The Hubble Space Telescope ( HST ) is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital research tool and a public relations boon for astronomy. The HST is a collaboration between NASA and the European Space Agency, and is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.
Space telescopes were proposed as early as 1923. The Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, scientists found that the main mirror had been ground incorrectly, severely compromising the telescope's capabilities. However, after a servicing mission in 1993, the telescope was restored to its intended quality. Hubble's orbit outside the distortion of Earth's atmosphere allows it to take extremely sharp images with almost no background light. Hubble's Ultra Deep Field image, for instance, is the most detailed visible-light image ever made of the universe's most distant objects. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.
The Hubble is the only telescope ever designed to be serviced in space by astronauts. There have been five servicing missions, the last occurring in May 2009. Servicing Mission 1 took place in December 1993 when Hubble's imaging flaw was corrected. Servicing missions 2, 3A, and 3B repaired various sub-systems and replaced many of the observing instruments with more modern and capable versions. However, following the 2003 Space Shuttle Columbia accident, the fifth servicing mission was canceled on safety grounds. After spirited public discussion, NASA reconsidered this decision, and administrator Mike Griffin approved one final Hubble servicing mission. STS-125 was launched in May 2009, and installed two new instruments and made numerous repairs.
The latest servicing should allow the telescope to function until at least 2014, when its successor, the James Webb Space Telescope (JWST), is due to be launched. The JWST will be far superior to Hubble for many astronomical research programs, but will only observe in infrared, so it will complement (not replace) Hubble's ability to observe in the visible and ultraviolet parts of the spectrum.
Conception, design and aims
Proposals and precursors
In 1923, Hermann Oberth—considered along with Robert H. Goddard and Konstantin Tsiolkovsky fathers of modern rocketry—published Die Rakete zu den Planetenräumen ("The Rocket into Planetary Space"), which mentioned how a telescope could be propelled into Earth orbit by a rocket.
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". In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes. First, the angular resolution (smallest separation at which objects can be clearly distinguished) would be limited only by diffraction, rather than by the turbulence in the atmosphere, which causes stars to twinkle and is known to astronomers as seeing. At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for a telescope with a mirror 2.5 m in diameter. Second, a space-based telescope could observe infrared and ultraviolet light, which are strongly absorbed by the atmosphere.
Spitzer devoted much of his career to pushing for a space telescope to be developed. In 1962 a report by the United States National Academy of Sciences recommended the development of a space telescope as part of the space program, and in 1965 Spitzer was appointed as head of a committee given the task of defining the scientific objectives for a large space telescope.
Space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. The first ultraviolet spectrum of the Sun was obtained in 1946, and NASA launched the Orbiting Solar Observatory to obtain UV, X-ray, and gamma-ray spectra in 1962. An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, and in 1966 National Aeronautics and Space Administration (NASA) launched the first Orbiting Astronomical Observatory (OAO) mission. OAO-1's battery failed after three days, terminating the mission. It was followed by OAO-2, which carried out ultraviolet observations of stars and galaxies from its launch in 1968 until 1972, well beyond its original planned lifetime of one year.
The OSO and OAO 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. These plans emphasized the need for manned maintenance missions to the telescope to ensure such a costly program had a lengthy working life, and the concurrent development of plans for the reusable space shuttle indicated that the technology to allow this was soon to become available.
Quest for funding
The continuing success of the OAO program encouraged increasingly strong consensus within the astronomical community that the LST should be a major goal. In 1970 NASA established two committees, one to plan the engineering side of the space telescope project, and the other to determine the scientific goals of the mission. Once these had been established, the next hurdle for NASA was to obtain funding for the instrument, which would be far more costly than any Earth-based telescope. The US Congress questioned many aspects of the proposed budget for the telescope and forced cuts in the budget for the planning stages, which at the time consisted of very detailed studies of potential instruments and hardware for the telescope. In 1974, public spending cuts instigated by Gerald Ford led to Congress cutting all funding for the telescope project.
In response to this, a nationwide lobbying effort was coordinated among astronomers. Many astronomers met congressmen and senators in person, and large scale letter-writing campaigns were organized. The National Academy of Sciences published a report emphasizing the need for a space telescope, and eventually the Senate agreed to half of the budget that had originally been approved by Congress.
The funding issues led to something of a reduction in the scale of the project, with the proposed mirror diameter reduced from 3 m to 2.4 m, both to cut costs and to allow a more compact and effective configuration for the telescope hardware. A proposed precursor 1.5 m space telescope to test the systems to be used on the main satellite was dropped, and budgetary concerns also prompted collaboration with the European Space Agency. ESA agreed to provide funding and supply one of the first generation instruments for the telescope, as well as the solar cells that would power it, and staff to work on the telescope in the United States, in return for European astronomers being guaranteed at least 15% of the observing time on the telescope. 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. In 1983 the telescope was named after Edwin Hubble, who made one of the greatest scientific breakthroughs of the 20th century when he discovered that the universe was expanding.
Construction and engineering
Once the Space Telescope project had been given the go-ahead, work on the program was divided among many institutions. Marshall Space Flight Center (MSFC) was given responsibility for the design, development, and construction of the telescope, while the Goddard Space Flight Center was given overall control of the scientific instruments and ground-control center for the mission. MSFC commissioned the optics company Perkin-Elmer to design and build the Optical Telescope Assembly (OTA) and Fine Guidance Sensors for the space telescope. Lockheed was commissioned to construct the spacecraft in which the telescope would be housed.
Optical Telescope Assembly (OTA)
Optically, the Hubble is a Cassegrain reflector of Ritchey-Chrétien design, as are most large professional telescopes. This design, with two hyperbolic mirrors, is known for good imaging performance over a wide field of view, with the disadvantage that the mirrors have shapes that are hard to fabricate and test. The mirror and optical systems of the telescope determine the final performance, and they were designed to exacting specifications. Optical telescopes typically have mirrors polished to an accuracy of about a tenth of the wavelength of visible light, but the Space Telescope was to be used for observations from the visible through
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