The Infrared Astronomy Satellite (IRAS), launched on Jan. 25, 1983, was the first satellite built to survey the sky at infrared
wavelengths (see infrared astronomy). The main instrument in the satellite was a 60-cm (24-in) Ritchey-Chretien Cassegrain telescope with a focal length of 5.5 m (220 in). In its focus were placed infrared detectors operating at wavelengths of 100, 60, 24, and 12 microns. In order to improve the signal-to-noise ratio, the detectors and the telescope were enclosed by a tank containing superfluid helium at a temperature of 2 K. The satellite was built by the Netherlands, and construction of the telescope was managed by the Jet Propulsion Laboratory in the United States. Among the infrared objects studied were sites of star formation and the center of the Galaxy.The operating time of the telescope was limited because of slow venting of liquid helium from the tank, and IRAS shut down on Nov. 21, 1983. A replacement for the satellite was launched in November 1995, when the European Space Agency sent the Infrared Space Observatory (ISO) on a nearly two-year mission. As with IRAS, its instruments are maintained at cryogenic temperatures.