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        <h1>The<br />Great<br />Observatories<br />Program</h1>
        <a class="attribution" href="#img-src-bg">Background images: Courtesy NASA/JPL-Caltech</a>
    <figure class="float-bottom" id="img-ref-a">
        <img src="resources/Ssc2004-15b.jpg" alt="A labeled space image comparing views of a supernova remnant by three different Great observatories."/>
            A labeled space image comparing views of a supernova remnant by three different Great
            observatories.<a href="#img-src-a"><sup>a</sup></a></figcaption>
    <div class="break-before multi-column">
                NASA's series of Great Observatories satellites are four large, powerful space-based
                astronomical telescopes launched between 1990 and 2003. They were built with different
                technology to examine specific wavelength/energy regions of the electromagnetic
                spectrum: gamma rays, X-rays, visible and ultraviolet light, and infrared light. 
                Two remain operational as of 2020.
            <h2>Great Observatories</h2>
                <b>The Hubble Space Telescope (HST)</b> primarily observes visible light and
                near-ultraviolet. It was launched in 1990 aboard the Space Shuttle Discovery during STS-31. In
                1997 the STS-82 servicing mission added capability in the near-infrared range, and in
                2009 the STS-125 mission fixed the telescope and extended its projected service life.
                <b>The Compton Gamma Ray Observatory (CGRO)</b> 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.
                <b>The Chandra X-ray Observatory (CXO)</b> 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).
                <b>The Spitzer Space Telescope (SST)</b> observed the infrared spectrum. It was
                launched in 2003 aboard a Delta II rocket into an Earth-trailing solar orbit. 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 early 2020.
            <figure class="float-bottom" id="img-ref-b">
                <img src="resources/Chandra_artist_illustration.jpg" alt="Illustration of Chandra"/>
                <figcaption>Illustration of Chandra<a href="#img-src-b"><sup>b</sup></a></figcaption>
                Hubble was originally intended to be retrieved and returned to Earth by the Space
                Shuttle, but the retrieval plan was later abandoned. On October 31, 2006, NASA Administrator
                Michael D. Griffin gave the go-ahead for a final refurbishment mission. The 11-day STS-125 mission
                by Space Shuttle Atlantis, launched on 11 May 2009,<a id="ref-1" href="#src-1"><sup>1</sup></a>
                installed fresh batteries, replaced all gyroscopes, replaced a command computer, fixed several instruments,
                and installed the Wide Field Camera 3 and the Cosmic Origins Spectrograph.<a id="ref-2" href="#src-2"><sup>2</sup></a>
                One of the three gyroscopes 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
                June 4, 2000.<a id="ref-3" href="#src-3"><sup>3</sup></a> Parts that survived reentry splashed
                into the Pacific Ocean.
                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 the Challenger disaster, the 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 canceled for cost reasons. After redesign and
                lightening, it was launched by a Delta II rocket instead. ; it was called the Space
                Infrared Telescope Facility (SIRTF) before launch.
            <h2>History of the program</h2>
            <h3>Hubble Space Telescope</h3>
                The history of the Hubble Space Telescope can be traced back to 1946, when the
                astronomer Lyman Spitzer wrote the paper Astronomical advantages of an extraterrestrial
                observatory.<a id="ref-4" href="#src-4"><sup>4</sup></a> Spitzer devoted much of his
                career to pushing for a space telescope.
                The 1966–1972 Orbiting Astronomical Observatory missions demonstrated the important role
                space-based observations could play in astronomy. In 1968, NASA developed firm plans for
                a space-based reflecting telescope with a 3-meter mirror, known provisionally as the
                Large Orbiting Telescope or Large Space Telescope (LST), with a launch slated for
                1979.<a id="ref-5" href="#src-5"><sup>5</sup></a> 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.
            <figure class="float-bottom" id="img-ref-c">
                <img src="resources/STS-31_Hubble_launch_roll_and_pitch.jpg" alt="Space Shuttle Discovery launching Hubble during the STS-31 mission on April 24, 1990."/>
                    Space Shuttle Discovery launching Hubble during the STS-31 mission on April 24,
                    1990.<a href="#img-src-c"><sup>c</sup></a>
            <h3>Gamma ray program</h3>
                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.<a id="ref-6" href="#src-6"><sup>6</sup></a>
            <h3>Chandra X-ray Observatory history</h3>
                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
                (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 the Space Shuttle but put the observatory above the Earth's radiation belts for most
                of its orbit.
            <figure class="float-top" id="img-ref-d">
                <img src="resources/CGRO_s37-96-010.jpg" alt="Compton Gamma Ray Observatory drifts away from the space shuttle Atlantis on April 7, 1991."/>
                    Compton Gamma Ray Observatory drifts away from the space shuttle Atlantis on April 7, 1991.<a href="#img-src-d"><sup>d</sup></a>
            <h3>Spitzer history</h3>
                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 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.
                <a id="ref-7" href="#src-7"><sup>7</sup></a><sup>,</sup><a id="ref-8" href="#src-8"><sup>8</sup></a>
            <h3>Great Observatory origin</h3>
                The concept of a Great Observatory program was first proposed in the 1979 NRC report "A
                Strategy for Space Astronomy and Astrophysics for the 1980's." This report laid the
                essential groundwork for the Great Observatories and was chaired by Peter Meyer (through
                June 1977) and then by Harlan J. Smith (through publication). In the mid-1980s it was
                further advanced by all of the astrophysics Division Directors at NASA 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
                <a id="ref-9" href="#src-9"><sup>9</sup></a><sup>,</sup><a id="ref-10" href="#src-10"><sup>10</sup></a>
                Each observatory was designed to push the state of technology in its intended wavelength
                region. Since the Earth's atmosphere prevents x-rays, gamma-rays and far-infrared
                radiation from reaching the ground, space missions were essential for the Compton,
                Chandra and Spitzer observatories.
            <figure class="float-top" id="img-ref-e">
                <img src="resources/spitzer.jpg" alt="Spitzer on Earth being readied for launch."/>
                <figcaption>Spitzer on Earth being readied for launch.<a href="#img-src-e"><sup>e</sup></a></figcaption>
                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
                using astronomical interferometers or adaptive 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 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 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 holes, pulsars, and supernovae.<a id="ref-11" href="#src-11"><sup>11</sup></a>
                Chandra similarly had no ground predecessors. It followed the three NASA HEAO Program
                satellites, notably the highly successful Einstein Observatory, which was the first to
                demonstrate the power of grazing-incidence, focusing X-ray optics, giving spatial
                resolution an order of magnitude better than collimated instruments (comparable to
                optical telescopes), with an enormous improvement in sensitivity. Chandra's large size,
                high orbit, and sensitive CCDs 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 ESA's large 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
                redshifted to infrared wavelengths, for cool objects which emit little visible light,
                and for regions optically obscured by dust.
                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.
                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.
            <figure class="float-top" id="img-ref-f">
                <img src="resources/Crab_Nebula_NGC_1952_(composite_from_Chandra,_Hubble_and_Spitzer).jpg" alt="Chandra, Hubble, and Spitzer composite image of the Crab Nebula (2009)."/>
                    Chandra, Hubble, and Spitzer composite image of the Crab Nebula (2009).
                    <a href="#img-src-f"><sup>f</sup></a>
                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. 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.
            <table class="float-top">
                        <th>Active Observatories</th>
                        <td>Late 1991</td>
                        <td>Hubble, Compton</td>
                        <td>Late 1999</td>
                        <td>Hubble, Compton, Chandra</td>
                        <td>Mid 2000</td>
                        <td>Hubble, Chandra</td>
                        <td>Late 2003</td>
                        <td>Hubble, Chandra, Spitzer</td>
                        <td>Early 2020</td>
                        <td>Hubble, Chandra</td>
                    Which Satellites were active and could be used together changed over time
                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 moons and asteroids, 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
                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
            <h3>Synergistic discoveries</h3>
                When great observatories worked together to make special discoveries or observations:
                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.
                <a id="ref-12" href="#src-12"><sup>12</sup></a><sup>,</sup><a id="ref-13" href="#src-13"><sup>13</sup></a>
                (List of the most distant astronomical objects)
            <h2>Successors to GO instruments</h2>
                James Webb Space Telescope (JWST) — the JWST, previously known as the NGST (Next
                Generation Space Telescope) is projected to launch in March 2021, and will work
                simultaneously with Hubble until its mission ends and the JWST replaces it.
                <a id="ref-14" href="#src-14"><sup>14</sup></a> Its segmented, deployable mirror 
                will be over twice as wide, 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 in the visible and especially the ultraviolet wavelengths.
            <figure class="float-bottom" id="img-ref-g">
                <img src="resources/James_Webb_Space_Telescope_Revealed_(26832090085).jpg" alt="Main mirror of the James Webb Space Telescope assembled at Goddard Space Flight Center, May 2016."/>
                    Main mirror of the James Webb Space Telescope assembled at Goddard Space Flight
                    Center, May 2016.<a href="#img-src-g"><sup>g</sup></a>
                The Fermi Gamma-ray Space Telescope, formerly GLAST, the Gamma Ray Large Area Space
                Telescope, is a follow-on to Compton launched on June 11, 2008.
                <a id="ref-15" href="#src-15"><sup>15</sup></a>
                GLAST is 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,
                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. INTEGRAL uses a fundamentally different telescope technology, coded-aperture
                masks. Thus, its capabilities are complementary to Compton and Fermi.
            <figure class="float-top" id="img-ref-h">
                <img src="resources/IXOFlyby1Large.jpg" alt="IXO was considered as a possible future X-ray observatory."/>
                    IXO was considered as a possible future X-ray observatory.<a href="#img-src-h"><sup>h</sup></a>
                JWST will exceed Spitzer's performance in near-infrared, and the European Space Agency's
                Herschel Space Observatory, operational from 2009 to 2013, has exceeded Spitzer in the
                far-infrared. The SOFIA (Stratospheric Observatory For Infrared Astronomy) airborne
                platform observes in near- and mid-infrared. SOFIA has a larger aperture than Spitzer,
                but lower relative sensitivity.
            <h2>Later programs</h2>
                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.
                <a id="ref-16" href="#src-16"><sup>16</sup></a>
                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.<a id="ref-17" href="#src-17"><sup>17</sup></a>
            <h2>Next Great Observatory</h2>
                In 2016, NASA began considering four different Flagship space telescopes,
                <a id="ref-18-1" href="#src-18"><sup>18</sup></a> they are
                the Habitable Exoplanet Imaging Mission (HabEx), Large UV Optical Infrared Surveyor
                (LUVOIR), Origins Space Telescope, and Lynx X-ray Surveyor. In 2019 the four teams will
                turn their final reports over to the National Academy of Sciences, whose independent
                Decadal Survey committee advises NASA on which mission should take top priority.
                Selection would take place in late 2020, and launch approximately in 2035.
                <a id="ref-18-2" href="#src-18"><sup>18</sup></a>
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            <a href="https://en.wikipedia.org/wiki/Great_Observatories_program">"Great Observatories program"</a>,
            which is released under the 
            <a href="https://creativecommons.org/licenses/by-sa/3.0/">Creative Commons Attribution-Share-Alike License 3.0</a>, retrieved on the 26.05.2020.
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    <div class="image-sources">
        <h2>Image sources</h2>
        <p id="img-src-bg">
            The background images used in this document are sections of <a href="https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA23647">&gt;&gt;this&lt;&lt;</a> image,
            courtesy NASA/JPL-Caltech, retrieved 26.05.2020.
            All other images used in this document were retrieved from Wikimedia Commons on the 26.05.2020
            <li id="img-src-a">
                <a href="#img-ref-a">^</a> <a href="https://commons.wikimedia.org/wiki/File:Ssc2004-15b.jpg">https://commons.wikimedia.org/wiki/File:Ssc2004-15b.jpg</a>
            <li id="img-src-b">
                <a href="#img-ref-b">^</a> <a href="https://commons.wikimedia.org/wiki/File:Chandra_artist_illustration.jpg">https://commons.wikimedia.org/wiki/File:Chandra_artist_illustration.jpg</a>
            <li id="img-src-c">
                <a href="#img-ref-c">^</a> <a href="https://commons.wikimedia.org/wiki/File:STS-31_Hubble_launch_roll_and_pitch.jpg">https://commons.wikimedia.org/wiki/File:STS-31_Hubble_launch_roll_and_pitch.jpg</a>
            <li id="img-src-d">
                <a href="#img-ref-d">^</a> <a href="https://commons.wikimedia.org/wiki/File:CGRO_s37-96-010.jpg">https://commons.wikimedia.org/wiki/File:CGRO_s37-96-010.jpg</a>
            <li id="img-src-e">
                <a href="#img-ref-e">^</a> <a href="https://commons.wikimedia.org/wiki/File:Faring0814_03.jpg">https://commons.wikimedia.org/wiki/File:Faring0814_03.jpg</a>
            <li id="img-src-f">
                <a href="#img-ref-f">^</a> <a href="https://commons.wikimedia.org/wiki/File:Crab_Nebula_NGC_1952_(composite_from_Chandra,_Hubble_and_Spitzer).jpg">https://commons.wikimedia.org/wiki/File:Crab_Nebula_NGC_1952_(composite_from_Chandra,_Hubble_and_Spitzer).jpg</a>
            <li id="img-src-g">
                <a href="#img-ref-g">^</a> <a href="https://commons.wikimedia.org/wiki/File:James_Webb_Space_Telescope_Revealed_(26832090085).jpg">https://commons.wikimedia.org/wiki/File:James_Webb_Space_Telescope_Revealed_(26832090085).jpg</a>
            <li id="img-src-h">
                <a href="#img-ref-h">^</a> <a href="https://commons.wikimedia.org/wiki/File:IXOFlyby1Large.jpg">https://commons.wikimedia.org/wiki/File:IXOFlyby1Large.jpg</a>