Great Observatories program
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[[NASA]]'s series of '''Great Observatories''' [[satellite]]s are four large, powerful space-based [[telescope]]s. Each of the Great Observatories has had a similar size and cost at program outset, and each has made a substantial contribution to [[astronomy]]. The four missions each examined a region of the [[electromagnetic spectrum]] to which it was particularly suited.<ref> {{cite web|url=http://www-istp.gsfc.nasa.gov/stargaze/Sun6new.htm |title=(S-6) Seeing the Sun in a New Light |accessdate=2007-12-07 |last=Stern |first=David P. |date=2004-12-12 |work=From Stargazers to Starships |publisher=NASA Goddard Space Flight Center }}</ref>
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== Great Observatories ==
* The [[Hubble Space Telescope]] (HST) primarily observes [[optical spectrum|visible light]] and [[Near ultraviolet|near-ultraviolet]]. A 1997 servicing mission added capability in the [[Near infrared|near-infrared]] range. It was launched in 1990 aboard the [[Space Shuttle Discovery]] during mission [[STS-31]].
* The [[Compton Gamma Ray Observatory]] (CGRO) primarily observed [[gamma ray]]s, though it extended into [[hard x-ray]]s as well. It was launched in 1991 aboard the [[Space Shuttle Atlantis]] during [[STS-37]]. It was deorbited in 2000 after failure of a gyroscope.
* The [[Chandra X-ray Observatory]] (CXO) was initially named the Advanced X-ray Astronomical Facility (AXAF). It primarily observes [[soft x-ray]]s. It was launched in 1999 aboard the [[Space Shuttle Columbia]] during [[STS-93]].
* The [[Spitzer space telescope|Spitzer Space Telescope]] (SST) was called the Space Infrared Telescope Facility (SIRTF) before launch. It observes the [[infrared]] spectrum, and was launched in 2003 aboard a Delta II rocket.
Of these satellites, only the Compton is not operating; one of its [[gyroscope]]s failed, and NASA ordered it to be de-orbited on [[June 4]], [[2000]]. Parts which survived reentry splashed into the Pacific Ocean. Hubble was originally intended to be retrieved and returned to Earth by the [[Space Shuttle]], but retrieval plan was later abandoned. On [[October 31]] [[2006]] NASA Administrator [[Mike Griffin]] gave the go-ahead for a final refurbishment mission. The 11-day [[STS-125]] mission by ''[[Space Shuttle Atlantis|Atlantis]]'', scheduled for launch on 8 October 2008,<ref>{{cite web|url=http://www.nasa.gov/home/hqnews/2008/may/HQ_08133_Shuttle_target_dates.html |title=NASA Updates Space Shuttle Target Launch Dates |publisher=NASA |accessdate=2008-05-22}}</ref> will install fresh batteries, replace all gyroscopes, and install [[Wide Field Camera 3]] and the [[Cosmic Origins Spectrograph]].<ref name="Green Light">{{cite news |url=http://www.msnbc.msn.com/id/15489217/ |title=NASA gives green light to Hubble rescue |first=Alan |last=Boyle |publisher=MSNBC |date=2006-10-31 |accessdate=2007-01-10}}</ref>
Spitzer was the only one of the Great Observatories not launched by the Space Shuttle. It was originally intended to be, but after the [[STS-51L|Challenger disaster]], the [[Centaur (rocket stage)|Centaur]] [[LH2]]/[[LOX]] upper stage that would have been required to push it into a [[heliocentric orbit]] was banned from Shuttle use. Titan and Atlas rockets were cancelled for cost reasons. After redesign and lightening, it was launched by a [[Delta II rocket]] instead.
== History of the program ==
=== Optical telescope program ===
The history of the Hubble Space Telescope can be traced back as far as 1946, when the [[astronomer]] [[Lyman Spitzer]] wrote the paper ''Astronomical advantages of an extraterrestrial observatory''<ref>Spitzer, L., ''REPORT TO PROJECT RAND: Astronomical Advantages of an Extra-Terrestrial Observatory'', reprinted in Astr. Quarterly, volume 7, p. 131, 1990.</ref>. Spitzer devoted much of his career to pushing for a space telescope to be developed.
The 1966-72 [[Orbiting Astronomical Observatory]] missions demonstrated the important role space-based observations could play in astronomy, and 1968 saw the development by NASA of firm plans for a space-based [[reflecting telescope]] with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope (LST), with a launch slated for 1979.<ref>Spitzer, Lyman S (1979), "History of the Space Telescope", ''Quarterly Journal of the Royal Astronomical Society'', v. 20, p. 29</ref> Congress eventually approved funding of US$36,000,000 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 after [[Edwin Hubble]].
=== Gamma ray program ===
Gamma rays had been examined above the atmosphere by several early space missions. During its [[High Energy Astronomy Observatory Program]] in 1977, NASA announced plans to build a "great observatory" for gamma-ray astronomy. The Gamma Ray Observatory (GRO), renamed [[Compton 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]].<ref> {{cite web|url=http://heasarc.gsfc.nasa.gov/docs/cgro/epo/brochures/compton/bro5.html |title=Gamma-Ray Astronomy in the Compton Era: The Instruments |accessdate=2007-12-07 |work=Gamma-Ray Astronomy in the Compton Era |publisher=NASA/ GSFC }}</ref>
=== Chandra history ===
In [[1976]] the Chandra X-ray Observatory (called AXAF at the time) was proposed to NASA by [[Riccardo Giacconi]] and [[Harvey Tananbaum]]. Preliminary work began the following year at [[Marshall Space Flight Center]] (MSFC) and the [[Smithsonian Astrophysical Observatory]] (SAO). In the meantime, in [[1978]], NASA launched the first imaging X-ray telescope, [[Einstein Observatory|Einstein]] (HEAO-2), into orbit. Work continued on the Chandra project through the 1980s and 1990's. 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 the [[Space Shuttle program|space shuttle]] but put the observatory above the Earth's [[Van Allen radiation belt|radiation belts]] for most of its orbit.
=== Spitzer history ===
By the early 1970s, astronomers began to consider the possibility of placing an infrared telescope above the obscuring effects of Earth's atmosphere. 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 the [[United States National Academy of Sciences|National Academy of Sciences]] report, ''A Strategy for Space Astronomy and Astrophysics for the 1980s'', identified a '''Shuttle Infrared Telescope Facility''' (SIRTF) as "one of two major astrophysics facilities [to be developed] for [[Spacelab]]," a Shuttle-borne platform.
The launch of the [[Infrared Astronomical Satellite]], an Explorer-class satellite designed to conduct the first infrared 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 1985 Spacelab-2 flight aboard [[STS-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 from ''Shuttle'' so it would be called the '''''Space''''' '''Infrared Telescope Facility'''.<ref> {{cite web|url=http://www.nasa.gov/mission_pages/spitzer/infrared/index.html |title=Studying the Universe in Infrared |accessdate=2007-12-08 |last=Watanabe |first=Susan |date=2007-11-22 |publisher=[[NASA]] }}</ref><ref>{{cite web
| last =Kwok
| first =Johnny
| title =Finding a Way: The Spitzer Space Telescope Story
| work =Academy Sharing Knowledge
| publisher =[[NASA]]
| date =Fall 2006
| url =http://appel.nasa.gov/ask/issues/25/25s_finding.php
| accessdate =2007-12-09 }}</ref>
=== Great Observatory origin ===
The concept of a Great Observatory program was developed in the mid-1980s by Charles Pellerin, NASA's Director of Astrophysics, working with Astronomer Martin Harwitt. [[Harlan James Smith]], the chairperson of the NASA Space Science Board, also participated in defining the program.<ref>{{cite web | title =Harlan James Smith, 1924-1991 | date =1994-08-12 | url =http://quasar.as.utexas.edu/BillInfo/HJSmithObits.html | accessdate =2007-12-09 }}</ref> Harwit pointed out in 1981 that many discoveries in astronomy were due to improved reception of the electromagnetic spectrum and recommended that NASA extend coverage of the electromagnetic spectrum. 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.<ref> {{cite web|url=http://www-istp.gsfc.nasa.gov/stargaze/Sun6new.htm |title=(S-6) Seeing the Sun in a New Light |accessdate=2007-12-07 |last=Stern |first=David P. |date=2004-12-12 |work=From Stargazers to Starships |publisher=NASA Goddard Space Flight Center }}</ref><ref>{{cite web | last =Roman | first =Nancy Grace | title =Exploring the Universe: Space-Based Astronomy and Astrophysics | work =Exploring the Cosmos | publisher =NASA | date =2001 | url =http://science.hq.nasa.gov/missions/docs/Chap3-essay.PDF | format =PDF | accessdate =2007-12-08 }}</ref>
== Strengths ==
Each observatory was designed to push the state of technology in its intended wavelength region. As [[x-ray astronomy|x-rays]], [[gamma-ray astronomy|gamma-rays]] and [[far infrared astronomy|far-infrared]] radiation do not pass through the Earth's atmosphere, 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 Hubble from the ground using [[astronomical interferometer]]s). 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 of [[airglow]], 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 at [[ultraviolet]] wavelengths which do not penetrate the atmosphere.
Compton observed in gamma rays, which do not penetrate the lower atmosphere. It was much larger than any gamma-ray instruments flown on the previous [[HEAO Program|HEAO]] missions, opening entirely new areas of observation. It had four instruments covering the 20 [[keV]] to 30 [[GeV]] 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 as [[black hole]]s, [[pulsar]]s, and [[supernovae]].
Chandra similarly had no ground predecessors. It followed the three NASA [[HEAO Program]] satellites, notably the highly successful [[HEAO 3|Einstein Observatory]], which was the first to demonstrate the power of [[Wolter telescope|grazing-incidence, focusing X-ray optics]], giving spatial resolution an order of magnitude better than [[collimator|collimated]] instruments (comparable to optical telescopes), with an enormous improvement in sensitivity. Chandra's large size, high orbit, and sensitive [[Charge-coupled device|CCD]]s 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 smaller [[IRAS]] mission and [[European Space Agency|ESA]]'s large [[Infrared Space Observatory|ISO]] 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 have been accordingly outstanding. Infrared observations are necessary for very distant astronomical objects where all the visible light is [[redshift]]ed to infrared wavelengths, for cool objects which emit little visible light, and for regions optically obscured by dust.
== Impact ==
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. The [[James Webb Space Telescope]] will provide an even greater step forward, providing rest-frame visible light images of even more distant galaxies which can be directly compared with images of nearby galaxies at visible light wavelengths.
== Synergies ==
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 a [[spiral galaxy]]. Similarly, the smaller aperture of Spitzer means that Hubble can add finer spatial information to a Spitzer image.
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 the [[Rossi X-ray Timing Explorer]], which can measure details in angular seconds or fractions of a second, due to different designs.
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 the [[field 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 is solar system and asteroid studies. Small bodies, such as small [[Natural satellite|moons]] and [[asteroid]]s, are too small and/or distant to be directly resolved even by Hubble; their image appears as a [[diffraction]] pattern determined by brightness, not size. However, the minimum size can be deduced by Hubble through knowledge of the body's [[albedo]]. 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. Further [[spectroscopy]] 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 the [[cosmic distance ladder]], observations made with Hubble, Spitzer and Chandra have been combined in the [[Great Observatories Origins Deep Survey]] to yield a multi-wavelength picture of [[galaxy formation and evolution]] in the early Universe.
* Late 1991: Operation of both Hubble and Compton
* Late 1999: Operation of Hubble, Compton, and Chandra
* Mid 2000: Operation of both Hubble and Chandra
* Late 2003-2008 (projected): Operation of Hubble, Chandra, and Spitzer
* Beyond 2008: Final servicing of Hubble in 2008, followed by operation beyond projected lifetime of other two telescopes.
== Successors to GO instruments ==
* [[James Webb Space Telescope]] (JWST) - the JWST, previously known as the NGST (Next Generation Space Telescope) is projected to replace Hubble (HST) around 2013. Its segmented, deployable mirror will be over twice as large, increasing angular resolution noticeably, and sensitivity dramatically. Unlike Hubble, JWST will observe in the infrared, in order to penetrate dust at cosmological distances. This means it will continue some Spitzer capabilities, while some Hubble capabilities will be lost. New advances in ground telescopes will take over some visible observations, but fewer in ultraviolet.
* [[Gamma-ray Large Area Space Telescope|GLAST]], the Gamma Ray Large Area Space Telescope, is a follow-on to Compton scheduled for launch on [[2008-06-11]].<ref>{{cite web |url=http://www.nasa.gov/missions/highlights/schedule.html |title=NASA's Shuttle and Rocket Missions - Launch Schedule |publisher=NASA |date=2008-06-05}}</ref> GLAST will be more narrowly defined, and much smaller; it will carry only one main instrument and a secondary experiment. Other missions, such as [[HETE]]-2, launched in 2000, and [[Swift Gamma-Ray Burst Mission|Swift]], launched in 2004, will complement GLAST. The Ramaty High-Energy Solar Spectroscopic Imager ([[RHESSI]]), launched in 2002, observes in some Compton and Chandra wavelengths, but is pointed at the Sun at all times. Occasionally it observes high-energy objects which happen to be in the view around the Sun.
* Another large, high-energy observatory is [[INTEGRAL]], Europe's INTErnational Gamma Ray Astrophysics Laboratory, launched in 2002. It observes in similar frequencies to Compton. But INTEGRAL uses a fundamentally different telescope technology, coded-aperture masks. Thus, its capabilities are complementary to Compton and GLAST, not a direct replacement.
* Spitzer has no direct successor planned. However, JWST will exceed its performance in near-infrared, and the [[ESA|European Space Agency's]] [[Herschel Space Observatory]] will exceed it in the far-infrared when launched around October 2008. The SOFIA ([[Stratospheric Observatory for Infrared Astronomy|Stratospheric Observatory For Infrared Astronomy]]) airborne platform will observe in near- and mid-infrared. SOFIA will have a larger aperture than Spitzer, but at lower relative sensitivities in restricted duty cycles. Also, smaller space missions will perform specialized infrared observations.
* [[Constellation-X]] - A mission to perform extremely sensitive x-ray observations, beginning around 2016. This is not a direct replacement for Chandra; Chandra is optimized for high angular resolution. Constellation-X is more of a follow-on to the [[XMM-Newton]] mission, which trades resolution for sensitivity. Constellation-X may be several times to several dozen times more sensitive than Chandra. It will also extend further into the hard x-ray regions, giving it some abilities of Compton.
Note that none of these missions are designed for Shuttle launch, or manned servicing. Most are in orbits beyond the Shuttle's capability, to allow new observing modes.
== Later programs ==
* The [[Beyond Einstein program]] will seek to develop new areas of science. [[Constellation-X]] and the [[Laser Interferometer Space Antenna]] (LISA) have been referred to by NASA as the [[Einstein Great Observatories]], to differentiate them from the current generation. However, they are not a part of the Great Observatories program. <ref>{{cite web
| title =Great Observatories
| work =Beyond Einstein
| publisher =NASA
| url =http://universe.nasa.gov/program/observatories.html
| accessdate =2007-11-28 }}</ref>
* The [[International Solar-Terrestrial Physics Science Initiative]], in the spirit of the Great Observatories program, is a group of instruments to study the Sun and related electromagnetic phenomena near Earth.<ref> {{cite web|url=http://pwg.gsfc.nasa.gov/istp/admin/proposal/1-1.html |title=The GGS Program |accessdate=2007-12-03 |last= Acu�a |first=Mario H. |coauthors=Keith W. Ogilvie, Robert A. Hoffman, Donald H. Fairfield, Steven A. Curtis, James L. Green, William H. Mish, and the GGS Science Teams |date=1997-05-01 |work=ISTP-GGS/SOLARMAX Proposal |publisher=[[Goddard Space Flight Center]] }}</ref>
== Gallery ==
<gallery>
Image:Hubble 01.jpg|Hubble Space Telescope
Image:Cartoon CGRO.jpg|Compton Gamma Ray Observatory
Image:Spitzer- Telescopio.jpg|Spitzer Space Telescope
Image:Chandra Obs Illust.jpg|Chandra X-Ray Observatory
</gallery>
== See also ==
* [[Space observatory]]
* [[Terrestrial Planet Finder]]
* [[Beyond Einstein program]]
== References ==
{{reflist}}
== External links ==
* {{cite web
| title =Preflight Videos (STS-93)
| publisher =NASA
| date =04/09/2002
| url =http://spaceflight1.nasa.gov/gallery/video/shuttle/sts-93/html/fd0.html
| quote=A detailed description of NASA's Great Observatories, including STS-93 primary payload, the Chandra X-ray Observatory. [http://spaceflight1.nasa.gov/gallery/video/shuttle/sts-93/net28/pac_3_28.asf (28K Media Player)] [http://spaceflight1.nasa.gov/gallery/video/shuttle/sts-93/net56/pac_3_56.asf (56K Media Player)] [http://spaceflight1.nasa.gov/gallery/video/shuttle/sts-93/real28/pac_3_28.rm (28K Real Video)] [http://spaceflight1.nasa.gov/gallery/video/shuttle/sts-93/real56/pac_2_56.rm (56K Real Video)]
| accessdate =2007-11-27 }}
{{Space telescopes}}
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