IRAS


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infrared astronomy

infrared astronomy, study of celestial objects by means of the infrared radiation they emit, in the wavelength range from about 1 micrometer to about 1 millimeter. All objects, from trees and buildings on the earth to distant galaxies, emit infrared (IR) radiation. The study of such radiation from celestial objects is of particular importance for several reasons. Cosmic dust particles effectively obscure parts of the visible universe, such as the center of our galaxy, the Milky Way, but this dust is transparent in the IR wavelengths. Most of the energy radiated by objects ranging from interstellar matter to planets lies in the IR wavelengths; IR observations are therefore significant in studying asteroids, comets, planetary satellites, and interstellar dust clouds where stars are forming. Finally, because the expansion of the universe shifts energy to longer wavelengths, most of the visible radiation emitted by stars and galaxies during the early stages of the formation of the universe is now shifted to the IR range; studies of the most distant objects in the IR spectrum are necessary if astronomers are to understand how the universe was formed.

The beginnings of IR astronomy can be traced to the discovery of IR radiation in the spectrum of the sun by English astronomer Sir William Herschel about 1800. It is reported that Irish astronomer Lord William Rosse detected IR radiation from the moon about 1845. As early as 1878 the American inventor Thomas Alva Edison observed a solar eclipse from a site in Wyoming using a sensitive IR detector, and during the 1920s the first systematic IR observations of celestial objects were made by Seth B. Nicholson, Edison Pettit, and other American astronomers. However, modern IR astronomy did not begin until the 1950s because of the lack of appropriate instrumentation. Since then, special interference filters and cryogenic systems (to minimize IR interference from the radiation emitted by the equipment itself) have been introduced for ground-based observations, and aircraft, balloons, rockets, and orbiting satellites have been successively employed to carry the equipment above the water vapor in the earth's atmosphere.

The Kuiper Airborne Observatory (KAO), operated by the National Aeronautics and Space Administration (NASA), had its first flight in 1975. Named for the American astronomer Gerard P. Kuiper, the KAO was a C-141 jet transport that carried its 36-inch (91-cm) telescope to altitudes of up to 45,000 ft (13,720 m). Before it flew its last mission in 1995, the KAO was instrumental in the discovery of the rings of Uranus, the atmosphere around Pluto, and the definitive detection of water during the crash of comet Shoemaker-Levy 9 into Jupiter. Also sponsored by NASA is the Infrared Telescope Facility, a 10-ft (3-m) IR telescope located at an altitude of 14,000 ft (4,270 m) on the summit of Mauna Kea in Hawaii; established in 1979, it effectively is the U.S. national IR observatory. Also near the summit of Mauna Kea is the 12.5-ft (3.8-m) United Kingdom Infrared Telescope (UKIRT), the largest telescope in the world used solely for IR observations.

The first IR satellite to be launched (1983) was the Infrared Astronomical Satellite (IRAS), a joint venture of the United States, Great Britain, and the Netherlands. Orbiting the earth for 10 months, IRAS performed an all-sky survey that yielded catalogs of hundreds of thousands of IR sources, more than half of these previously unknown, including asteroids and comets; detected a new class of long-lived “cool” galaxies that are dim in the visible region of the spectrum; located a protoplanetary disk around a nearby star; and showed clearly for the first time the bulge near the center of the Milky Way. In 1989 the second IR satellite, the Cosmic Background Explorer (COBE), was launched by NASA. Operating through 1993, COBE detected small temperature variations in the cosmic microwave background radiation that provided vital clues to the nature of the early universe and its evolution since the “big bang.” The European Space Agency (ESA) launched the Infrared Space Observatory (ISO) in 1995. Operating until May, 1998, ISO monitored nearby planets, asteroids, and comets. It found water vapor in the atmospheres of Saturn, Neptune, Uranus, and Titan, Saturn's largest moon; detected water vapor and fluorides in the interstellar medium; and studied the “cool” galaxies first seen by IRAS. The near-infrared camera multiobject spectrometer (NICMOS) was placed aboard the Hubble Space Telescope in 1997. Consisting of three cameras and three spectrometers, it has been used to study interstellar clouds where stars are being formed, young stars, and the atmospheres of Jupiter and Uranus.

The Spitzer Space Telescope, a cryogenically cooled satellite observatory with a 2.8-ft (0.85-m) telescope, was launched in Aug., 2003, and placed in a solar orbit in which it trails the earth by 5.4 million mi (8.7 million km); the lifetime of its main instruments ended in 2009, but it continued operating until 2020. In May, 2009, ESA launched the Herschel Space Telescope, with a 138-in. (3.5-m) mirror; it also was cryogenically cooled. Positioned some 930,000 mi (1.5 million km) from earth on a mission that lasted until 2013, it observed wavelengths from the infrared to the submillimeter. NASA's Wide-field Infrared Survey Explorer (WISE) was launched in Dec., 2009, on a six-month mission to survey the entire sky at infrared wavelengths. A KAO replacement, the Stratospheric Observatory for Infrared Astronomy (SOFIA), flew its first official science mission in 2010. Consisting of a Boeing 747-SP aircraft modified to accommodate a 8.2-ft (2.5-m) reflecting telescope (the largest airborne telescope in the world), it is a joint project of NASA and the German space agency, DLR.

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IRAS

(ÿ -rass) Abbrev. for Infrared Astronomical Satellite. A satellite that has made an all-sky survey in the infrared wavelength range 10–100 μm using liquid helium-cooled optics and detectors. The project was developed and operated by the Netherlands Agency for Aerospace Programs (NIVR), NASA, and the UK Science and Engineering Research Council (SERC). The satellite was launched into a 900 km orbit on Jan. 26 1983 and operated until Nov. 22 1983 when the helium supply was exhausted. By the end of the mission 95% of the sky had been surveyed with confirming scans, 3% surveyed only once, and the remaining 2% not surveyed at all.

IRAS consists of a spacecraft that pointed a liquid helium cryostat containing a 0.57-meter Ritchey–Chrétien telescope cooled to less than 10 K. An array of cooled detectors was located in the focal plane at the Cassegrain focus of the telescope. The 62 photoconductive detectors responded to one of four wavebands centered on 100, 60, 25, or 12 μm. They were arranged so that every source crossing the field of view could be seen by at least two detectors in each of the wavebands. The low resolution spectrometer (LRS) covered the range 8–23 μm with a spectral resolution, λ/δλ, of about 20. The chopped photometric channel (CPC) mapped infrared sources on a 9 × 9 arcmin raster at 50 and 100 μm with a spatial resolution of about 1 arcmin.

The operation center for the satellite was at the SERC Rutherford-Appleton Laboratory, near Oxford, England where a preliminary analysis of the data was made. The final reduction of data was done in the USA and the Netherlands. The IRAS data has been issued in various catalogs, including the IRAS Point Source Catalog (IPSC) and the IRAS Faint Source Survey (IFSS) and corresponding catalog (IFSC). The IRAS Sky Survey Atlas (ISSA) for ecliptic latitude over 50° was released on CD–ROM in 1992 the remainder scheduled in 1994.

The results obtained so far relate to almost every aspect of astronomy. Some of the highlights are: the discovery of a dust shell around Vega, which may be an early planetary system; the discovery of infrared cirrus in our Galaxy; the first detection from space using infrared of comets (IRAS discovered 5 new comets); the detailed mapping of the infrared emission from the galactic plane; observations on galaxies showing that most of the galaxies detected in the 60- and 100-μm wavebands are spirals; observations on low-luminosity protostars in several molecular cloud complexes.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
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