NASA - National Aeronautics and Space Administration

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 (visible, gamma rays, X-rays, and infrared). 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.

Image above: Three of NASA's four Great Observatories are still in service. Credit: NASA

Hubble Space Telescope

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.

Image to right: This image of the Hubble Space Telescope was taken by the Space Shuttle crew during a servicing mission. Credit: NASA

Since its preliminary inception, HST was designed to be a different type of mission for NASA -- a long term space-based observatory. From its position 380 miles above Earth's surface, the Hubble Space Telescope has contributed enormously to astronomy. It has expanded our understanding of star birth, star death, and galaxy evolution, and has helped move black holes from scientific theory to fact. Credited with thousands of images and the subject of thousands of research papers, the space telescope is helping astronomers answer a wide range of intriguing questions about the origin and evolution of the universe.

Compton Gamma Ray Observatory

The Compton Gamma Ray Observatory (CGRO) was the second of NASA's Great Observatories. Compton, at 17 tons, was the heaviest astrophysical payload ever flown at the time of its launch on April 5, 1991, aboard the space shuttle Atlantis. This mission collected data on some of the most violent physical processes in the Universe, characterized by their extremely high energies.

Image to left: The Compton Gamma Ray Observatory was the second of NASA's Great Observatories. Credit: NASA

Compton had four instruments that covered an unprecedented six decades of the electromagnetic spectrum, from 30 keV to 30 GeV. In order of increasing spectral energy coverage, these instruments were the Burst And Transient Source Experiment (BATSE), the Oriented Scintillation Spectrometer Experiment (OSSE), the Imaging Compton Telescope (COMPTEL), and the Energetic Gamma Ray Experiment Telescope (EGRET). For each of the instruments, an improvement in sensitivity of better than a factor of ten was realized over previous missions.

Compton was safely deorbited and re-entered the Earth's atmosphere on June 4, 2000.

Chandra X-ray Observatory

The third member of the Great Observatory family, the Chandra X-Ray Observatory (CXO), was deployed from a Space 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.

Image above: The Chandra X-ray Observatory can observe particles right before they fall into a black hole! Credit: NASA

Chandra detects and images X-ray sources that are billions of light years away. The mirrors on Chandra are the largest, most precisely shaped and aligned, and smoothest mirrors ever constructed. If the surface of Earth was as smooth as the Chandra mirrors, the highest mountain would be less than six feet tall! The images Chandra makes are twenty-five times sharper than the best previous X-ray telescope. This focusing power is equivalent to the ability to read a newspaper at a distance of half a mile. Chandra's improved sensitivity is making possible more detailed studies of black holes, supernovas, and dark matter. Chandra will increase our understanding of the origin, evolution, and destiny of the universe.

Spitzer Space Telescope

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.

Image to right: This artist's rendition shows the Spitzer Space Telescope against an infrared sky. Credit: NASA

The Spitzer Space Telescope was launched into space by a Delta rocket on August 25, 2003. Spitzer obtains images and spectra by detecting the infrared energy, or heat, radiated by objects in space between wavelengths of 3 and 180 microns (1 micron is one-millionth of a meter). Most of this infrared radiation is blocked by the Earth's atmosphere and cannot be observed from the ground.

Consisting of a 0.85-meter telescope and three cryogenically-cooled science instruments, Spitzer is the largest infrared telescope ever launched into space. Its highly sensitive instruments give us a unique view of the Universe and allow us to peer into regions of space which are hidden from optical telescopes. Many areas of space are filled with vast, dense clouds of gas and dust which block our view. Infrared light, however, can penetrate these clouds, allowing us to peer into regions of star formation, the centers of galaxies, and into newly forming planetary systems. Infrared also brings us information about the cooler objects in space, such as smaller stars which are too dim to be detected by their visible light, extrasolar planets, and giant molecular clouds. Also, many molecules in space, including organic molecules, have their unique signatures in the infrared.

For more information about NASA's Great Observatories, visit these sites:

Hubble Space Telescope (HST)
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Compton Gamma Ray Telescope (CGRO)
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Chandra X-ray Observatory (CXRO)
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Spitzer Space Telescope (SST)
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Adapted from HST (pdf),CGRO, CXRO and the SST