NASA's series ofGreat Observatoriessatellites are four large, powerfulspace-based astronomical telescopes launched between 1990 and 2003. They were built with different technology to examine specific wavelength/energy regions of theelectromagnetic spectrum:gamma rays,X-rays,visible andultraviolet light, andinfrared light.
TheHubble Space Telescope (HST) primarily observesvisible light andnear-ultraviolet. It was launched in 1990 aboard theSpace ShuttleDiscovery duringSTS-31, but its main mirror had been ground incorrectly, resulting inspherical aberration that compromised the telescope's capabilities. The optics were corrected to their intended quality by theSTS-61 servicing mission in 1993. In 1997, theSTS-82 servicing mission added capability in thenear-infrared range, and in 2009 theSTS-125 servicing mission refurbished the telescope and extended its projected service life. It remains in active operation as of October 2024[update].
TheCompton Gamma Ray Observatory (CGRO) primarily observedgamma rays, though it extended intohard x-rays as well. It was launched in 1991 aboardAtlantis duringSTS-37. It wasde-orbited in 2000 after a gyroscope failed.
TheChandra X-ray Observatory (CXO) primarily observessoft X-rays. It was launched in 1999 aboardColumbia duringSTS-93 into an elliptical high-Earth orbit, and was initially named the Advanced X-ray Astronomical Facility (AXAF). It remains in active operation as of October 2024[update].
TheSpitzer Space Telescope (SST) observed theinfrared spectrum. It was launched in 2003 aboard aDelta II rocket into an Earth-trailing solar orbit. Depletion of itsliquid 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 concept of a Great Observatory program was first proposed in the 1979NRC report "A Strategy for Space Astronomy and Astrophysics for the 1980s".[1] This report laid the essential groundwork for the Great Observatories and was chaired byPeter Meyer (through June 1977) and then byHarlan J. Smith (through publication). In the mid-1980s, it was further advanced by all of the astrophysics Division Directors atNASA headquarters, including Frank Martin and Charlie Pellerin. NASA's "Great Observatories" program used four separate satellites, each designed to cover a different part of the spectrum in ways which terrestrial systems could not. This perspective enabled the proposed X-ray and InfraRed observatories to be appropriately seen as a continuation of the astronomical program begun with Hubble and CGRO rather than competitors or replacements.[2][3] Two explanatory documents published by NASA and created for the NASA Astrophysics Division and the NASA Astrophysics Management Working Group laid out the rationale for the suite of observatories and questions that could be addressed across the spectrum.[4][5] They had an important role in the campaign to win and sustain approval for the four telescopes.[citation needed]
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The history of the Hubble Space Telescope can be traced back to 1946, when theastronomerLyman Spitzer wrote the paperAstronomical advantages of an extraterrestrial observatory.[6] Spitzer devoted much of his career to pushing for a space telescope.
The 1966–1972Orbiting Astronomical Observatory missions demonstrated the important role space-based observations could play in astronomy. In 1968, NASA developed firm plans for a space-basedreflecting telescope with a 3-meter mirror, known provisionally as the Large Orbiting Telescope or Large Space Telescope (LST), with a launch slated for 1979.[7] Congress eventually approved funding of US$36 million for 1978, and the design of the LST began in earnest, aiming for a launch date of 1983. During the early 1980s, the telescope was named afterEdwin Hubble.
Hubble was originally intended to be retrieved and returned toEarth by theSpace Shuttle, but the retrieval plan was later abandoned. On 31 October 2006,NASA AdministratorMichael D. Griffin gave the go-ahead for a final refurbishment mission. The 11-daySTS-125 mission bySpace ShuttleAtlantis, launched on 11 May 2009,[8] installed fresh batteries, replaced all gyroscopes, replaced a command computer, fixed several instruments, and installed theWide Field Camera 3 and theCosmic Origins Spectrograph.[9]
Gamma rays had been examined above the atmosphere by several early space missions. During itsHigh Energy Astronomy Observatory Program in 1977, NASA announced plans to build a "great observatory" forgamma-ray astronomy. The Gamma Ray Observatory (GRO), renamedCompton Gamma-Ray Observatory (CGRO), was designed to take advantage of the major advances in detector technology during the 1980s. Following 14 years of effort, the CGRO was launched on 5 April 1991.[10] One of the threegyroscopes on the Compton Gamma Ray Observatory failed in December 1999. Although the observatory was fully functional with two gyroscopes, NASA judged that failure of a second gyroscope would result in inability to control the satellite during its eventual return to Earth due to orbital decay. NASA chose instead to preemptively de-orbit Compton on 4 June 2000.[11] Parts that survived reentry splashed into thePacific Ocean.
In 1976 theChandra X-ray Observatory (called AXAF at the time) was proposed to NASA byRiccardo Giacconi andHarvey Tananbaum. Preliminary work began the following year atMarshall Space Flight Center (MSFC) and theSmithsonian Astrophysical Observatory (SAO). In the meantime, in 1978, NASA launched the first imaging X-ray telescope,Einstein Observatory (HEAO-2), into orbit. Work continued on the Chandra project through the 1980s and 1990s. In 1992, to reduce costs, the spacecraft was redesigned. Four of the twelve planned mirrors were eliminated, as were two of the six scientific instruments. Chandra's planned orbit was changed to an elliptical one, reaching one third of the way to the Moon's at its farthest point. This eliminated the possibility of improvement or repair by theSpace Shuttle but put the observatory above the Earth'sradiation belts for most of its orbit.
By the early 1970s, astronomers began to consider the possibility of placing an infrared telescope above the obscuring effects ofatmosphere of Earth. Most of the early concepts, envisioned repeated flights aboard the NASA Space Shuttle. This approach was developed in an era when the Shuttle program was presumed to be capable of supporting weekly flights of up to 30 days duration. In 1979, a National Research Council of theNational Academy of Sciences report,A Strategy for Space Astronomy and Astrophysics for the 1980s, identified aShuttle Infrared Telescope Facility (SIRTF) as "one of two major astrophysics facilities [to be developed] forSpacelab," a Shuttle-borne platform.
The launch of theInfrared Astronomical Satellite, an Explorer-class satellite designed to conduct the firstinfrared survey of the sky led to anticipation of an instrument using new infrared detector technology. By September 1983, NASA was considering the "possibility of a long duration [free-flyer] SIRTF mission". The 1985Spacelab-2 flight aboardSTS-51-F confirmed the Shuttle environment was not well suited to an onboard infrared telescope, and a free-flying design was better. The first word of the name was changed fromShuttle so it would be called the Space Infrared Telescope Facility.[12][13]
Spitzer was the only one of the Great Observatories not launched by the Space Shuttle. It was originally intended to be so launched, but after theChallenger disaster, theCentaurLH2/LOXupper stage that would have been required to push it into aheliocentric orbit was banned from Shuttle use.Titan andAtlas launch vehicles were canceled for cost reasons. After redesign and lightening, it was launched in 2003 by aDelta IIlaunch vehicle instead. It was called theSpace Infrared Telescope Facility (SIRTF) before launch. The telescope was deactivated when operations ended on 30 January 2020.
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Since the Earth's atmosphere preventsX-rays,gamma-rays[14] andfar-infraredradiation from reaching the ground, space missions were essential for the Compton, Chandra and Spitzer observatories. Hubble also benefits from being above the atmosphere, as the atmosphere blurs ground-based observations of very faint objects, decreasing spatial resolution (however brighter objects can be imaged in much higher resolution than by Hubble from the ground usingastronomical interferometers oradaptive optics). Larger, ground-based telescopes have only recently matched Hubble in resolution for near-infrared wavelengths of faint objects. Being above the atmosphere eliminates the problem ofairglow, allowing Hubble to make observations of ultrafaint objects. Ground-based telescopes cannot compensate for airglow on ultrafaint objects, and so very faint objects require unwieldy and inefficient exposure times. Hubble can also observe atultraviolet wavelengths which do not penetrate the atmosphere.
Each observatory was designed to push the state of technology in its region of the electromagnetic spectrum. Compton was much larger than any gamma-ray instruments flown on the previousHEAO missions, opening entirely new areas of observation. It had four instruments covering the 20keV to 30GeV energy range, which complemented each other's sensitivities, resolutions, and fields of view. Gamma rays are emitted by various high-energy and high-temperature sources, such asblack holes,pulsars, andsupernovae.
Chandra similarly had no ground predecessors. It followed the three NASAHEAO Program satellites, notably the highly successfulEinstein Observatory, which was the first to demonstrate the power ofgrazing-incidence, focusing X-ray optics, giving spatial resolution an order of magnitude better thancollimated instruments (comparable to optical telescopes), with an enormous improvement in sensitivity. Chandra's large size, high orbit, and sensitiveCCDs allowed observations of very faint X-ray sources.
Spitzer also observes at wavelength largely inaccessible to ground telescopes. It was preceded in space by NASA's smallerIRAS mission andEuropean Space Agency (ESA)'s largeISO telescope. Spitzer's instruments took advantage of the rapid advances in infrared detector technology since IRAS, combined with its large aperture, favorable fields of view, and long life. Science returns were accordingly outstanding.[citation needed] Infrared observations are necessary for very distant astronomical objects where all the visible light isredshifted to infrared wavelengths, for cool objects which emit little visible light, and for regions optically obscured by dust.
Aside from inherent mission capabilities (particularly sensitivities, which cannot be replicated by ground observatories), the Great Observatories program allows missions to interact for greater science return. Different objects shine in different wavelengths, but training two or more observatories on an object allows a deeper understanding.
High-energy studies (in X-rays and gamma rays) have had only moderate imaging resolutions so far. Studying X-ray and gamma-ray objects with Hubble, as well as Chandra and Compton, gives accurate size and positional data. In particular, Hubble's resolution can often discern whether the target is a standalone object, or part of a parent galaxy, and if a bright object is in the nucleus, arms, or halo of aspiral galaxy. Similarly, the smaller aperture of Spitzer means that Hubble can add finer spatial information to a Spitzer image. Reported in March 2016, Spitzer and Hubble were used to discover the most distant-known galaxy,GN-z11. This object was seen as it appeared 13.4 billion years ago.[15][16] (List of the most distant astronomical objects)
Ultraviolet studies with Hubble also reveal the temporal states of high-energy objects. X-rays and gamma rays are harder to detect with current technologies than visible and ultraviolet. Therefore, Chandra and Compton needed long integration times to gather enough photons. However, objects which shine in X-rays and gamma rays can be small, and can vary on timescales of minutes or seconds. Such objects then call for followup with Hubble or theRossi X-ray Timing Explorer, which can measure details in angular seconds or fractions of a second, due to different designs. Rossi's last full year of operation was 2011.
The ability of Spitzer to see through dust and thick gases is good for galactic nuclei observations. Massive objects at the hearts of galaxies shine in X-rays, gamma rays, and radio waves, but infrared studies into these clouded regions can reveal the number and positions of objects.
Hubble, meanwhile, has neither thefield of view nor the available time to study all interesting objects. Worthwhile targets are often found with ground telescopes, which are cheaper, or with smaller space observatories, which are sometimes expressly designed to cover large areas of the sky. Also, the other three Great Observatories have found interesting new objects, which merit diversion of Hubble.
One example of observatory synergy isSolar System andasteroid studies. Small bodies, such as smallmoons and asteroids, are too small and/or distant to be directly resolved even by Hubble; their image appears as adiffraction pattern determined by brightness, not size. However, the minimum size can be deduced by Hubble through knowledge of the body'salbedo. The maximum size can be determined by Spitzer through knowledge of the body's temperature, which is largely known from its orbit. Thus, the body's true size is bracketed. Furtherspectroscopy by Spitzer can determine the chemical composition of the object's surface, which limits its possible albedos, and therefore sharpens the low size estimate.
At the opposite end of thecosmic distance ladder, observations made with Hubble, Spitzer and Chandra have been combined in theGreat Observatories Origins Deep Survey to yield a multi-wavelength picture ofgalaxy formation and evolution in the earlyUniverse.
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. TheHubble 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.
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In 2016,NASA began considering four differentFlagshipspace telescopes,[21] they are theHabitable Exoplanet Imaging Mission (HabEx),Large UV Optical Infrared Surveyor (LUVOIR),Origins Space Telescope (OST), andLynx X-ray Observatory. In 2019, the four teams will turn their final reports over to theNational Academy of Sciences, whose independentDecadal Survey committee advises NASA on which mission should take top priority.[21]
NASA announced theHabitable Worlds Observatory (HWO) in 2023, a successor building on theLarge UV Optical Infrared Surveyor (LUVOIR) andHabitable Exoplanet Imaging Mission (HabEX) proposals.[22] The administration also created theGreat Observatory Maturation Program for the development of theHabitable Worlds Observatory.[23]
This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain.A detailed description of NASA's Great Observatories, including STS-93 primary payload, the Chandra X-ray Observatory
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