An orbiting observatory, a joint project of the European Space Agency and NASA, launched in 1990 with a 2·4 m (94 in) aperture telescope; named after Edwin Hubble. It was expected to image objects more sharply than telescopes on Earth, and detect fainter sources. However, following the launch, a defect was discovered in the main mirror, which limited its performance. A space shuttle mission added optics to correct the defect and carried out other repairs in 1993. Further new equipment was installed by astronauts on servicing missions in 1997 and 2002, but in 2004 NASA announced it was halting its maintenance programme. This decision was reversed in 2006, with a shuttle mission to service and upgrade the telescope planned for 2008.
The Hubble Space Telescope (HST) is a telescope in orbit around the Earth, named after astronomer Edwin Hubble for his discovery of galaxies outside the Milky Way and his creation of Hubble's Law, which calculates the rate at which the universe is expanding.
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.
The future of Hubble is uncertain. The repairs to the Hubble will allow the telescope to function until at least 2013, when its successor is to be launched.
Hubble's successor telescope, the James Webb Space Telescope (JWST), is due to be launched in 2013 and will be far superior to Hubble for many astronomical research programs.
Conception, design and aims
Proposals and precursors
The history of the Hubble Space Telescope can be traced back as far as 1946, when astronomer Lyman Spitzer wrote a paper entitled Astronomical advantages of an extra-terrestrial 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 0.1 arcsec for a telescope with a mirror 2.5 m in diameter. The second major advantage would be that 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 U.S. 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 the developments in rocket technology that had taken place. An orbiting solar telescope was launched in 1962 by the UK as part of the Ariel space program, and 1966 saw National Aeronautics and Space Administration 's (NASA) launch of the first Orbiting Astronomical Observatory (OAO) mission.
The 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.
The 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 science 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 co-ordinated among astronomers. The National Academy of Sciences published a report emphasizing the need for a space telescope, and eventually the Senate agreed to a budget half that originally refused 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 some of the instruments for the telescope as well as the solar cells which would power it, in return for European astronomers being guaranteed at least 15% of observing time on the telescope. During the early 1980s, 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 between 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. Marshall commissioned optics company Perkin-Elmer to design and build the Optical Telescope Assembly (OTA) and Fine Guidance Sensors for the space telescope.
Optical Telescope Assembly (OTA)
The mirror and optical systems of the telescope were the most crucial part, and were designed to exacting specifications. Telescopes typically have mirrors polished to an accuracy of about a tenth of the wavelength of visible light, but because the Space Telescope was to be used for observations ranging from ultraviolet to near-infrared with ten times better resolution than the best previous telescopes, its mirror needed to be polished to an accuracy of 1/20 of the wavelength of visible light, or about 30 nanometers.
Perkin-Elmer intended to use extremely sophisticated computer-controlled polishing machines to grind the mirror to the required shape, but in case their cutting-edge technology ran into difficulties, Kodak was commissioned to construct a back-up mirror using traditional mirror-polishing techniques (the Kodak mirror is now on permanent display at the Smithsonian Institution.).
Mirror polishing began in 1979 and continued until May 1981. To save money, NASA halted work on the back-up mirror and put the launch date of the telescope back to October 1984.
However, doubts continued to be expressed about Perkin-Elmer's competence on a project of this importance as their budget and timescale for producing the rest of the OTA continued to inflate. In response to a schedule described as "unsettled and changing daily", NASA postponed the launch date of the telescope until April 1985.
Spacecraft systems
The spacecraft in which the telescope and instruments were to be housed was another major engineering challenge. It would have to adequately withstand frequent passages from direct sunlight into the darkness of Earth's shadow which would generate major changes in temperature, while being stable enough to allow the extremely accurate pointing of the telescope that would be required. A shroud of multi-layered insulation keeps the temperature within the telescope stable, and surrounds a light aluminum shell in which the telescope and instruments sit. Within the shell, a graphite-epoxy frame keeps the working parts of the telescope firmly aligned.
While construction of the spacecraft in which the telescope and instruments would be housed proceeded somewhat more smoothly than the construction of the OTA, Lockheed still experienced some budget and schedule slippage, and by the summer of 1985, construction of the spacecraft was 30% over budget and three months behind schedule.
Ground support
In 1983, the Space Telescope Science Institute (STScI) was established after something of a power struggle between NASA and the scientific community at large.
STScI is responsible for the scientific operation of the telescope and delivery of data products to astronomers, a function which NASA had wanted to keep "in-house", but which scientists were keen to see based in an academic establishment. Hubble's operation is monitored 24 hours per day by four teams of flight controllers who make up Hubble's Flight Operations Team.
The Space Telescope European Coordinating Facility was established at Garching bei München near Munich in 1984 to provide similar support primarily for European astronomers.
Challenger disaster
In early 1986, the planned launch date of October that year looked feasible, but the Challenger disaster brought the U.S. space program to a halt, grounding the Space Shuttle fleet and forcing the launch of Hubble to be postponed for several years. All telescope parts had to be kept in clean rooms until a launch could be rescheduled, a costly situation which pushed the overall costs of the project still higher.
Eventually, following the resumption of Shuttle flights in 1988, the launch of the telescope was scheduled for 1990. Finally, on 24 April 1990, shuttle mission STS-31 saw Discovery launch the telescope successfully into its planned orbit.
From its original total cost estimate of about 400 million dollars, the telescope had by now cost over US$2 billion to construct.
Instruments
When launched, the HST carried five scientific instruments: the Wide Field and Planetary Camera (WF/PC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS).
The final instrument was the HSP, designed and built at the University of Wisconsin-Madison.
HST's guidance system can also be used as a scientific instrument. Its three Fine Guidance Sensors (FGS) are primarily used to keep the telescope accurately pointed during an observation, but can also be used to carry out extremely accurate astrometry;
Flawed mirror
Within weeks of the launch of the telescope, the images returned showed that there was a serious problem with the optical system. Although the first images appeared to be sharper than ground-based images, the telescope failed to achieve a final sharp focus, and the best image quality obtained was drastically lower than expected.
Analysis of the flawed images showed that the cause of the problem was that the primary mirror had been ground to the wrong shape. The aberration meant that images from the Space Telescope were only marginally better than the best images obtainable from the ground.
Origin of the problem
Working backwards from images of point sources, astronomers determined that the conic constant of the mirror was −1.0139, instead of the intended −1.00229.
During the polishing of the mirror, Perkin-Elmer had analyzed its surface with two other null correctors, both of which (correctly) indicated that the mirror was suffering from spherical aberration. Relations between NASA and the optics company had been severely strained during the telescope construction due to frequent schedule slippage and cost overruns. NASA found that Perkin-Elmer had not regarded the telescope mirror as a crucial part of their business and were also secure in the knowledge that NASA could not take its business elsewhere once the polishing had begun.
Design of a solution
The flaw meant that Hubble could obtain data about as good as that achievable with a large ground-based telescope on a night of good seeing, but at a vastly greater cost. NASA and the telescope became the butt of many jokes, and the project was popularly regarded as a white elephant. However, the design of the telescope had always incorporated servicing missions, and astronomers immediately began to seek potential solutions to the problem which could be applied at the first servicing mission, scheduled for 1993.
While Kodak had ground a back-up mirror for Hubble, it would have been impossible to replace the mirror in orbit, and too expensive and time-consuming to bring the telescope temporarily back to Earth for a refit. Instead, the fact that the mirror had been ground so precisely to the wrong shape led to the design of new optical components with exactly the same error but in the opposite sense, to be added to the telescope at the servicing mission, effectively acting as "spectacles" to correct the spherical aberration.
Because of the way the instruments were designed, two different sets of correctors were required.
COSTAR
The system designed to correct the spherical aberration for light focused at the FOC, FOS and GHRS was called the "Corrective Optics Space Telescope Axial Replacement" (COSTAR) and consisted essentially of two mirrors in the light path, one of which would be figured to correct the aberration. To fit the COSTAR system onto the telescope, one of the other instruments had to be removed, and astronomers selected the High Speed Photometer to be sacrificed.
During the first three years of the Hubble mission, before the optical corrections could be fitted, the telescope still carried out a large number of observations. Spectroscopic observations in particular were not too badly affected by the aberration, but many imaging projects were cancelled as the space telescope no longer gave decisive advantages over ground-based observations.
Servicing missions and new instruments
Servicing mission 1
The telescope had always been designed so that it could be regularly serviced, but after the problems with the mirror came to light, the first servicing mission assumed a much greater importance, as the astronauts would have to carry out extensive work on the telescope to install the corrective optics.
Most importantly, the High Speed Photometer was replaced with the COSTAR corrective optics package, and WFPC was replaced with the Wide Field and Planetary Camera 2 (WFPC2), with its internal optical correction system. In addition, the solar arrays and their drive electronics were replaced, as well as four of the gyroscopes used in the telescope pointing system, two electrical control units and other electrical components, and two magnetometers. The onboard computers were upgraded, and finally, the telescope's orbit was boosted, having been slowly decaying for three years due to drag in the tenuous upper atmosphere.
On January 13, 1994, NASA declared the mission a complete success and showed the first of many much sharper images. The mission had been one of the most complex ever undertaken, involving five lengthy periods of extravehicular activity, and its resounding success was an enormous boon for NASA, as well as for the astronomers who now had a fully capable space telescope.
Subsequent servicing missions
Subsequent servicing missions were less dramatic, but each gave the space telescope new capabilities. Servicing Mission 2 Discovery (STS-82) in February 1997 replaced the GHRS and the FOS with the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), replaced an Engineering and Science Tape Recorder with a new Solid State Recorder, repaired thermal insulation and again boosted Hubble's orbit.
Servicing Mission 3A Discovery (STS-103) took place in December 1999, replaced all six gyroscopes (one had failed and rendered the telescope unusable just weeks before the mission), replaced a Fine Guidance Sensor and the computer, installed a Voltage/temperature Improvement Kit (VIK) to prevent battery overcharging, and replaced thermal insulation blankets.
Servicing Mission 3B Columbia (STS-109) in March 2002 saw the installation of a new instrument, with the FOC being replaced with the Advanced Camera for Surveys (ACS), and also saw the revival of NICMOS, which had run out of coolant in 1999.
The mission replaced the solar arrays for a third time, with the new arrays being smaller but generating more power.
The completion of this servicing mission considerably enhanced Hubble's capabilities.
Scientific results
Important 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. 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 the age of the universe, it also cast doubt on 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 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.
Other major discoveries made using Hubble data include proto-planetary disks (proplyds) in the Orion Nebula;
A unique legacy of Hubble is Hubble Deep Field and Hubble Ultra Deep Field images, which utilized Hubble's unmatched sensitivity at visible wavelengths to create images of small patches of sky which are the deepest ever obtained at optical wavelengths.
Impact on astronomy
Many objective measures show the enormous impact of Hubble data on astronomy.
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 and 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. Ground-based imaging can be done at much lower cost, and this has been a key consideration in the debate about the future of space telescopes (see below).
Using the telescope
Anyone can apply for time on the telescope; Competition for time on the telescope is extremely intense, and the ratio of time requested to time available (the oversubscription ratio) typically ranges between 6 and 9.
Calls for proposals are issued roughly annually, with time allocated for a 'cycle' lasting approximately one year. 'Snapshot observations' are those in which targets require only 45 minutes or less of telescope time, including the overheads of acquiring the target and so on; snapshot observations are used to fill in gaps in the telescope schedule which cannot be filled by regular GO programs.
Astronomers may make 'Target of Opportunity' proposals, in which observations are scheduled if a transient event covered by the proposal occurs during the scheduling cycle. In addition, up to 10% of the telescope time is designated Director's Discretionary (DD) Time. Other uses of DD time have included the observations that led to the production of the Hubble Deep Field and Hubble Ultra Deep Field, and in the first four cycles of telescope time, observations carried out by amateur astronomers (discussed below).
Observation scheduling
Scheduling observations for Hubble is not a simple matter. Observations cannot take place when the telescope passes through the South Atlantic Anomaly due to elevated radiation levels, and there are also sizable exclusion zones around the Sun (precluding observations of Mercury), Moon and Earth, which cannot be observed.
Because Hubble orbits in the upper atmosphere, its orbit changes over time in a way that is not accurately predictable.
Amateur observations
The first director of the STScI, Riccardo Giacconi, announced in 1986 that he intended to devote some of his Director Discretionary time to allowing amateur astronomers to use the telescope.
Proposals for amateur time were stringently peer reviewed by a committee of leading amateur astronomers, and time was awarded only to proposals with genuine scientific merit which did not duplicate proposals made by professionals and which required the unique capabilities of the space telescope. In total, 13 amateur astronomers were awarded time on the telescope, with observations being carried out between 1990 and 1997.
Hubble data
Transmission to Earth
Hubble data are initially stored on the spacecraft.
Archive
All Hubble data are eventually made available via a public archive at http://archive.stsci.edu/hst.
Observations made on Director's Discretionary Time are exempt from the proprietary period, and are released to the public immediately.
Pipeline reduction
Astronomical data taken with CCDs must undergo several calibration steps before it is suitable for astronomical analysis.
Astronomers may if they wish retrieve the calibration files themselves and run the pipeline reduction software locally.
Data analysis
Hubble data can be analyzed using many different packages, but STScI develops the custom-made STSDAS (Space Telescope Science Data Analysis System) software.
Outreach activities
It has always been important for the Space Telescope to capture the public's imagination, given the considerable contribution of taxpayers to its construction and operational costs. After the difficult early years when the faulty mirror severely dented Hubble's reputation with the public, the first servicing mission allowed its rehabilitation as the corrected optics produced numerous remarkable images.
Several initiatives have helped to keep the public informed about Hubble activities.
In addition, STScI maintains several comprehensive websites for the general public containing Hubble images and information about the observatory. The outreach efforts are coordinated by the Office for Public Outreach, which was established in 2000 to ensure that U.S. taxpayers saw the benefits of their investment in the space telescope program.
The Heritage Project is granted a small amount of time to observe objects which, for scientific reasons, may not have images taken at enough wavelengths to construct a full color image. In 2001, to celebrate the 11th anniversary of the launch of Hubble, NASA polled internet users to find out what they would most like Hubble to observe, and they overwhelmingly selected the Horsehead Nebula .
Future
Equipment failure
Past servicing missions have exchanged old instruments for new ones, both avoiding failure and making possible new types of science. On August 3, 2004, the power system of the Space Telescope Imaging Spectrograph (STIS) failed, rendering the instrument inoperable.
Hubble uses gyroscopes to stabilize itself in orbit and point accurately and steadily at astronomical targets. In 2005, it was decided to switch to two-gyroscope mode for regular telescope operations as a means of extending the lifetime of the mission. Estimates of the failure rate of the gyros indicate that Hubble may be down to one gyro by 2008, after which the telescope would be rendered unusable.
In addition to predicted gyroscope failure, Hubble will eventually require a change of batteries.
Orbital decay
Hubble orbits the Earth in the extremely tenuous upper atmosphere, and over time its orbit decays due to drag. The state of Hubble's gyros also impacts the re-entry date, as a controllable telescope can be made to minimize atmospheric drag.
Addition of an external propulsion module to allow controlled re-entry is currently being investigated by NASA.
NASA's original plan for safely de-orbiting Hubble was to retrieve it using a space shuttle (see STS-144). The Hubble telescope would then have most likely been displayed in the Smithsonian Institution.
Debate over final servicing mission
The Space Shuttle Columbia was originally scheduled to visit Hubble again in February 2005.
This decision was assailed by numerous astronomers, who felt that the Hubble telescope was valuable enough to merit the risk; The break in space observing capabilities between the decommissioning of Hubble and the commissioning of a successor is of major concern to some astronomers, given the great scientific impact of many space telescope observations. On 29 January 2004, Sean O'Keefe said that he would review his decision to cancel the final servicing mission of the Hubble Space Telescope due to public outcry and requests from Congress for NASA to look for a way to save the Hubble Space Telescope.
On 13 July 2004, an official panel from the National Academy of Sciences made the recommendation that the Hubble telescope be preserved despite the apparent risks. Their report urged "NASA should take no actions that would preclude a space shuttle servicing mission to the Hubble Space Telescope".
The arrival, in April 2005, of the new NASA Administrator, Mike Griffin, has changed the status of both of the manned and unmanned rescue missions. Soon after his appointment, he authorized NASA's Goddard Space Flight Center to proceed with preparing for a manned Hubble maintenance flight, saying he would make the final decision on this flight after the next two shuttle missions. If all goes well, Hubble will be serviced on mission STS-125, currently scheduled to send Discovery to the telescope sometime in 2008.
On October 31, 2006 the final go-ahead was given for the mission by NASA Administrator Mike Griffin.
Hubble Origins Probe
NASA and ESA are currently investigating building a follow-on to the Hubble Space Telescope called the Hubble Origins Probe ).
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