NASA's Great Observatories
To grasp the wonders of the cosmos, and understand its infinite variety and splendor, we must collect and analyze radiation emitted by phenomena throughout the entire electromagnetic (EM) spectrum. Towards that end, NASA proposed the concept of Great Observatories, a series of four space-borne observatories designed to conduct astronomical studies over many different wavelengths. An important aspect of the Great Observatory program was to overlap the operations phases of the missions to enable astronomers to make contemporaneous observations of an object at different spectral wavelengths.
The first element of the program -- and arguably the best known -- is the Hubble Space Telescope (HST). The Hubble telescope was deployed by a NASA Space Shuttle in 1990. A subsequent Shuttle mission in 1993 serviced HST and recovered its full capability. A second successful servicing mission took place in 1997. Subsequent servicing missions have added additional capabilities to HST, which observes the Universe at ultraviolet, visual, and near-infrared wavelengths.
The second Great Observatory was launched and deployed by a Shuttle in 1991: the Compton Gamma-Ray Observatory (CGRO). This mission continues to collect data on some of the most violent physical processes in the Universe, characterized by their extremely high energies.
The third member of the Great Observatory family, the Chandra X-Ray Observatory(CXO), was deployed from a Shuttle and boosted into a high-Earth orbit, in July 1999. This observatory is observing such objects as black holes, quasars, and high-temperature gases throughout the x-ray portion of the EM spectrum.
The Spitzer Space Telescope represents the fourth and final element in NASA's Great Observatory program. Spitzer fills in an important gap in wavelength coverage not available from the ground -- the thermal infrared.