X-Ray Satellite), launched in mid-1999, has much higher resolution and sensitivity than its predecessors.
III GAMMA-RAY OBSERVATORIES
Studying gamma rays offers scientists answers to some of the most perplexing questions about the explosive and dynamic physical processes in the universe. Gamma-ray observation also provides clues about the structure and dynamics of the Milky Way and other galaxies; the nature of pulsars, quasars, black holes, and neutron stars; and the origin and history of the universe itself. See also Gamma-Ray Astronomy.
Early gamma-ray satellites included NASA's Explorer 48 (also called Small Astronomy Satellite 2 or SAS 2), launched in 1972, and the Celestial Observation Satellite B (COS B) of the European Space Agency (ESA), launched in 1975. SAS 2 and COS B were the first satellites to detect specific sources of gamma rays. NASA launched HEAO 1 and 3 in 1978 and 1979. These satellites produced a survey of gamma-ray sources throughout the sky. NASA's Solar Maximum Mission, launched in 1980, observed gamma rays from the Sun and from other cosmic sources.
In 1991 NASA's Compton Gamma Ray Observatory (GRO) discovered that the mysterious gamma-ray bursts are evenly distributed across the sky, suggesting they originate in distant galaxies scattered across the universe. This idea was confirmed in 1997 when the ground-based Keck telescope measured the distance to the optical counterpart of a gamma-ray burst and found it to be billions of light-years away. Astronomers now believe that gamma-ray bursts are caused by extremely violent supernovas.
Compton's mission ended in 2000. The ESA launched an even more sensitive gamma-ray observatory, the International Gamma-Ray Astrophysics Laboratory (Integral), in 2002.
IV ULTRAVIOLET OBSERVATORIES
Some of the hottest and most energetic stars in the universe are visible in the ultraviolet region of the electromagnetic spectrum. Sources include the atmospheres of young stars, the surfaces of hot white dwarf stars and the cores of active galaxies. See also Ultraviolet Astronomy.
The International Ultraviolet Explorer (IUE), a joint project of the United Kingdom, NASA, and the ESA, was one of the longest lasting and productive space telescopes, operating from 1978 to 1996. IUE observed several novas and supernovas, providing much more information about the stars than was known. The satellite identified a new type of nova created by an exploding white dwarf star. IUE was followed by a series of free-flying and space-shuttle-mounted ultraviolet telescopes.
V INFRARED OBSERVATORIES
Earth's atmosphere emits infrared radiation, called infrared background glow, which interferes with readings by ground-based infrared telescopes. The atmosphere also contains water vapor that absorbs some infrared radiation, preventing it from reaching ground-based infrared telescopes. Infrared telescopes in space are not hampered by background glow and are extraordinarily sensitive to faint infrared sources. See also Infrared Astronomy.
The Infrared Astronomy Satellite (IRAS), launched in January 1983, made an infrared survey of the entire sky, cataloging hundreds of thousands of infrared sources. IRAS was a joint project of NASA, the United Kingdom, and The Netherlands. The satellite revealed the births of stars; found disks of dust around stars, which could be where planets are formed; uncovered new clues about quasars; discovered a class of dusty infrared galaxies; and detected new comets.
The ESA's Infrared Space Observatory (ISO) was launched in 1995 and provided significant information on subjects ranging from the weather on the planet Saturn to details about distant galaxies that are producing stars at rates up to 1,000 times that of the Milky Way Galaxy. NASA launched another infrared observatory, the Spitzer Space Telescope, in 2003. Spitzer has even higher sensitivity and resolution than IRAS.
VI VISIBLE-LIGHT AND OTHER OBSERVATORIES
Visible-light observations from space have the advantage of a clearer image. Ground-based telescopes are hampered by interference by Earth's atmosphere, which produces fuzzy images.
NASA's 12-ton Hubble Space Telescope (HST) was lofted into Earth orbit in 1990 to undertake a 15-year mission of discovery. The HST's 94.5-in (240-cm) mirror produces stunningly clear images that are ten times sharper than what is routinely seen from the ground. In addition to its visible-light observations, the HST views the universe in adjacent portions of the electromagnetic spectrum, from ultraviolet to near-infrared light.
NASA's Cosmic Background Explorer (COBE), launched in 1989, goes beyond the HST's range to view far-infrared and microwave radiation. COBE found a nearly uniform background radiation coming from all parts of the universe, helping to support the big bang theory that the universe expanded from a super-hot, ultra-dense fireball and is still cooling.
The ESA's Hipparcos satellite, launched in 1989, returned data on the color, positions, and distances of over 100,000 stars.
Contributed By:
Ray Villard
Hubble Space Telescope
The Hubble Space Telescope, free of the distorting effects of the earth's atmosphere, has an unprecedented view of distant galaxies. Placed in orbit in 1990, scientists discovered soon after the telescope became operational that its 240-cm (94.5-in) primary mirror was flawed. However, a repair mission completed by space shuttle astronauts in December 1993 successfully installed corrective optics which compensated for the flawed mirror.
Photo Researchers, Inc./NASA/Science Source
