moderate projects in key areas. Table 4.2 compares the contributions of the major recommended projects to the scientific themes. More detailed descriptions of the instruments and the science that they might accomplish can be found in the Working Papers (NRC, 1991).

THE DECADE OF THE INFRARED

The infrared and submillimeter portion of the spectrum, from 1 µm to 1000 µm, is poorly explored but is of fundamental importance for almost all aspects of astronomy, from solar system studies to cosmology. Four factors make these wavelengths critical: (1) the expansion of the universe shifts the radiation from primeval objects out of the ultraviolet and visible bands into the infrared; (2) most of the known mass in galaxies is in the form of cool stars that are brightest at wavelengths in the range 1 to 10 µm, depending on the distance to the galaxy; (3) the dust associated with cold gas and star-forming regions obscures objects at wavelengths shorter than 1 µm, but glows brightly at longer wavelengths due to the absorbed energy; and (4) atoms and molecules have rich infrared spectra that can be used to probe the density, temperature, and elemental abundances of astronomical objects.

The technology for detecting infrared and submillimeter radiation has been revolutionized in the last 10 years. A decade ago, astronomers used single detectors on ground-based telescopes to observe in a few spectral windows between 1 and 30 µm. Radio astronomers struggled to detect radiation at wavelengths as short as 1,000 µm. Pioneering astronomers used balloons or airborne telescopes to work between 30 and 1000 µm. In 1983 the Infrared Astronomical Satellite (IRAS) demonstrated that a telescope cooled with liquid helium could approach the theoretical sensitivity limit set by the faint light emitted by interplanetary dust grains. The 1,000-fold increase in sensitivity compared with that of earth-bound telescopes permitted IRAS to survey the entire sky at wavelengths from 12 to 100 µm and to discover important new phenomena, including trails of solid material behind comets, disks of solid material orbiting nearby stars —possibly the remnants of planet formation (Plate 2.1 and Plate 4.1)—and luminous galaxies emitting more than 90 percent of their energy in the infrared. At wavelengths between 100 and 1000 µm, new techniques for detecting radiation using the Kuiper Airborne Observatory (KAO) and ground-based telescopes like the Caltech Submillimeter Observatory (CSO) led to the discovery of spectral lines from atoms and molecules that brought new information about planetary atmospheres, star formation, and interstellar chemistry. Most recently, the development of arrays of detectors operating from 1 to 200 µm has led to the replacement of single-channel photometers by cameras and two-dimensional spectrographs with 50,000 or more individual detectors.

The large initiative accorded the highest priority by this committee is



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