Infrared Astronomy

Throughout most of history astronomers were confined to using optical light, the light we can detect with our eyes. The advent of electronic detectors has, in the past few decades, opened up new vistas to astronomers, allowing them to utilize the entire electromagnetic spectrum. Infrared astronomers use traditional optical telescopes equipped with special detectors that can detect infrared light. Earth's atmosphere is, for the most part, only mildly transparent to infrared light, so infrared astronomers work from high, dry mountain tops, airplanes, high altitude balloons, or space. The infrared spectral window allows astronomers to probe dusty regions of the universe that obscure optical light.

Ground-based infrared astronomy

Infrared light is heavily absorbed by both carbon dioxide and water vapor, major components of Earth's atmosphere. Accordingly, the atmosphere is opaque to many infrared wavelengths. There are a few specific wavelength bands between one and five micrometers, around 10 micrometers, and sometimes near 20 micrometers at which the atmosphere is partially transparent. These bands make up the standard ground based infrared bands. Still, astronomers must build infrared observatories at very dry, high-altitude sites to get above as much atmosphere as possible. One of the best infrared sites in the world is the 14,000-ft (4,200-m) summit of Mauna Kea in Hawaii. On a clear night half a dozen large telescopes may probe the infrared sky, although some of the telescopes are used for optical astronomy. The high altitude at Mauna Kea makes observation at its summit very rigorous.

There are special difficulties to infrared astronomy, especially from the ground. The heat radiation from the telescope, telescope building, and atmosphere are all very bright in the infrared. They combine into an infrared background that is at least a million times brighter than strong astronomical infrared sources. To account for this strong background astronomers rapidly oscillate the telescope field of view from the star to a region of sky nearby. Taking the difference of the two intensities allows astronomers to subtract the background.

Airborne and space infrared astronomy

To conduct experiments in infrared astronomy at wavelengths other than those observable from the ground, astronomers must place their telescopes above the atmosphere. Options include mounting telescopes on high-altitude balloons, airplanes, rockets, or satellites. High-altitude balloons are less expensive than the other options, but astronomers cannot ride with the telescope and have little control over the flight path of the balloon. Today aircraft are more frequently used. Since 1974, NASA has operated the Kuiper Airborne Observatory (KAO), which is a 36 in (91 cm) infrared telescope in a military cargo plane. It flies at high altitudes in a controlled path with the astronomers along to operate the telescope. Astronomers can make observations at far-infrared wavelengths with more control than from a balloon. Beginning in 2001, NASA is replacing the KAO with the Stratospheric Observatory for Infrared Astronomy (SOFIA), a 100 in (254 cm) telescope that will be flown on a 747.

To record long-term images from space, astronomers must place infrared telescopes on orbiting satellites. Such experiments are quite expensive, but allow astronomers to record a large number of observations. Infrared observatories in space have a more limited lifetime than other space observatories because they run out of liquid helium. Space is cold, but not cold enough for infrared detectors, so they must still be cooled with liquid helium, which evaporates after a year or two. Astronomers must carefully plan their observations to get the most out of the limited lifetime.

In the early 1980s the Infrared Astronomical Satellite (IRAS) surveyed the entire sky at four infrared wavelengths not accessible from the ground (12, 25, 60, and 100 micrometers). The helium ran out in 1983 after a successful mission. Astronomers are still mining the vast amounts of data accumulated from that experiment. The satellite charted the positions of 15,000 galaxies, allowing a sky survey team to produce a three-dimensional map that covers a sphere with a radius of 700 million lightyears. Of particular interest to astronomers is the presence of massive superclusters, consisting of formed of galactic clusters containing dozens to thousands of galaxies like our own. Between these superclusters lie vast voids that are nearly galaxy-free, provoking great interest from scientists.

In 1995, the European Space Agency launched the Infrared Space Observatory (ISO), an astronomical satellite that operated at wavelengths from 2.5 to 240 micrometers. ISO allowed astronomers to study comet Hale-Bopp in detail. The satellite discovered protostars, planet-forming nebula around dying stars, and water throughout the universe, including in star-forming regions and in the atmospheres of the gas giants like Saturn and Uranus. The telescope was live until 1998, when it ran out of liquid helium.

Future infrared satellites planned include the NASA's Space Infrared Telescope Facility (SIRTF), slated for launch in late 2001.