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NASA's four great observatories were designed to conduct astronomical studies over many different wavelengths (visible, gamma rays, X-rays, and infrared).

Hubble Space Telescope(HST)

Compton Gamma Ray Observatory(CGRO), the second element, launched into Earth orbit from space shuttle Atlantis in 1991, covered an unprecedented six decades of the electromagnetic spectrum, from 30 keV to 30 GeV. It was re-entered the Earth's atmosphere on June 4, 2000.

Chandra X-ray Observatory(CXO), the third member of the Great Observatory family, launched in July 1999.

Spitzer Space Telescope(SST), the fourth Great Observatory, was launched on August 25, 2003. It obtains images and spectra between wavelengths of 3 and 180 microns. Spitzer fills in an important gap in wavelength coverage not available from the ground -- the thermal infrared.