자료 1: 망원경으로 떠나는 4백 년의 여행, http://book.nate.com/detail.html?sbid=513312
오늘날의 우주 망원경들은 매우 다양하다. 가장 유명한 것이 NASA의 ‘대천문대(Great Observatory)’라고 불리는 네 개의 망원경으로,
- 허블 우주 망원경(1990년 발사, 자외선과 가시광선의 관찰),
- 콤프튼 감마선 망원경(Compton Gamma-Ray Observatory, 1991년 발사, 2000년에 궤도 이탈),
- 찬드라 X선 망원경(Chandra X-ray Observatory, 1999년 발사),
- 스피처 우주 망원경(Spitzer Space Telescope, 2003년 발사, 적외선 관찰)이 그것이다.
NASA's series of Great Observatories satellites are four large, powerful space-based telescopes. 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.[1]
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[edit]Great Observatories
- The Hubble Space Telescope (HST) primarily observes visible light and near-ultraviolet. A 1997 servicing mission added capability in the 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 rays, though it extended into hard x-rays as well. It was launched in 1991 aboard the Space Shuttle Atlantisduring 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-rays. It was launched in 1999 aboard the Space Shuttle Columbia during STS-93.
- The 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 gyroscopes 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 theSpace 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 Atlantis, launched on 11 May 2009,[2] installed fresh batteries, replaced all gyroscopes, and installed the Wide Field Camera 3 and theCosmic Origins Spectrograph.[3]
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 orbitwas 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.
[edit]History of the program
[edit]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 paperAstronomical advantages of an extraterrestrial observatory[4]. 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.[5]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.
[edit]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.[6]
[edit]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 (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.
[edit]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 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.[7][8]
[edit]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.[9] 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.[10][11]
[edit]Strengths
Each observatory was designed to push the state of technology in its intended wavelength region. As x-rays, gamma-rays and 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 interferometers). 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 20keV 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.
Chandra similarly had no ground predecessors. It followed the three NASA HEAO Program satellites, notably the highly successfulEinstein 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 IRASmission 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.
[edit]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.
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