Hubble Space Telescope

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The Hubble Space Telescope, from the Space Shuttle Discovery during the second servicing mission, STS-82
Organization	NASA/ESA
Wavelength regime	Optical, ultraviolet, near-infrared
Type of Orbit	Elliptical
Orbit height	589  km
Orbit period	96-97  min
Orbit velocity	7,500  m/s
Acceleration due to gravity	8.169  m/s²
Launch date	April 24, 1990
Deorbit date	Around 2020
NSSDC ID	1990-037B
Mass	11,000  kg (24,250  lb)
Websites	http://www.nasa.gov/hubble http://hubble.nasa.gov http://hubblesite.org http://www.spacetelescope.org
Physical characteristics
Telescope style	Ritchey-Chretien reflector
Diameter	2.4  m (94  in)
Collecting area	approx. 4.3  m² (46  ft²)
Effective focal length	57.6 m (189 ft)
Instruments
NICMOS	infrared camera/spectrometer
ACS	optical survey camera
WFPC2	wide field optical camera
STIS	optical spectrometer/camera (failed)
FGS	three fine guidance sensors

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. Its position outside the Earth's atmosphere allows it to take sharp optical images of very faint objects, and 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 is uncertain. Its stabilising gyroscopes are failing such that by today (2006) its redundancy was exhausted and with another failure, its ability to point will be compromised. These must be replaced by a manned service mission. In addition, without a reboost to increase the diameter of its orbit, drag will cause it to re-enter the Earth's atmosphere sometime after 2010. Following the Columbia Space Shuttle disaster, NASA decided that a repair mission by astronauts would be unreasonably dangerous. 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 Discovery no earlier than May 2008. 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. However, the JWST will only observe in infrared, so it will not replace Hubble's ability to observe in the visible and ultraviolet parts of the spectrum. JWST is a project of international collaboration between NASA, ESA and the Canadian Space Agency.

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. The first ultraviolet spectrum of the Sun was obtained in 1946. 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. 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 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 emphasised 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. Many astronomers met congressmen and senators in person, and large scale letter-writing campaigns were organised. The National Academy of Sciences published a report emphasising 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. 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, 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 Centre (MSFC) was given responsibility for the design, development and construction of the telescope, while the Goddard Space Flight Centre was given overall control of the scientific instruments and ground control centre 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. Lockheed was commissioned to construct the spacecraft in which the telescope would be housed.

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  nanometres.

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.). Construction of the Perkin-Elmer mirror began in 1979, using ultra-low expansion glass. To keep the mirror's weight to a minimum it consisted of inch-thick top and bottom plates sandwiching a honeycomb lattice.

Mirror polishing began in 1979 and continued until May 1981. NASA reports at the time questioned Perkin-Elmer's managerial structure, and the polishing began to slip behind schedule and over budget. To save money, NASA halted work on the back-up mirror and put the launch date of the telescope back to October 1984. The mirror was completed by the end of 1981 with the addition of a reflective coating of aluminium 75 nm thick and a protective coating of magnesium fluoride 25 nm thick, which increased the mirror's reflectivity in ultraviolet light.

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. Perkin-Elmer's schedules continued to slip at a rate of about one month per quarter, and at times delays reached one day for each day of work. NASA was forced to postpone the launch date until first March and then September 1986. By this time the total project budget had risen to US$1.175 billion.

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 aluminium 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. An MSFC report said that Lockheed tended to rely on NASA directions rather than take their own initiative in the construction.

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 operated by the Association of Universities for Research in Astronomy (AURA) and is physically located on the Homewood campus of Johns Hopkins University in Baltimore, which is one of the 32 U.S. universities and seven international affiliates that comprise the AURA consortium.

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. Engineering support is provided by NASA and contractor personnel at the Goddard Space Flight Centre in Greenbelt, Maryland, 30 miles south of the STScI. 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. In preparation for its final launch, dust which had accumulated on the mirror since its completion had to be removed with jets of nitrogen, and all systems were tested extensively to ensure they were fully functional. 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. Hubble's cumulative costs up to this day are estimated to be several times higher still, with U.S. expenditure estimated at between 4.5 and 6 billion USD and Europe's financial contribution at 593 million Euros (1999 estimate) .

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). WF/PC was a high-resolution imaging device primarily intended for optical observations. It was built by NASA's Jet Propulsion Laboratory, and incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest. The instrument contained four CCD chips, three of which were "wide field" chips while the fourth was the "planetary camera" (PC). The PC took images at a longer effective focal length than the WF chips, giving it a greater magnification.

The GHRS was a spectrograph designed to operate in the ultraviolet. It was built by the Goddard Space Flight Centre in conjunction with Ball Aerospace, and could achieve a spectral resolution of 90,000. Also optimised for ultraviolet observations were the FOC and FOS, both of which were also capable of the highest spatial resolution of any instrument on Hubble. Rather than CCDs these three instruments used photon-counting digicons as their detectors. FOC was constructed by ESA, while the Martin Marietta corporation built the FOS.

The final instrument was the HSP, designed and built at the University of Wisconsin-Madison. It was optimised for visible and ultraviolet light observations of variable stars and other astronomical objects varying in brightness. It could take up to 100,000 measurements per second with a photometric accuracy of about 2% or better.

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; measurements accurate to within 0.0003 arcseconds have been achieved.

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. Images of point sources spread out over a radius of more than one arcsecond, instead of having a point spread function concentrated within a circle 0.1 arcsec in diameter as had been specified in the design criteria.

Analysis of the flawed images showed that the cause of the problem was that the primary mirror had been ground to the wrong shape. Although it was probably the most accurately figured mirror ever made, with variations from the prescribed curve of no more than 1/20 of the wavelength of light, it was too flat at the edges. The mirror was barely 2 micrometres out from the required shape, but the difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edge of a mirror focuses on a different point from the light reflecting off its centre. 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. The same number was also derived by analysing the null correctors (instruments which accurately measure the curvature of a polished surface) used by Perkin-Elmer to figure the mirror, as well as by analysing interferograms obtained during ground testing of the mirror.

A commission headed by Lew Allen, director of the Jet Propulsion Laboratory, was established to determine how the error could have arisen. The Allen Commission found that the null corrector used by Perkin-Elmer had been incorrectly calibrated, as a spot on a metering scale where an end cap had worn away was wrongly believed to be a valid scale. The null corrector had then been wrongly spaced by 1.3 mm.

During the polishing of the mirror, Perkin-Elmer had analysed its surface with two other null correctors, both of which (correctly) indicated that the mirror was suffering from spherical aberration. These tests were specifically designed to eliminate the possibility of major optical aberrations. Against written quality guidelines, the company ignored these test results as it believed that the two null correctors were less accurate than the primary device which was reporting that the mirror was perfectly figured.

The commission blamed the failings primarily on Perkin-Elmer. 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. While the commission heavily criticised Perkin-Elmer for these managerial failings, NASA was also criticised for not picking up on the quality control shortcomings such as relying totally on test results from a single instrument.

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. The design of the Wide Field and Planetary Camera (WFPC) included four relay mirrors to direct light onto the four separate CCD chips making up the camera, and so the relay mirrors on the replacement Wide Field and Planetary Camera 2 could be figured to correct the aberration. However, the other instruments lacked any intermediate surfaces which could be figured in this way, and so required an external correction device.

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. COSTAR was built by Ball Aerospace.

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. Despite the setbacks, the first three years saw numerous scientific advances as astronomers worked to optimise the results obtained using sophisticated image processing techniques such as deconvolution.

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. The seven astronauts selected for the mission were trained intensively in the use of the hundred or so specialised tools which would need to be used. The mission ( STS-61) Endeavour took place in December 1993, and involved installation of several instruments and other equipment over a total of 10 days.

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. NICMOS, built by Ball Aerospace contained a heat sink of solid nitrogen to reduce the thermal noise from the instrument, but shortly after it was installed, an unexpected thermal expansion resulted in part of the heat sink coming into contact with an optical baffle. This led to an increased warming rate for the instrument and reduced its original expected lifetime of 4.5 years to about 2 years.

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. The new computer was based on a radiation hardened Intel 486 and permits some computing tasks that were previously performed by computers on the ground to be handled on board the spacecraft.

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 ACS was built for NASA by Ball Aerospace. A new cooling system was installed which reduced the instrument's temperature enough for it to be usable again, although it was not as cold as its original design called for .

The mission replaced the solar arrays for a third time, with the new arrays being smaller but generating more power. The new arrays were derived from those built for the Iridium comsat system and were only two-thirds the size of the old arrays, resulting in less drag against the tenuous reaches of the upper atmosphere, while providing 30% more power. The additional power allowed all instruments on board the Hubble to be run simultaneously, and reduced a vibration problem that occurred when the old, less rigid arrays entered and left direct sunlight. Hubble's Power Distribution Unit was also replaced in order to correct a problem with sticky relays, a procedure that required the complete electrical power down of the spacecraft for the first time since it was launched.

The completion of this servicing mission considerably enhanced Hubble's capabilities. The two instruments primarily affected by the mission, ACS and NICMOS, together imaged the Hubble Ultra Deep Field in 2003 to 2004.

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. 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 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. 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.

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. The images reveal galaxies billions of light years away, and have generated a wealth of scientific papers, providing a new window on the early Universe.

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 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; there are no restrictions on nationality or academic affiliation. 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. Proposals are divided into several categories; 'general observer' proposals are the most common, covering routine observations. '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. Astronomers can apply to use DD time at any time of year, and it is typically awarded for study of unexpected transient phenomena such as supernovae. 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. It is situated in a low-Earth orbit so that it can be reached by the Space Shuttle for servicing missions, but this means that most astronomical targets are occulted by the Earth for slightly less than half of each orbit. 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. However, there is a so-called continuous viewing zone (CVZ), at roughly 90 degrees to the plane of Hubble's orbit, in which targets are not occulted for long periods. Due to the precession of the orbit, the location of the CVZ moves slowly over a period of eight weeks. Because the limb of the Earth is always within about 30° of regions within the CVZ, the brightness of scattered earthshine may be elevated for long periods during CVZ observations.

Because Hubble orbits in the upper atmosphere, its orbit changes over time in a way that is not accurately predictable. The density of the upper atmosphere varies according to many factors, and this means that Hubble's predicted position for six week's time could be in error by up to 4,000 km. Observation schedules are typically finalised only a few days in advance, as a longer lead time would mean there was a chance that the target would be unobservable by the time it was due to be observed.

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. The total time to be allocated was only a few hours per cycle, but excited great interest among amateur astronomers.

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. One such study was Transition Comets -- UV Search for OH Emissions in Asteroids. After that time, however, budget reductions at STScI made the support of work by amateur astronomers untenable, and no further amateur programs have been carried out.

Hubble data

Transmission to Earth

Hubble data are initially stored on the spacecraft. When launched, the storage facilities were old-fashioned reel-to-reel tape recorders, but these were replaced by solid state data storage facilities during servicing missions 2 and 3A. From the onboard storage facilities, data are transferred to the ground via the Tracking and Data Relay Satellite System, a system of satellites designed so that satellites in low-Earth orbit can communicate with their mission control facilities during about 85% of their orbit. Data is transmitted to the TDRSS ground station and then on to the Goddard Space Flight Centre for archiving.

Archive

All Hubble data are eventually made available via a public archive at http://archive.stsci.edu/hst. Data are usually proprietary—available only to the Principal Investigator (PI) and astronomers designated by the PI—for one year after being taken. The PI can apply to the director of the STScI to extend or reduce the proprietary period in some circumstances.

Observations made on Director's Discretionary Time are exempt from the proprietary period, and are released to the public immediately. Calibration data such as flat fields and dark frames are also publicly available straight away. All data in the archive are in the FITS format, which is suitable for astronomical analysis but not for public use. The Hubble Heritage Project processes and releases to the public a small selection of the most striking images in JPEG and TIFF formats.

Pipeline reduction

Astronomical data taken with CCDs must undergo several calibration steps before it is suitable for astronomical analysis. STScI has developed sophisticated software which automatically calibrates data when it is requested from the archive using the best calibration files available. This 'on-the-fly' processing means that large data requests can take a day or more to be processed and returned. The process by which data is calibrated automatically is known as 'pipeline reduction', and is increasingly common at major observatories.

Astronomers may if they wish retrieve the calibration files themselves and run the pipeline reduction software locally. This may be desirable when calibration files other than those selected automatically need to be used.

Data analysis

Hubble data can be analysed using many different packages, but STScI develops the custom-made STSDAS (Space Telescope Science Data Analysis System) software. The software contains all the programs needed to run pipeline reduction on raw data files, as well as many other astronomical image processing tools, tailored to the requirements of Hubble data. The software runs as a module of IRAF, a popular astronomical data reduction program.

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. The Hubble Heritage Project was established to produce high-quality images for public consumption of the most interesting and striking objects observed. The Heritage Team is composed of amateur and professional astronomers, as well as people with backgrounds outside astronomy, and emphasises the artistic nature of Hubble images.

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 colour 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 . A Heritage Project image of the nebula was released on 24 April 2001, the 11th anniversary of the launch.

Future

Equipment failure

Past servicing missions have exchanged old instruments for new ones, both avoiding failure and making possible new types of science. Without servicing missions, all of the instruments will eventually fail. On August 3, 2004, the power system of the Space Telescope Imaging Spectrograph (STIS) failed, rendering the instrument inoperable. The electronics had originally been fully redundant, but the first set of electronics failed in May 2001. It seems unlikely that any science functionality can be salvaged without a servicing mission.

Hubble uses gyroscopes to stabilize itself in orbit and point accurately and steadily at astronomical targets. Normally, three gyroscopes are required for operation; observations are still possible with two gyros, but the area of sky that can be viewed would be somewhat restricted, and observations requiring very accurate pointing would be more difficult. 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. The switch to this mode was made on August 31, 2005, leaving Hubble with two gyroscopes in use and two on backup. 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. A robotic servicing mission including this would be tricky, as it requires many operations, and a failure in any might result in irreparable damage to Hubble. However, the observatory was designed so that during Shuttle servicing missions it would receive power from a connection to the Space Shuttle, and this fact may be utilized by adding an external power source (an additional battery) rather than changing the internal ones.

On 25 June 2006 the main camera (the ACS) stopped working. The third-generation instrument had been installed by Columbia space shuttle crew in 2002. It was built with a redundant set of electronics, which was brought into use on 30 June, and science operations resumed on 4 July. On 23 September 2006, the ACS again failed, though by 9 October 2006 the problem had been diagnosed and resolved.

Orbital decay

Hubble orbits the Earth in the extremely tenuous upper atmosphere, and over time its orbit decays due to drag. If it is not re-boosted by a shuttle or other means, it will re-enter the Earth's atmosphere sometime between 2010 and 2032, with the exact date depending on how active the Sun is and its impact on the upper atmosphere. The state of Hubble's gyros also impacts the re-entry date, as a controllable telescope can be made to minimize atmospheric drag. Not all of the telescope would burn up on re-entry. Parts of the main mirror and its support structure would probably survive, leaving the potential for damage or even human fatalities (estimated at up to a 1 in 700 chance of human fatality for a completely uncontrolled re-entry).

Addition of an external propulsion module to allow controlled re-entry is currently being investigated by NASA. It would not have to be executed until the expected natural re-entry date, potentially as late as 2030.

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. The problems with this method are the cost of a shuttle flight (about US$500 million by some estimates) and the mandate to retire the space shuttles by 2010 and risk to a shuttle's crew.

Debate over final servicing mission

The Space Shuttle Columbia was originally scheduled to visit Hubble again in February 2005. The tasks of this servicing mission would include replacing a fine guidance sensor and two broken gyroscopes, placing protective "blankets" on top of torn insulation, replacing the Wide Field and Planetary Camera 2 with a new Wide Field Camera 3 and installing the Cosmic Origins Spectrograph (COS). However, then-NASA Administrator Sean O'Keefe decided that, in order to prevent a repeat of the Columbia disaster, all future shuttles must be able to reach the 'safe-haven' of the International Space Station (ISS) should an in-flight problem develop which would preclude the shuttle from landing safely. The shuttle is incapable of reaching both HST and ISS during the same mission, and so future manned service missions were cancelled.

This decision was assailed by numerous astronomers, who felt that the Hubble telescope was valuable enough to merit the risk; its successor the JWST will not be ready until well after the 2010 deadline to stop shuttle flights, and whilst Hubble can image in the ultraviolet, JWST is limited to the infra-red. However, many astronomers feel strongly that the servicing of Hubble should not take place if the costs of the servicing come from the JWST budget. 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". On August 11, 2004, Sean O'Keefe requested the Goddard Space Flight Centre to prepare a detailed proposal for a robotic service mission. It is expected that the proposal will take 12 months to produce—any such mission, likely to cost in excess of $1 billion, will not take place before 2007.

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. Griffin has stated that he will reconsider the possibility of a manned servicing mission. Soon after his appointment, he authorized NASA's Goddard Space Flight Centre 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. At the same time, Griffin canceled plans for a robotic rescue mission, calling it "not feasible."

On October 31, 2006 the final go-ahead was given for the mission by NASA Administrator Mike Griffin. The 11-day mission tentatively scheduled for launch during the spring to fall of 2008, will install fresh batteries, fix the guidance system, and install Wide Field Camera 3 and the Cosmic Origin Spectrograph. Both instruments have been developed and built by Ball Aerospace, Boulder, Colorado. With the installation of the Wide Field Camera 3 and the Cosmic Origin Spectrograph, all of the instruments on board HST will be Ball Aerospace-built.

Hubble Origins Probe

NASA and ESA are currently investigating building a follow-on to the Hubble Space Telescope called the Hubble Origins Probe . If approved, it would not be ready for launch until 2010. The probe would probably use an Atlas V rocket for its ride to orbit. It would also incorporate new technology into its design to reduce its weight with respect to the original. The mission would be a one-time five-year run and would receive no servicing from the Space Shuttle. The mission is still being debated and is currently unfunded.