| | | The Hubble Space Telescope
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NASA: The Hubble Telescope
The Hubble Space Telescope (HST) is a telescope in orbit around the Earth, named after astronomer Edwin Hubble. Its position outside the Earth's atmosphere provides significant advantages over ground based telescopes - images are not blurred by the atmosphere, there is no background light scattered from the atmosphere, and the Hubble can observe in ultra-violet light that is absorbed by the ozone layer. Since its launch in 1990, it has become one of the most important instruments in the history of astronomy. It has been responsible for many ground-breaking observations and has helped astronomers achieve a better understanding of many fundamental problems in astrophysics. Hubble's Ultra Deep Field is the deepest (most sensitive) astronomical optical image ever taken.
From its original conception in 1946 until its launch, the project to build a space telescope was beset by delays and budget problems. Immediately after its launch, it was found that the main mirror suffered from spherical aberration, severely compromising the telescope's capabilities. However, after a servicing mission in 1993, the telescope was restored to its planned quality and became a vital research tool as well as a public relations boon for astronomy. The HST is part of NASA's Great Observatories series, with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope. Hubble is a collaboration between NASA and the European Space Agency.
The future of Hubble depends on the success of its next servicing mission. Several of its stabilizing gyroscopes have already failed, so that currently (2007), their redundancy has been exhausted and with another failure the ability to point the telescope will be compromised. The gyroscopes require a manned service mission to replace. On January 30, 2007, ACS, (the main camera) stopped working, and without manned servicing, only its ultraviolet channel will be usable. In addition, without a reboost to increase the diameter of its orbit, drag will cause Hubble to re-enter the Earth's atmosphere sometime after 2010. Following the 2003 Columbia Space Shuttle disaster, NASA decided that a manned repair mission to Hubble would be unreasonably dangerous, due primarily to the lack of access to the safe haven of the International Space Station for astronauts in the event of emergency because of the highly differing orbits of the two satellites. The organization later reconsidered this position, and, on October 31, 2006, NASA administrator Mike Griffin gave the green light for a final Hubble servicing mission to be flown by Atlantis. The mission is now planned for September 2008. As a safety precaution, NASA will have the orbiter Discovery standing by at LC-39B to provide rescue in the event of an emergency. The planned repairs to the Hubble will allow the telescope to function until at least 2013, 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 that it will not replace Hubble's ability to observe in the visible and ultraviolet parts of the spectrum. Yet another space telescope would be needed for that.
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Hubble Discoveries
Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required whole new theories to explain them. Among its primary mission targets was to measure distances to Cepheid variable stars more accurately than ever before, and thus constrain the value of the Hubble constant, the measure of the rate at which the universe is expanding, which is also related to its age. Before the launch of Hubble, estimates of the Hubble constant typically had errors of up to 50%, but Hubble measurements of Cepheid variables in the Virgo cluster and other distant galaxy clusters provided a measured value with an accuracy of 10%, which is consistent with other more accurate measurements made since Hubble's launch using other techniques.
While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project used the telescope to observe distant supernovae and uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating. This acceleration was later measured more accurately by other ground-based and space-based telescopes which confirmed Hubble's finding, but the cause of this acceleration is currently very poorly understood.
The high-resolution spectra and images provided by the Hubble have been especially well suited to establishing the prevalence of black holes in the nuclei of nearby galaxies. While it had been hypothesized in the early 1960s that black holes would be found at the centers of some galaxies, and work in the 1980s identified a number of good black hole candidates, it fell to work conducted with the Hubble to show that black holes are probably common to the centers of all galaxies. The Hubble programs further established that the masses of the nuclear black holes and properties of the galaxies are closely related. The legacy of the Hubble programs on black holes in galaxies is thus to demonstrate a deep and profound connection between galaxies and their central black holes.
The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was very fortuitously timed for astronomers, coming just a few months after Servicing Mission 1 had restored Hubble's optical performance. Hubble images of the planet were sharper than any taken since the passage of Voyager 2 in 1979, and were crucial in studying the dynamics of the collision of a comet with Jupiter, an event believed to occur once every few centuries. It has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto and Eris.
Other major discoveries made using Hubble data include proto-planetary disks (proplyds) in the Orion Nebula; evidence for the presence of extrasolar planets around sun-like stars; and the optical counterparts of the still-mysterious gamma-ray bursts.
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The Impact On Astronomy
Many objective measures show the enormous impact of Hubble data on astronomy. Over 4,000 papers based on Hubble data have been published in peer-reviewed journals, and countless more have appeared in conference proceedings. Looking at papers several years after their publication, about one-third of all astronomy papers have no citations, while only 2% of papers based on Hubble data have no citations. On average, a paper based on Hubble data receives about twice as many citations as papers based on non-Hubble data. Of the 200 papers published each year which receive the most citations, about 10% are based on Hubble data.
Although the HST has clearly had a significant impact on astronomical research, the financial cost of this impact has been very large. A study on the relative impacts on astronomy of different sizes of telescopes found that while papers based on HST data generate 15 times as many citations as a 4 m ground-based telescope such as the William Herschel Telescope, the HST cost about 100 times as much to build and maintain. Even before Hubble's launch, ground-based speckle imaging could provide higher resolution images of bright objects than Hubble can achieve. More recently the development of adaptive optics has extended the high-resolution imaging capabilities of ground-based telescopes to the infrared imaging of faint objects. The field of view over which high-quality adaptive optics corrections is limited however, especially in optical colors. HST retains the unique ability to do high-resolution optical imaging over a wide field. However, since ground-based imaging can be done at a much lower cost, and the quality of adaptive optics has steadily improved in the infra-red since the launch of HST, the role of adaptive optics has been a key consideration in the debate about the future of space telescopes (see below). This issue has often been cast simplistically in general terms as a competition between HST and ground-telescopes, but in practice the usefulness of adaptive optics versus HST observations depends strongly on the particular details of the research questions being asked.
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