Hubble Space Telescope

From Academic Kids

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The Hubble Space Telescope

The Hubble Space Telescope, from the Space Shuttle Discovery during the second servicing mission, STS-82
OrganizationNASA, ESA
Wavelength regimeoptical, ultraviolet, near-infrared
Orbit height600 km
Orbit period97 min
Launch date24 April 1990
Deorbit datecirca 2010
Mass11,000 kg
Webpagehttp://hubble.nasa.gov
Physical characteristics
Telescope stylereflector
Diameter2.4 m
Collecting areaapprox. 4.3 m2
Effective focal Length57.6 m (189 ft)
Instruments
NICMOScamera/spectrometer
ACSsurvey camera
WFPC2wide field camera
STISspectrometer/camera (failed)

The Hubble Space Telescope is one of the most important telescopes in the history of astronomy. Since its launch in 1990 it has been responsible for many ground-breaking observations and has helped astronomers achieve a better understanding of many fundamental problems in astrophysics.

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 future of Hubble is currently uncertain. Its final servicing mission was cancelled following the Space Shuttle Columbia disaster, and without intervention it will re-enter the Earth's atmosphere some time after 2010. Its successor telescope, the James Webb Space Telescope, is due to be launched in 2012.

Contents

Conception, design and aims

Proposals and precursors

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Lyman Spitzer, 'father' of the Space Telescope

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. Ground-based telescopes are typically 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 US 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 NASA's launch of the first Orbiting Astronomical Observatory (OAO) mission. OAO-1's battery failed after three days, terminating the mission, but OAO-2 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, 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 re-usable 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 1.5m 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 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

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Polishing of Hubble's primary mirror begins at Perkin-Elmer corporation, Danbury, Connecticut, May 1979

Once the Space Telescope project had been given the go-ahead, work on the program was divided between many institutions. Marshall Space Flight Center 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 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 were 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 were commissioned to construct a back-up mirror using traditional mirror-polishing techniques. Construction of the 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 aluminum 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 USD 1.175 billion.

Spacecraft systems

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Early stages of Hubble's construction, 1980

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. It was not part of NASA, but was instead attached to Johns Hopkins University in Baltimore, after something of a power struggle between NASA and the scientific community at large. STSci was to become 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. The Space Telescope European Coordinating Facility was established at Garching bei M? 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 Space Shuttle Challenger disaster brought the US 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 the Space Shuttle 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 US expenditure estimated at between 4.5 to 6 billion USD and Europe's financial contribution at 593 million Euros (1999 estimate) [1] (http://www.spacetelescope.org/about/faq.html).

Instruments

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Shuttle mission STS-31 lifts off, carrying Hubble into orbit

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 (FOC). 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 Center 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. 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.

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 must be that the 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 edges of a mirror reaches a different focus to the light reflecting off the 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

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An extract from a WF/PC shows the light from a star spread over a wide area instead of being concentrated on a few pixels

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 was established to determine how the error could have arisen, and was headed by Lew Allen, director of the Jet Propulsion Laboratory. 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 retain data about as good as 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, or 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 focussed 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. Despite the setbacks, the first three years saw numerous scientific advances as astronomers worked to optimise the results obtained using sophisticated image processing techniques.

Servicing missions and new instruments

Servicing mission 1

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Astronauts work on Hubble during the first servicing mission

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 100 or so specialised tools which would need to be used. The mission (STS-61) took place in December 1993, and over a total of 10 days installed several instruments and other equipment.

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, and of course 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 (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 contained a heat sink of solid nitrogen to reduce the infrared '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 (STS-103) took place in December 1999, and 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 (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. 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 also 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, more rigid arrays entered and left direct sunlight.

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

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One of Hubble's most famous images: pillars of creation where stars are forming in the Eagle Nebula

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 accurate measurements made since Hubble's launch using other techniques.

While Hubble helped to refine the age of the universe, it also threw doubt on its future. Astronomers using the telescope to observe distant supernovae uncovered evidence that far from decelerating under the influence of gravity, the universe may in fact be accelerating. This acceleration was later confirmed by other ground-based and space-based telescopes, 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.

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.

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. The development of adaptive optics in recent years means that ground-based telescopes can take images approaching the sharpness of Hubble images, 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

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Hubble's low orbit means many targets spend much of each orbit behind the Earth

Scheduling observations for Hubble is not a simple matter. Because it is situated in a low-Earth orbit, most astronomical targets are occulted by the Earth for slightly less than half of each orbit. Observations do not take place when the telescope passes through the South Atlantic Anomaly due to elevated radiation levels, and there is a also a sizable exclusion zone around the Sun, 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. 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 DD 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. 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 is 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 is 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 Center for archiving.

Archive

All Hubble data is eventually made available via a public archive at http://archive.stsci.edu/hst. Data are usually proprietory - available only to the Principlal Investigator 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 proprietory period in some circumstances.

Observations made on Director's Discretionary Time are exempt from the proprietory 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, which runs only under various flavours of Linux, and Mac OS X.

Outreach activities

The , selected by the public to be observed by Hubble for its 11th anniversary
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The Horsehead Nebula, selected by the public to be observed by Hubble for its 11th anniversary

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 operation 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 as well as 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 US 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 [2] (http://heritage.stsci.edu/2001/12/caption.html). A Heritage Project image of the nebula was released on 24 April 2001, the 11th anniversary of the launch.

The future

Equipment failure

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A WFPC2 image of a small region of the Tarantula Nebula in the Large Magellanic Cloud

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. As of 2005, astronomers estimate that the telescope has about a 50% chance of having to move into two-gyro mode before the end of Cycle 14 in June 2006. Any further gyroscope failures after that would render the telescope 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 [3] (http://news.bbc.co.uk/2/hi/science/nature/3652627.stm).

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 reboosted by a shuttle or other means, it will reenter 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 impact the reentry date, as a controllable telescope can be made to minimize atmospheric drag. Not all of the telescope would burn up on reentry. Parts of the main mirror and its support structure would probably survive, leaving the potential for damage or even human fatalies (estimated at up to a 1 in 700 chance of human fatality for a completely uncontrolled reentry).

Addition of an external propulsion module to allow controlled re-entry, and this option is currently being investigated by NASA. It would not have to be executed until the expected natural reentry date, after Hubble has completed its operational lifetime. One potential model involves a Pac-Man shaped unit entirely enclosing the satellite. Alternatively, instead of being used to control re-entry, the propulsion module could boost the telescope into a much higher orbit, in which it could remain indefinitely.

Another possibility for safely de-orbiting Hubble is retrieval by a space shuttle. The Hubble telescope would then most likely be displayed in the Smithsonian Institution. The problems with this method are the cost of a shuttle flight (about $500 million by some estimates) and risk to a shuttle's crew. In the wake of the Space Shuttle Columbia disaster, NASA's astronaut office is wary of risking a shuttle crew simply to retrieve a museum-bound telescoe [4] (http://www.space.com/businesstechnology/technology/hubble_grunsfeld_0306731.html). Also, this mission would require a rebuild of the cargo space of the space shuttle sent to retrieve Hubble, since the only space shuttle unmodified since Hubble's launch (and therefore able to hold it in its cargo space) was the destroyed Columbia shuttle.

Will Hubble be serviced one final time?

The Space Shuttle was originally scheduled to visit Hubble again in February 2005. The tasks of this servicing mission would include adding fresh gyroscopes and replacing the Wide Field and Planetary Camera 2 with a new Wide Field Camera 3. However, then-NASA Administrator Sean O'Keefe decided that, in order to prevent a repeat of the Space Shuttle Columbia disaster, all future shuttles must be inspected externally on orbit before reentry, a task which cannot be done without the facilities of the International Space Station (ISS). 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. On 29 January 2004, Sean O'Keefe said that 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 Center 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 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. At the same time, Griffin decided to cancel the plans for a robotic rescue mission, calling it "not feasible." [5] (http://www.washingtonpost.com/wp-dyn/content/article/2005/04/12/AR2005041201646.html)

References

  1. Benn C.R., Sᮣhez S.F. (2001), Scientific Impact of Large Telescopes, Publications of the Astronomical Society of the Pacific, v. 113, p.385
  2. Bless R.C., Walter L.E., White R.L. (1992), High Speed Photometer Instrument Handbook, v 3.0, STSci
  3. Brandt J.C. et al (1994), The Goddard High Resolution Spectrograph: Instrument, goals, and science results, Publications of the Astronomical Society of the Pacific, v. 106, p. 890-908
  4. Burrows C.J. et al (1991), The imaging performance of the Hubble Space Telescope, Astrophysical Journal, v.369, p.21
  5. Dunar A.J., Waring S.P. (1999), Power To Explore -- History of Marshall Space Flight Center 1960-1990, US Government Printing Office, ISBN 0160589924 (Chapter 12, Hubble Space telescope: [6] (http://history.msfc.nasa.gov/book/chpttwelve.pdf)}
  6. Jedrzejewski R.I., Hartig G., Jakobsen P., Crocker J.H., Ford H. C. (1994), In-orbit performance of the COSTAR-corrected Faint Object Camera, Astrophysical Journal Letters, v. 435, p. L7-L10
  7. O'Meara S. (1997), The Demise of the HST Amateur Program, Sky and Telescope, June 1997, p.97.
  8. Sembach, K. R., et al. 2004, HST Two-Gyro Handbook, Version 1.0, (Baltimore: STScI)
  9. Spitzer, Lyman S (1979), History of the Space Telescope, Quarterly Journal of the Royal Astronomical Society, v. 20, p. 29
  10. Trauger J.T., Ballester G.E., Burrows C.J., Casertano S., Clarke J.T., Crisp D. (1994), The on-orbit performance of WFPC2, Astrophysical Journal Letters, v. 435, p. L3-L6
  11. HST Primer for Cycle 14, (2004), eds Diane Karakla, Editor and Susan Rose, Technical Editor
  12. Hubble Space Telescope Call for Proposals for Cycle 14, (2004), eds. Neill Reid and Jim Younger
  13. Selected Documents in the History of the U.S. Civil Space Program Volume V: Exploring the Cosmos, (2001), John M. Logsdon, Editor
  14. STSCi newsletter, v. 20, issue 2, Spring 2003

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