A high-performance infrared telescope that is one of the four Great Observatories launched by the National Aeronautics and Space Administration (NASA) between 1990 and 2003. It takes advantage of dramatic advances in infrared detectors that have occurred over the last 20 years; it utilizes modern detector arrays in space, where they are limited only by the faint glow of the zodiacal dust cloud. Ground-based infrared telescopes can operate only at the wavelengths where the atmosphere is transparent, lying between 1 and 25 micrometers. Even within these windows, the thermal emission of the atmosphere is more than a million times greater than the dilute emission of the zodiacal cloud; there is additional foreground thermal emission from the telescope itself. High-sensitivity detectors are blinded by these bright foreground signals. Operating in space eliminates the atmospheric absorption and emission; also, a telescope in the vacuum of space can be cooled sufficiently to virtually eliminate its emission.