NASA’s series of Great Observatories satellites are four large, powerful space-based astronomical telescopes. Each of the four missions was designed to examine a specific wavelength/energy region of the electromagnetic spectrum (gamma rays, X-rays, visible and ultraviolet light, infrared light) using very different technologies.
The four Great Observatories were launched between 1990 and 2003.
1 . The Hubble Space Telescope (HST) primarily observes visible light and near-ultraviolet. It was launched in 1990 aboard Discovery during STS-31. A servicing mission in 1997 added capability in the near-infrared range.
2 . The Compton Gamma Ray Observatory (CGRO) primarily observed gamma rays, though it extended into hard x-rays as well. It was launched in 1991 aboard Atlantis during STS-37 and was de-orbited in 2000 after a gyroscope failed.
3 . The Chandra X-ray Observatory (CXO) primarily observes soft x-rays. It was launched in 1999 aboard Columbia during STS-93 into an elliptical high-earth orbit, and was initially named the Advanced X-ray Astronomical Facility (AXAF).
4 . The Spitzer Space Telescope (SST) observes the infrared spectrum. It was launched in 2003 aboard a Delta II rocket into an earth-trailing solar orbit; it was called the Space Infrared Telescope Facility (SIRTF) before launch. Depletion of its liquid helium coolant in 2009 reduced its functionality, leaving it with only two short-wavelength imaging modules. It was removed from service and placed into safe-mode on January 30, 2020.
The Hubble Space Telescope and Chandra X-ray Observatory continue to operate as of April 2022.
Significance of Great Observatories program
All four telescopes have had a substantial impact on astronomy.
The opening up of new wavebands to high resolution, high sensitivity observations by the Compton, Chandra and Spitzer has revolutionized our understanding of a wide range of astronomical objects, and has led to the detection of thousands of new, interesting objects.
Hubble has had a much larger public and media impact than the other telescopes, although at optical wavelengths Hubble has provided a more modest improvement in sensitivity and resolution over existing instruments.
Hubble’s capability for uniform high-quality imaging of any astronomical object at any time has allowed accurate surveys and comparisons of large numbers of astronomical objects.
The Hubble Deep Field observations have been very important for studies of distant galaxies, as they provide rest-frame ultraviolet images of these objects with a similar number of pixels across the galaxies as previous ultraviolet images of closer galaxies, allowing direct comparison.
