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Hubble Space Telescope

2007 Schools Wikipedia Selection. Related subjects: Space transport

   The NASA/ESA Hubble Space Telescope
   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

   Lyman Spitzer, "father" of the Space Telescope.
   Enlarge
   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. 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

   Polishing of Hubble's primary mirror begins at Perkin-Elmer
   corporation, Danbury, Connecticut, May 1979. The engineer pictured is
   Dr. Martin Yellin, an optical engineer working for Perkin-Elmer on the
   project.
   Enlarge
   Polishing of Hubble's primary mirror begins at Perkin-Elmer
   corporation, Danbury, Connecticut, May 1979. The engineer pictured is
   Dr. Martin Yellin, an optical engineer working for Perkin-Elmer on the
   project.

   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

   Early stages of Hubble's construction, 1980.
   Enlarge
   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
   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

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

   An extract from a WF/PC image shows the light from a star spread over a
   wide area instead of being concentrated on a few pixels.
   Enlarge
   An extract from a WF/PC image 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 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.
   Astronauts work on Hubble during the first servicing mission.
   Enlarge
   Astronauts work on Hubble during the first servicing mission.
   Improvement in Hubble images after the first service mission.
   Enlarge
   Improvement in Hubble images after the first service mission.

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.
   Hubble on the payload bay just prior to release with beautiful glowing
   color of earth in the background. SM3B : STS-109.
   Enlarge
   Hubble on the payload bay just prior to release with beautiful glowing
   colour of earth in the background. SM3B : STS-109.

   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

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

   Distant galaxies in deep space in the Hubble Ultra Deep Field
   photograph.
   Enlarge
   Distant galaxies in deep space in the Hubble Ultra Deep Field
   photograph.

   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

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

   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

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

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