Infrared Space Observatory - astronomy. I INTRODUCTION Infrared Space Observatory (ISO), satellite dedicated to the study of astronomical objects through the infrared radiation that the objects emit (see Infrared Astronomy). Launched by the European Space Agency (ESA) in 1995, the ISO transmitted data about the molecular structure of some of the most far-reaching objects in the universe. The ISO's mission ended in 1998. Infrared radiation, like visible light, X rays, and radio waves, is a form of electromagnetic radiation. All objects emit electromagnetic radiation because of their temperature. In cooler objects, the most intense of this heat or thermal electromagnetic radiation falls in the infrared part of the spectrum. Therefore, the amount of infrared radiation that an object emits is a good measure of its temperature. Infrared radiation is unique from visible light and ultraviolet radiation in that it is not altered as it passes through interstellar matter, the fine gases and dust found between stars. But measuring infrared wavelengths from a ground-based telescope on the earth is difficult. The earth's atmosphere blocks some infrared wavelengths--water vapor and carbon dioxide in the atmosphere absorb many parts of the infrared region, leaving only a few sections of the infrared spectrum that can be studied from the ground. Furthermore, heat radiation emitted by a ground-based telescope itself tends to mask faint astronomical sources. The ISO had the advantage of being in orbit above the earth's atmosphere, where there is nothing to block infrared light. To eliminate its own satellite's telescope emission of infrared radiation due to heat, the satellite's telescope was cooled to a very low temperature. II THE SATELLITE ISO was a large, cylindrical satellite open at one end that was 5.3 m (17 ft) high, 3.6 m (12 ft) wide, and 2.8 m (9.2 ft) deep. At the time of launch it weighed 2200 kg (4900 lb). The main body of the satellite was a cryostat, a double-walled, cup-shaped cylinder with 2000 liters (500 gallons) of liquid helium between the walls. The liquid helium acted as a refrigerant to keep the satellite's telescope and scientific instruments at a temperature of -271° to -265° C (-456° to -445° F). Positioned on the outside of the cryostat was a V-shaped shield that shaded the cryostat from the Sun. An array of solar cells (see Solar Energy), which produced the satellite's electricity, was mounted on the outside surface of the Sun shield. The service module, a boxlike region below the cryostat, held instruments needed for spacecraft operations. The telescope was mounted in the center of the satellite and was surrounded by the cryostat. The telescope was a reflecting telescope with a 24-in (60-cm) primary mirror coated in gold, which reflects infrared light well. There was a central hole in the primary mirror and the infrared light was brought to a focus behind the telescope. Infrared light collected by the telescope was analyzed by any one of four instruments. The ISO camera (ISOCAM) included two cameras that took electronic pictures at various infrared wavelengths. The short wavelength spectrometer (SWS) and the long wavelength spectrometer (LWS) each analyzed the infrared light gathered from an object to determine the object's chemical composition and temperature (see Spectroscopy). The ISO imaging photo-polarimeter (ISOPHOT) was an aperture photometer, or brightness-measuring instrument, with three subsystems that each contained filters and polarizers to manipulate the infrared light gathered. The data gathered by ISO was sent back to Earth by radio. ISO contained no onboard data storage devices, so the satellite needed to be in direct radio contact with a ground station on Earth to carry out scientific observations. The lifespan of the satellite was determined by the supply of liquid helium on board. When the helium boiled off and escaped into space, the instruments and the telescope itself gradually warmed up and became less and less sensitive to faint infrared sources. At launch time, the expected lifetime of ISO was 18 months, but after analysis of the boil-off rate in space, scientists extended this estimate to 24 months. The satellite actually functioned for 28 months before its telescope became too warm to make useful measurements. ISO's controllers kept the spacecraft switched on for another month after observations ended to make engineering tests and prepare the telescope to be shut down. III THE MISSION ISO was launched on November 17, 1995, from French Guiana on the northeastern coast of South America. An Ariane 44P launch rocket boosted the satellite into a highly elliptical orbit. At closest approach to Earth, known as the perigee, the satellite's altitude was only about 1000 km (about 600 mi). At this altitude the ISO passed through the Van Allen radiation belt, at which time the satellite's infrared detectors were not functional. When at its farthest point from Earth, known as the apogee, ISO was at an altitude of about 70,500 km (about 43,800 mi). The satellite made one orbit around the planet every 24 hours, of which about 16 hours was spent outside the radiation belts. ISO was designed for planetary science, especially studies of atmospheres within our solar system. Planetary atmospheres contain molecules of various gases, such as methane (CH4) and molecular hydrogen (H2), which produce characteristic absorption and emission features in the infrared part of the spectrum. ISO was also particularly useful for analyzing the composition of comets, which are believed to retain some of the original material from which the solar system condensed. ISO provided information about regions that are not visible to telescopes that only use optical wavelengths. Star-forming regions are always shrouded in clouds of gas and dust, called nebulas. The gas is mainly hydrogen and the dust is tiny grains of solid particles such as silicates and carbon. The large expanse of nebulas cause almost total extinction of the visible light from stars within or beyond the cloud. Infrared waves are unaffected as they pass through these nebulas, however, and emission from the dust itself can be seen at the longest infrared wavelengths. All of the ISO instruments were important in studying star formation. Usually the ISOCAM and ISOPHOT were used to detect areas where stars are forming and the spectrometers were used for detailed analysis of the composition of the area. One of the primary ISO targets was a region called the Chameleon Dark Clouds, which is one of the closest regions of current star formation. ISOCAM obtained over 23,000 images of this region and detected 65 young stars, 40 percent of which were not known before. ISO proved useful for studying both very young and very old stars. Clouds of gas and dust often obscure stars at the beginning and end of their lives, but ISO could detect infrared radiation from these objects. Stars begin their life cycle by collapsing under the pull of gravity out of the denser parts of giant interstellar molecular clouds, and they end their lives by expanding into red giant stars and throwing off up to 80 percent of their original mass. This material is cool and enriches the interstellar medium with heavy elements such as oxygen, carbon, and silicon. In both cases, the young and the old source are accompanied by this cool, dusty material. Infrared imaging and spectroscopy penetrate these regions and reveal the composition and structure of the stars. ISO contributed to the search for planets around other stars by looking for the flattened disks of gas and dust out of which planets may form. Astronomers need to know how often these disks occur around stars to help them understand how common it is for planets to form. ISO found several previously unknown stars with disks. ISO also detected olivine, a silicate mineral found in Earth's own rocky mantle, in the comet Hale-Bopp, which was visible from Earth in 1996 and 1997. The discovery of olivine in the comet suggests that the comet and Earth have a similar origin. The satellite also detected the first evidence of water outside of the solar system in planetary nebula NGC 7027, a ring of gas thrown off by a dying star. In October 1997, ISO found that a cloud of interstellar gas near the Orion Nebula produces enough water each day to fill Earth's oceans 60 times. In 1998 scientists announced that ISO had found evidence of water in the atmosphere of Saturn's moon Titan. Theoretical models of the universe and measured gravitational effects show that there must be a large amount of matter in the universe that has not yet been detected (see Dark Matter). If this "missing matter" is the right temperature to radiate at shorter wavelengths, then ISO should have been able to find signs of it. ISOCAM was the primary instrument for these studies. ISOCAM systematically surveyed a section of the Milky Way but was unable to identify any significant component of dark matter. Contributed By: Ian S. McLean Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
