hubble
Autor: eu | Publicado: 2012-01-07T13:42:00-08:00


The Hubble Space Telescope (HST) is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter (7.9&nbsp;ft) aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared. The telescope is named after the astronomer Edwin Hubble.

Hubble's orbit outside the distortion of Earth's atmosphere allows it to take extremely sharp images with almost no background light. Hubble's Ultra Deep Field image, for instance, is the most detailed visible-light image ever made of the universe's most distant objects. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.

Although not the first space telescope, Hubble is one of the largest 
and most versatile, and is well known as both a vital research tool and a
 public relations boon for astronomy. The HST was built by the United States space agency NASA, with contributions from the European Space Agency, and is operated by the Space Telescope Science Institute. The HST is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.[5]

Space telescopes were proposed as early as 1923. Hubble was funded in
 the 1970s, with a proposed launch in 1983, but the project was beset by
 technical delays, budget problems, and the Challenger disaster.
 When finally launched in 1990, scientists found that the main mirror 
had been ground incorrectly, significantly compromising the telescope's 
capabilities. However, after a servicing mission in 1993, the telescope 
was restored to its intended quality.

Hubble is the only telescope designed to be serviced in space by 
astronauts. Between 1993 and 2002, four missions repaired, upgraded, and
 replaced systems on the telescope, but a fifth mission was canceled on 
safety grounds following the Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved one final servicing mission, completed in 2009. The telescope is now expected to function until at least 2014. Its scientific successor, the James Webb Space Telescope (JWST), is to be launched in 2018 or possibly later.

Proposals and precursors
In 1923, Hermann Oberth—considered along with Robert H. Goddard and Konstantin Tsiolkovsky fathers of modern rocketry—published Die Rakete zu den Planetenräumen ("The Rocket into Planetary Space"), which mentioned how a telescope could be propelled into Earth orbit by a rocket.[6]

The history of the Hubble Space Telescope can be traced back as far as 1946, when the astronomer Lyman Spitzer wrote the paper "Astronomical advantages of an extraterrestrial observatory".[7] 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&nbsp;arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05&nbsp;arcsec for a telescope with a mirror 2.5&nbsp;m in diameter. Second, 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 United States 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.[8]

Space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. The first ultraviolet spectrum of the Sun was obtained in 1946,[9] and NASA launched the Orbiting Solar Observatory to obtain UV, X-ray, and gamma-ray spectra in 1962.[10] An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, and in 1966 National Aeronautics and Space Administration (NASA) launched 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.[11]

The OSO and 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&nbsp;m in diameter, known provisionally as the Large 
Orbiting Telescope or Large Space Telescope (LST), with a launch slated 
for 1979. These plans emphasized the need for manned maintenance 
missions to the telescope to ensure such a costly program had a lengthy 
working life, and the concurrent development of plans for the reusable space shuttle indicated that the technology to allow this was soon to become available.[12]


[edit] 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 
scientific 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 led to Congress cutting all funding for the telescope project.[13]

In response to this, a nationwide lobbying effort was coordinated among astronomers. Many astronomers met congressmen and senators
 in person, and large scale letter-writing campaigns were organized. The
 National Academy of Sciences published a report emphasizing the need 
for a space telescope, and eventually the Senate agreed to half of the 
budget that had originally been approved by Congress.[14]

The funding issues led to something of a reduction in the scale of 
the project, with the proposed mirror diameter reduced from 3&nbsp;m to 
2.4&nbsp;m, both to cut costs [15]
 and to allow a more compact and effective configuration for the 
telescope hardware. A proposed precursor 1.5&nbsp;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 one of the first generation instruments for the telescope, as well as the solar cells
 that would power it, and staff to work on the telescope in the United 
States, in return for European astronomers being guaranteed at least 15%
 of the observing time on the telescope.[16] 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.[14] In 1983 the telescope was named[17] after Edwin Hubble, who made one of the greatest scientific breakthroughs of the 20th century when he discovered that the universe is expanding.[18]


[edit] Construction and engineering








Polishing of Hubble's primary mirror begins at Perkin-Elmer corporation, Danbury, Connecticut, March 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 among many institutions. Marshall Space Flight Center (MSFC) was given responsibility for the design, development, and construction of the telescope, while the Goddard Space Flight Center was given overall control of the scientific instruments and ground-control center for the mission.[19] MSFC commissioned the 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 and integrate the spacecraft in which the telescope would be housed.[20]


[edit] Optical Telescope Assembly (OTA)
Optically, the HST is a Cassegrain reflector of Ritchey-Chrétien design,
 as are most large professional telescopes. This design, with two 
hyperbolic mirrors, is known for good imaging performance over a wide 
field of view, with the disadvantage that the mirrors have shapes that 
are hard to fabricate and test. The mirror and optical systems of the 
telescope determine the final performance, and they were designed to 
exacting specifications. Optical telescopes typically have mirrors 
polished to an accuracy of about a tenth of the wavelength of visible light,
 but the Space Telescope was to be used for observations from the 
visible through the ultraviolet (shorter wavelengths) and was specified 
to be diffraction limited to take full advantage of the space environment. Therefore its mirror needed to be polished to an accuracy of 10&nbsp;nanometers, or about 1/65 of the wavelength of red light.[21]
 On the long wavelength end, the OTA was not designed with optimum IR 
performance in mind&nbsp;— for example, the mirrors are kept at stable (and 
warm, about 15&nbsp;°C) temperatures by heaters. This limits Hubble's 
performance as an infrared telescope.[22]

Perkin-Elmer intended to use custom-built and extremely sophisticated computer-controlled polishing machines to grind the mirror to the required shape.[20] However, in case their cutting-edge technology ran into difficulties, NASA demanded that PE sub-contract to Kodak to construct a back-up mirror using traditional mirror-polishing techniques.[23] (The team of Kodak and Itek
 also bid on the original mirror polishing work. Their bid called for 
the two companies to double-check each other's work, which would have 
almost certainly caught the polishing error that later caused such problems.[24]) The Kodak mirror is now on permanent display at the Smithsonian Institution.[25][26] An Itek mirror built as part of the effort is now used in the 2.4&nbsp;m telescope at the Magdalena Ridge Observatory.[27]

Construction of the Perkin-Elmer mirror began in 1979, starting with a blank manufactured by Corning from their 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. Perkin-Elmer simulated microgravity
 by supporting the mirror on both sides with 138 rods that exerted 
varying amounts of force. This ensured that the mirror's final shape 
would be correct and to specification when finally deployed. Mirror 
polishing 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.[28] The mirror was completed by the end of 1981; it was washed using 2,400 gallons (9,100 L) of hot, deionized water and then received a reflective coating of aluminum 65&nbsp;nm-thick and a protective coating of magnesium fluoride 25&nbsp;nm-thick.[22][29]









Construction of Hubble. The optical metering truss and secondary baffle are visible.


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.[30]


[edit] Spacecraft systems
The spacecraft in which the telescope and instruments were to be 
housed was another major engineering challenge. It would have to 
adequately withstand frequent passages from direct sunlight into the 
darkness of Earth's shadow,
 which would generate major changes in temperature, while being stable 
enough to allow extremely accurate pointing of the telescope. A shroud 
of multi-layer insulation
 keeps the temperature within the telescope stable, and surrounds a 
light aluminum shell in which the telescope and instruments sit. Within 
the shell, a graphite-epoxy frame keeps the working parts of the telescope firmly aligned.[31] Because graphite composites are hygroscopic,
 there was a risk that water vapor absorbed by the truss while in 
Lockheed's clean room would later be expressed in the vacuum of space; 
the telescope's instruments would be covered in ice. To reduce that 
risk, a nitrogen gas purge was performed prior to launching the 
telescope into space.[32]









Exploded view of the Hubble Telescope.


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.[33]


[edit] Initial instruments

Main articles: Wide Field and Planetary Camera, Goddard High Resolution Spectrograph, High Speed Photometer, Faint Object Camera, and Faint Object Spectrograph
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 eight charge-coupled device
 (CCD) chips divided between two cameras, each using four CCDs. The 
"wide field camera" (WFC) covered a large angular field at the expense 
of resolution, while the "planetary camera" (PC) took images at a longer
 effective focal length than the WF chips, giving it a greater magnification.[34]

The GHRS was a spectrograph designed to operate in the ultraviolet. It was built by the Goddard Space Flight Center and could achieve a spectral resolution of 90,000.[35]
 Also optimized for ultraviolet observations were the FOC and FOS, which
 were capable of the highest spatial resolution of any instruments on 
Hubble. Rather than CCDs these three instruments used photon-counting digicons as their detectors. The FOC was constructed by ESA, while the University of California, San Diego and the Martin Marietta corporation built the FOS.[34]

The final instrument was the HSP, designed and built at the University of Wisconsin–Madison. It was optimized 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.[36]

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&nbsp;arcseconds have been achieved.[37]


[edit] Ground support








Hubble's low orbit means many targets are visible for somewhat less than
 half of elapsed time, since they are blocked from view by the Earth for one-half of each orbit.



Main article: Space Telescope Science Institute
The Space Telescope Science Institute (STSI)
 is responsible for the scientific operation of the telescope and 
delivery of data products to astronomers. STScI is operated by the Association of Universities for Research in Astronomy (AURA) and is physically located in Baltimore, Maryland on the Homewood campus of Johns Hopkins University, one of the 33 US universities and 7 international affiliates that make up the AURA consortium. STScI was established in 1981 [38]
 after something of a power struggle between NASA and the scientific 
community at large. NASA had wanted to keep this function "in-house", 
but scientists wanted it to be based in an academic establishment.[39][40] The Space Telescope European Coordinating Facility (ST-ECF), established at Garching bei München near Munich in 1984, provides similar support for European astronomers.

One rather complex task that falls to STScI is scheduling observations for the telescope.[41]
 Hubble 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. The solar avoidance angle is about 50°, which is 
specified to keep sunlight from illuminating any part of the OTA. Earth 
and Moon avoidance is to keep bright light out of the FGSs and to keep 
scattered light from entering the instruments. If the FGSs are turned 
off, however, the Moon and Earth can be observed. Earth observations 
were used very early in the program to generate flat-fields for the 
WFPC1 instrument. There is a so-called continuous viewing zone (CVZ), at
 roughly 90&nbsp;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 weeks' time could be in error by up 
to 4,000&nbsp;km. Observation schedules are typically finalized 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.[42]

Engineering support for HST is provided by NASA and contractor personnel at the Goddard Space Flight Center in Greenbelt, Maryland,
 48&nbsp;km south of the STScI. Hubble's operation is monitored 24&nbsp;hours per 
day by four teams of flight controllers who make up Hubble's Flight 
Operations Team.[43]


[edit] Challenger disaster, delays, and eventual launch
By early 1986, the planned launch date of October that year looked feasible, but the Challenger accident
 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. The telescope had to be kept in a clean room, powered up and 
purged with nitrogen, until a launch could be rescheduled. This costly 
situation (about $6 million per month) pushed the overall costs of the 
project even higher. This delay did allow time for engineers to perform 
extensive tests, swap out a possibly failure-prone battery, and make 
other improvements.[44]
 Furthermore, the ground software needed to control Hubble was not ready
 in 1986, and in fact was barely ready by the 1990 launch.[45]









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


Eventually, following the resumption of shuttle flights in 1988, the 
launch of the telescope was scheduled for 1990. On April 24, 1990, 
shuttle mission STS-31 saw Discovery launch the telescope successfully into its planned orbit.[46]

From its original total cost estimate of about US$400 million, the 
telescope had by now cost over $2.5 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 and $6 billion, 
and Europe's financial contribution at €593 million (1999 estimate).[47]


[edit] Flawed mirror
Within weeks of the launch of the telescope, the returned images 
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 (PSF) concentrated within a circle 0.1&nbsp;arcsec in diameter as had been specified in the design criteria.[48] The detailed performance is shown in graphs from STScI illustrating the mis-figured PSFs compared to post-correction and ground-based PSFs.[49]

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 precisely figured mirror ever made, with 
variations from the prescribed curve of only 10 nanometers,[21] it was too flat at the edges by about 2200 nanometers (2.2 micrometres).[50] This 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 center.[51]









Hubble is deployed from Discovery.


The effect of the mirror flaw on scientific observations depended on 
the particular observation—the core of the aberrated PSF was sharp 
enough to permit high-resolution observations of bright objects, and 
spectroscopy was largely unaffected. However, the loss of light to the 
large, out of focus halo severely reduced the usefulness of the 
telescope for faint objects or high-contrast imaging. This meant that 
nearly all of the cosmological programs were essentially impossible, 
since they required observation of exceptionally faint objects.[51] NASA and the telescope became the butt of many jokes, and the project was popularly regarded as a white elephant. (For instance, in the 1991 comedy The Naked Gun 2½: The Smell of Fear, the Hubble was pictured with the Titanic, the Hindenburg, and the Edsel).[52]
 Nonetheless, during the first three years of the Hubble mission, before
 the optical corrections, the telescope still carried out a large number
 of productive observations. The error was well characterized and 
stable, enabling astronomers to optimize the results obtained using 
sophisticated image processing techniques such as deconvolution.[53]


[edit] 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.










The correctly ground backup mirror built by Eastman Kodak for the Hubble
 space telescope (the mirror was never coated with a reflective surface,
 hence its inner support structure can be seen). It now resides in the National Air and Space Museum in Washington, DC.[54]


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 main null corrector, a device used to measure the exact shape of the mirror, had been incorrectly assembled—one lens was wrongly spaced by 1.3&nbsp;mm.[55]
 During the polishing of the mirror, Perkin-Elmer had analyzed its 
surface with two other null correctors, both of which correctly 
indicated that the mirror was suffering from spherical aberration.
 The company ignored these test results, as it believed that the two 
null correctors were less accurate than the primary device that was 
reporting that the mirror was perfectly figured.[56]

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 did not review or 
supervise the mirror construction adequately, did not assign its best 
optical scientists to the project (as it had for the prototype), and in 
particular did not involve the optical designers in the construction and
 verification of the mirror. While the commission heavily criticized 
Perkin-Elmer for these managerial failings, NASA was also criticized for
 not picking up on the quality control shortcomings, such as relying 
totally on test results from a single instrument.[57]


[edit] Design of a solution








COSTAR on display at the National Air and Space Museum


The design of the telescope had always incorporated servicing 
missions, and astronomers immediately began to seek potential solutions 
to the problem that could be applied at the first servicing mission, 
scheduled for 1993. While Kodak and Itek had each ground back-up mirrors
 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.[58][59]

The first step was a precise characterization of the error in the 
main mirror. Working backwards from images of point sources, astronomers
 determined that the conic constant of the mirror as built was −1.01390±0.0002, instead of the intended −1.00230.[60][61] The same number was also derived by analyzing the null corrector used by Perkin-Elmer to figure the mirror, as well as by analyzing interferograms obtained during ground testing of the mirror.[62]

Because of the way the HST's instruments were designed, two different sets of correctors were required. The design of the Wide Field and Planetary Camera 2, already planned to replace the existing WF/PC, included relay mirrors to direct light onto the eight separate CCD
 chips making up its two cameras. An inverse error built into their 
surfaces could completely cancel the aberration of the primary. However,
 the other instruments lacked any intermediate surfaces that could be 
figured in this way, and so required an external correction device.[63]

The Corrective Optics Space Telescope Axial Replacement
 (COSTAR) system was designed to correct the spherical aberration for 
light focused at the FOC, FOS, and GHRS. It consists of two mirrors in 
the light path with one ground to correct the aberration.[64] 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.[63] By 2002 all of the original instruments requiring COSTAR had been replaced by instruments with their own corrective optics.[65]
 COSTAR was removed and returned to Earth in 2009 where it is exhibited 
at the National Air and Space Museum. The area previously used by COSTAR
 is now occupied by the Cosmic Origins Spectrograph.


[edit] Servicing missions and new instruments


















[edit] Servicing Mission 1









Astronauts Musgrave and Hoffman installing corrective optics during SM1






Improvement in Hubble images after SM1












Astronauts replacing gyroscopes during SM3A






Hubble on the payload bay just prior to release during SM3B




Main article: STS-61
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 more 
specialized tools that would be needed.[66] The Space Shuttle&nbsp;Endeavour mission STS-61 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,
 to compensate for the orbital decay from 3 years of drag in the tenuous upper atmosphere.[50]

On January 13, 1994, NASA declared the mission a complete success and showed the first of many much sharper images.[67] At the time, the mission had been one of the most complex ever undertaken, involving five lengthy periods of extra-vehicular 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.


[edit] Servicing Mission 2

Main article: STS-82
Servicing Mission 2, flown by 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.[68] NICMOS 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.[69]


[edit] Servicing Mission 3A

Main article: STS-103
Servicing Mission 3A flown by Discovery (STS-103),
 took place in December 1999, and was a split-off from Servicing Mission
 3 after three of the six onboard gyroscopes had failed. (A fourth 
failed a few weeks before the mission, rendering the telescope incapable
 of performing science observations.) The mission replaced all six 
gyroscopes, replaced a Fine Guidance Sensor
 and the computer, installed a Voltage/temperature Improvement Kit (VIK)
 to prevent battery overcharging, and replaced thermal insulation 
blankets.[70] Although the new computer is hardly a powerhouse (a 25&nbsp;MHz radiation hardened Intel 486 with two megabytes of RAM), it is still 20 times faster, with six times more memory, than the DF-224
 it replaced. The new computer increases throughput by moving some 
computing tasks from the ground to the spacecraft, and saves money by 
allowing the use of modern programming languages.[71]


[edit] Servicing Mission 3B

Main article: STS-109
Servicing Mission 3B flown by Columbia (STS-109) in March 2002 saw the installation of a new instrument, with the FOC (the last original instrument) being replaced by the Advanced Camera for Surveys
 (ACS). This meant that COSTAR was no longer required, since all new 
instruments had correction for the main mirror aberration built in.[65]

The mission also saw the revival of NICMOS, which had run out of coolant
 in 1999. A new cooling system was installed that reduced the 
instrument's temperature enough for it to be usable again. Although not 
as cold as its original design called for, the temperature is more 
stable, in many ways a better tradeoff.[69] ACS in particular enhanced Hubble's capabilities; it and the revived NICMOS together imaged the Hubble Ultra Deep Field.

The mission replaced the solar arrays for the second time. 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 percent more power. The additional power 
allowed all instruments on board the HST 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.[72]




[edit] Servicing Mission 4

Main article: STS-125




Wikinews has related news: NASA launches Space Shuttle Atlantis










Astronauts work on Hubble during SM4










Hubble floats free from Atlantis after SM4.


Servicing Mission 4 (SM4), which took place in May 2009, was the last scheduled shuttle mission (STS-125) for the Hubble Space Telescope.[73] The mission was first planned for October 14, 2008.[74]
 However on September 27, 2008, the Science Instrument Command and Data 
Handling (SI C&amp;DH) unit on HST failed. All science data pass through
 this unit before they can be transmitted to Earth. Although it had a 
backup unit, if the backup were to fail, the HST's useful life would be 
over.[75]
 Therefore on September 29, 2008, NASA announced that the launch of SM4 
would be postponed until 2009 so the SI C&amp;DH unit could be replaced 
as well.[76] SM4, with a replacement SI C&amp;DH unit,[77] was launched aboard Space Shuttle&nbsp;Atlantis on May 11, 2009.[78]

On SM4, astronauts, over the course of five spacewalks, installed two new instruments, Wide Field Camera 3 (WFC3), and the Cosmic Origins Spectrograph
 (COS). WFC3 will increase Hubble's observational capabilities in the 
ultraviolet and visible spectral ranges by up to 35 times due to its 
higher sensitivity and wider field of view. The telephone-booth sized 
COS assembly replaced the Corrective Optics Space Telescope Axial 
Replacement (COSTAR) that was installed in 1993 to correct Hubble's 
spherical aberration problems. (COSTAR was no longer needed after the 
replacement of the FOC in 2002, the last original instrument without the necessary correction built in.[65])
 The COS will do observations in the ultraviolet parts of the spectrum, 
complementing the measurements done by the repaired STIS system.

The mission repaired two instruments that had failed, the Advanced Camera for Surveys (ACS) and the Space Telescope Imaging Spectrograph
 (STIS). The astronauts also performed other component replacements, 
including all three Rate Sensor Units (each containing two gas-bearing 
gyroscopes); one of three Fine Guidance Sensor (FGS) units used to help 
keep pointing accuracy and increase platform stability; the SI C&amp;DH 
unit; all six of the 125-pound (57&nbsp;kg) nickel-hydrogen batteries used to
 provide all Hubble's electrical power to support operations during the 
night portion of its orbit; and three New Outer Blanket Layer (NOBL) 
thermal insulation protective blankets. The batteries had never been 
replaced and were more than 13 years over their original design life.[79]

Atlantis released the Hubble Space Telescope on May 19, 2009 
back into space after all repairs were successfully made. After testing 
and calibration, Hubble resumed routine operation in September 2009.[80] These efforts are expected to keep the telescope fully functioning at least into 2014, and perhaps longer.[81]

Hubble was originally designed to be returned to earth on board a shuttle. With the retirement of the shuttle fleet,
 in July 2011, this is no longer possible. NASA engineers developed the 
Soft Capture and Rendezvous System (SCRS), a ring-like device that was 
attached to Hubble’s aft bulkhead during SM4, which will enable the 
future rendezvous, capture, and safe disposal of Hubble by either a crewed or robotic mission.[82] NASA is currently (as of 2011) studying the best method for deorbiting Hubble, projected to be needed around 2025.[83]




[edit] Scientific results

[edit] Key Projects
In the early 1980s, NASA and StScI convened four panels to discuss 
Key Projects. These were projects that were both scientifically 
important and would require significant telescope time, which would be 
explicitly dedicated to each project. This guaranteed that these 
particular projects would be completed early, in case the telescope 
failed sooner than expected. The panels identified three such projects: 
(1) a study of the nearby intergalactic medium using quasar absorption 
lines to determine the properties of the intergalactic medium and the 
gaseous content of galaxies and groups of galaxies;[84] (2) a medium deep survey using the Wide Field Camera to take data whenever one of the other instruments was being used[85] and (3) a project to determine the Hubble Constant within ten percent by reducing the errors, both external and internal, in the calibration of the distance scale.[86]


[edit] Important discoveries








One of Hubble's most famous images, Pillars of Creation shows stars forming in the Eagle Nebula


The Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required 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 HST, 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.[87]

While Hubble helped to refine estimates of the age of the universe, 
it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project[88] 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, confirming Hubble's finding. 
The cause of this acceleration remains poorly understood;[89] the most common cause attributed is dark energy.[90]

The high-resolution spectra and images provided by the HST have been especially well-suited to establishing the prevalence of black holes
 in the nuclei of nearby galaxies. While it had been hypothesized in the
 early 1960s that black holes would be found at the centers of some 
galaxies, and work in the 1980s identified a number of good black hole 
candidates, it fell to work conducted with Hubble to show that black 
holes are probably common to the centers of all galaxies.[91][92][93]
 The Hubble programs further established that the masses of the nuclear 
black holes and properties of the galaxies are closely related. The 
legacy of the Hubble programs on black holes in galaxies is thus to 
demonstrate a deep connection between galaxies and their central black 
holes.


Main article: Comet Shoemaker-Levy 9
The collision of Comet Shoemaker-Levy 9 with Jupiter
 in 1994 was 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;[94] evidence for the presence of extrasolar planets around sun-like stars;[95] and the optical counterparts of the still-mysterious gamma ray bursts.[96] HST has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto[97] and Eris.[98]


Main articles: Hubble Deep Field and Hubble Ultra-Deep Field
A unique legacy of Hubble are the 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 that 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.

The non-standard object SCP 06F6 was discovered by the Hubble Space Telescope (HST) in February 2006.[99][100]


[edit] Impact on astronomy








Distant galaxies in a Hubble Ultra-Deep Field photograph


Many objective measures show the positive impact of Hubble data on 
astronomy. Over 9,000 papers based on Hubble data have been published in
 peer-reviewed journals,[101] 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 that receive the most citations, about 10% are based
 on Hubble data.[102]

Although the HST has clearly had a significant impact on astronomical
 research, the financial cost of this impact has been 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&nbsp;m ground-based telescope such as the William Herschel Telescope, the HST costs about 100 times as much to build and maintain.[103]

Making the decision between investing in ground-based versus 
space-based telescopes in the future is complex. Even before Hubble was 
launched, specialized ground-based techniques such as aperture masking interferometry had obtained higher-resolution optical and infrared images than Hubble would achieve, though restricted to targets about 108 times brighter than the faintest targets observed by Hubble.[104][105] Since then, advances in adaptive optics
 have extended the high-resolution imaging capabilities of ground-based 
telescopes to the infrared imaging of faint objects. The usefulness of 
adaptive optics versus HST observations depends strongly on the 
particular details of the research questions being asked. In the visible
 bands, adaptive optics can only correct a relatively small field of 
view, whereas HST can conduct high-resolution optical imaging over a 
wide field. Only a small fraction of astronomical objects are accessible
 to high-resolution ground-based imaging; in contrast Hubble can perform
 high-resolution observations of any part of the night sky, and on 
objects that are extremely faint.


[edit] Usage








The Hubble Space Telescope as seen from Space Shuttle&nbsp;Discovery during its second servicing mission (STS-82).










To commemorate Hubble Telescope's 20th Birthday, NASA, with ESA and STI,
 released this Wide Field Camera 3 shot of a portion of Carina Nebula, 
dubbed 'mystic mountain'. Taken in February 2010, oxygen is colored 
blue, hydrogen and nitrogen green, and sulfur with red


Anyone can apply for time on the telescope; there are no restrictions on nationality or academic affiliation.[106]
 Competition for time on the telescope is intense, and the ratio of time
 requested to time available (the oversubscription ratio) typically 
ranges between 6 and 9.[107]

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 overheads such as acquiring the target; snapshot observations 
are used to fill in gaps in the telescope schedule that cannot be filled
 by regular GO programs.[108]

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.[109]
 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.


[edit] Amateur observations
The first director of 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.[110]

Proposals for amateur time were stringently peer reviewed
 by a committee of leading amateur astronomers, and time was awarded 
only to proposals that were deemed to have genuine scientific merit, did
 not duplicate proposals made by professionals, and 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&nbsp;— UV Search for OH Emissions in Asteroids.
 The very first proposal, A Hubble Space Telescope Study of Post Eclipse
 Brightening and Albedo Changes on Io, was published in Icarus,[111]
 a journal devoted to solar system studies. 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.[112]


[edit] 20th birthday
The Hubble Telescope celebrated its 20th birthday on April 22, 2010. 
To commemorate the occasion, NASA, ESA, and Space Telescope Institute 
(STScI) released an image from the Carina Nebula.[113]


[edit] Hubble data

[edit] Transmission to Earth








Hubble Control Center, Goddard Space Flight Center, 1999


Hubble data was 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. 
Approximately twice daily, the Hubble Space Telescope radios data to a 
satellite in the geosynchronous Tracking and Data Relay Satellite System (TDRSS).[114]
 The TDRSS then downlinks the science data to one of two 60-foot 
(18-meter) diameter high gain microwave antennas located at the White Sands Test Facility in White Sands, New Mexico.[114] From there they are sent to the Goddard Space Flight Center and finally to the Space Telescope Science Institute for archiving.[115]

These data are then transmitted to the Space Telescope Operations Control Center (STOCC) located in Greenbelt, Maryland.[114]


[edit] Archives
All Hubble data is eventually made available via the archives at STScI,[116] CADC[117] and ESO.[118] Data is 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.[119]

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 is in the FITS format, which is suitable for astronomical analysis but not for public use.[120] The Hubble Heritage Project processes and releases to the public a small selection of the most striking images in JPEG and TIFF formats.[121]


[edit] Pipeline reduction
Astronomical data taken with CCDs
 must undergo several calibration steps before they are suitable for 
astronomical analysis. STScI has developed sophisticated software that 
automatically calibrates data when they are 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 are 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.[122]


[edit] Data analysis








Data analysis of a spectrum reveals the chemistry of hidden clouds


Hubble data can be analysed using many different packages. STScI 
maintains the custom-made STSDAS (Space Telescope Science Data Analysis 
System) software, which 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.[123]


[edit] Outreach activities








In 2001, NASA polled internet users to find out what they would most like Hubble to observe; they overwhelmingly selected the Horsehead Nebula.










A small scale replica of the Hubble Space Telescope in Marshfield, Missouri.










The "Twenty Years at the Frontier of Science" exhibition at the Istituto Veneto di Scienze.


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.[124]
 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 emphasizes the aesthetic
 nature of Hubble images. The Heritage Project is granted a small amount
 of time to observe objects which, for scientific reasons, may not have 
images taken at enough wavelengths to construct a full-color image.[125]

In addition, STScI maintains several comprehensive websites for the 
general public containing Hubble images and information about the 
observatory.[126][127][128][129]
 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.

Since 1999, the leading Hubble outreach activities group in Europe has been the Hubble European Space Agency Information Centre (HEIC).[130] This office was established at the Space Telescope - European Coordinating Facility
 (ST-ECF) in Munich, Germany. HEIC's mission statement is to fulfill the
 Hubble Space Telescope outreach and education tasks for the European Space Agency
 (ESA). The work is centered on the production of news and photo 
releases that highlight interesting Hubble science results and images. 
These are often European in origin, and so not only increase the 
awareness of ESA’s Hubble share (15%), but the contribution of European 
scientists to the observatory. The group also produces video releases 
and other innovative educational material.

The Hubble Space Telescope has won two Space Achievement Awards from the Space Foundation for its outreach activities, in 2001 and 2010.[131]

There is a replica of the Hubble Telescope on the courthouse lawn in Marshfield, Missouri, the hometown of namesake Edwin P. Hubble.


[edit] Hubblecast
The Hubblecast
 is a vodcast series made by the ESA's Hubble team to bring world-class 
science news to everyone about Hubble's latest discoveries. Hosted by 
"Dr. J" (Joe Liske) who is a research scientist at ESO. The episodes are available in several formats and resolutions for different platforms.


[edit] Future

[edit] Equipment failure








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. In 
August 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.[132] This power supply was fixed during servicing mission 4 in May 2009. Similarly, the main camera (the ACS) primary electronics failed in June 2006, and the power supply for the backup electronics failed on January 27, 2007.[133]
 Only the instrument's Solar Blind Channel (SBC) was operable using the 
side-1 electronics. A new power supply for the wide angle channel was 
added during SM 4, but quick tests revealed this did not help the high 
resolution channel.[134] As of late May 2009, tests of both repaired instruments are still ongoing.

HST 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, but the area of sky
 that can be viewed would be somewhat restricted, and observations 
requiring very accurate pointing are more difficult.[135] There are further contingency plans for science with just one gyro,[136]
 but if all gyros fail, continued scientific observations will not be 
possible. 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 in August 2005, leaving 
Hubble with two gyroscopes in use, two on backup, and two inoperable.[137] One more gyro failed in 2007.[138]
 By the time of the final repair mission, during which all six gyros 
were replaced (with two new pairs and one refurbished pair), only three 
gyros were still working. Engineers are confident that they have 
identified the root causes of the gyro failures, and the new models 
should be much more reliable.[139]

In addition to predicted gyroscope failure, Hubble eventually required a change of nickel hydrogen 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. Alternatively, the observatory was 
designed so that during shuttle servicing missions it would receive 
power from a connection to the space shuttle, and this capability could 
have been utilized by adding an external power source (an additional 
battery) rather than changing the internal ones.[140] In the end, however, the batteries were simply replaced during service mission 4.


[edit] 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 2019 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 affects the re-entry date, 
as a controllable telescope can be oriented 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).[141] With the success of STS-125,
 the natural re-entry date range has been extended further as the 
mission replaced its gyroscopes, even though Hubble was not re-boosted 
to a higher orbit.

NASA's original plan for safely de-orbiting Hubble was to retrieve it using a space shuttle. The Hubble telescope would then have most likely been displayed in the Smithsonian Institution. This is no longer possible since the space shuttle has been retired,
 and would have been unlikely in any case due to the cost of the mission
 and risk to the crew. Instead NASA looked at adding an external 
propulsion module to allow controlled re-entry.[142] The final decision was not to attach a de-orbit module on STS-125, but to add a grapple fixture so a robotic mission could more easily attach such a module later.[143]


[edit] Debate over final servicing mission

Main article: STS-125
Columbia was originally scheduled to visit Hubble again in 
February 2005. The tasks of this servicing mission would have included 
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 accident, all future shuttles must be able to reach the 'safe-haven' of the International Space Station
 (ISS) should an in-flight problem develop that would preclude the 
shuttle from landing safely. The shuttle is incapable of reaching both 
the Hubble Space Telescope and the International Space Station during 
the same mission, and so future manned service missions were canceled.[144]

This decision was assailed by numerous astronomers, who felt that 
Hubble was valuable enough to merit the human risk. HST's successor, the
 James Webb Space Telescope (JWST), will not be ready until well after the 2010 scheduled retirement of the space shuttle. While Hubble can image in the ultraviolet and visible wavelengths, JWST is limited to the infrared.
 The break in space-observing capabilities between the decommissioning 
of Hubble and the commissioning of a successor is of major concern to 
many astronomers, given the great scientific impact of HST taken as a 
whole.[145] The consideration that the JWST will not be located in low Earth orbit,
 and therefore cannot be easily repaired in the event of an early 
failure, only makes these concerns more acute. Nor can JWST's 
instruments be easily upgraded. On the other hand, many astronomers felt
 strongly that the servicing of Hubble should not take place if the costs of the servicing come from the JWST budget.

In January 2004, O'Keefe said he would review his decision to cancel 
the final shuttle servicing mission to HST due to public outcry and 
requests from Congress for NASA to look for a way to save it. On 13 July
 2004 an official panel from the National Academy of Sciences
 made the recommendation that the HST should 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". In August 2004, O'Keefe requested the Goddard Space Flight 
Center to prepare a detailed proposal for a robotic service mission. These plans were later canceled, the robotic mission being described as "not feasible".[146]
 In late 2004, several Congressional members, led by Sen. Barbara 
Mikulski (D-MD), held public hearings and carried on a fight with much 
public support (including thousands of letters from school children 
across the country) to get the Bush Administration and NASA to 
reconsider the decision to drop plans for a Hubble rescue mission.[147]

The arrival in April 2005 of the new NASA Administrator, Michael D. Griffin,
 changed the status of the proposed shuttle rescue mission. At the time,
 Griffin stated he would reconsider the possibility of a manned 
servicing mission. Soon after his appointment, he authorized Goddard 
Space Flight Center to proceed with preparations for a manned Hubble 
maintenance flight, saying he would make the final decision on this 
flight after the next two shuttle missions. In October 2006 Griffin gave
 the final go-ahead for the mission. The 11-day STS-125 mission by Atlantis was scheduled for launch in October 2008.[148][149]
 However, the main data-handling unit failed in late September 2008, 
halting all reporting of scientific data. This unit has a backup, and on
 October 25, 2008 Hubble was successfully rebooted and was reported to 
be functioning normally.[150]
 However, since a failure in the backup unit would now leave the HST 
helpless, the service mission was postponed to allow astronauts to 
repair this problem. This mission got underway on May 11, 2009[151] and completed all the long planned replacements as well as additional repairs, including replacing the main data-handling unit.


[edit] Successors

Main article: James Webb Space Telescope








Some JWST mirror segments are tested










M1 area: 4.5 m2 (HST) and 25 m2 (JWST)


There is no direct successor to the Hubble as an ultraviolet and 
visible-light space telescope, as near term space telescopes do not 
duplicate Hubble's wavelength coverage (near ultra-violet to near 
infrared wavelength), instead concentrating on the farther infrared
 bands. These bands are preferred for studying high Z and low 
temperature objects, objects generally farther away and older in the 
universe. These wavelengths are also difficult or impossible to study 
from the ground, justifying the expense of a space-based telescope. 
Alternatively, large ground based telescopes include many of the same 
wavelengths as Hubble, sometimes challenge HST in terms of resolution 
(via adaptive optics), have much larger light gathering power, and can 
be upgraded more easily. However, they cannot yet match the Hubble's 
excellent resolution over a wide field of view, and the very dark 
background of space.

Plans for a Hubble successor materialized as the Next Generation Space Telescope (NGST) project, which culminated in the James Webb Space Telescope (JWST), the formally planned successor of Hubble.[152] Very different from a scaled up Hubble, it is designed to operate colder and farther away from the Earth at the L2 Lagrangian point,
 where thermal and optical interference from the Earth and Moon are 
lessened. It is not engineered to be fully serviceable (such as 
replaceable instruments), but the design includes a docking ring to 
enable visitation by other spacecraft.[153]
 A main scientific goal of the telescope is to observe the most distant 
objects in the universe, beyond the reach of existing instruments. It is
 expected to detect stars in the early Universe approximately 280 million years older than stars HST now detects.[154][155] The telescope is an international collaboration between NASA, the European Space Agency and the Canadian Space Agency since 1996,[156] and is planned for launch on a Ariane 5 rocket.[157]
 Although JWST is primarily an infrared instrument, its wavelength 
coverage extends down to 600&nbsp;nm wavelength light, or roughly orange in 
the visible spectrum.
 A typical human eye can see to about 750&nbsp;nm wavelength light, so there 
is some overlap with the longest wavelength visible bands, including 
orange and red light.




Color
Wavelength


violet
380–450&nbsp;nm


blue
450–475&nbsp;nm


cyan
476–495&nbsp;nm


green
495–570&nbsp;nm


yellow
570–590&nbsp;nm


orange
590–620&nbsp;nm


red
620–750&nbsp;nm





Selected space telescopes &amp; instruments[158]


Name
Year
Wavelength
Aperture


Human Eye
-
0.39-0.75 μm
0.01 m


Spitzer
2003
3-180 μm
0.85 m


Hubble STIS
1997
0.115-1.03 μm
2.4 m


Hubble WFC3
2009
0.2-1.7 μm
2.4 m


Herschel
2009
60-672 μm
3.5 m


JWST
Planned
0.6-28.5 μm
6.5 m


A complementary telescope, looking at even longer wavelengths than Hubble or JWST, is the European Space Agency's Herschel Space Observatory, launched on May 14, 2009. Like JWST, Herschel has a mirror substantially larger than Hubble's, but observes only in the far-infrared. Like JWST, it is not designed to be serviced after launch.

Longer term, concepts for advanced 21st century space telescopes exist, such as the Advanced Technology Large-Aperture Space Telescope (AT-LAST)[159] A conceptualized 8 to 16-meter (320 to 640-inch) optical space telescope that if approved, built, and launched (perhaps using the Space Launch System),
 could be a more direct successor Hubble Space Telescope (HST); with the
 ability to observe and photograph astronomical objects in the optical, ultraviolet, and Infrared wavelengths, but with substantially better resolution than Hubble or Spitzer. This effort is being planned for the 2025-2035 time frame.









Comparison focusing on observation of the early Universe


Existing ground based telescopes, and various proposed Extremely Large Telescopes,
 can exceed the HST in terms of sheer light gathering power and 
diffraction limit due to larger mirrors, but other factors affect 
telescopes. In some cases, they may also be able to match or beat Hubble
 in resolution by using adaptive optics
 (AO). However, AO on large ground-based reflectors will not make Hubble
 and other space telescopes obsolete. Most AO systems sharpen the view 
over a very narrow field&nbsp;– Lucky Cam,
 for example, produces crisp images just 10" to 20" wide, whereas 
Hubble's cameras are super sharp across a 2½' (150") field. Furthermore,
 space telescopes can study the heavens across the entire 
electromagnetic spectrum, most of which is blocked by Earth's 
atmosphere. Finally, the background sky is darker in space than on the 
ground, because air absorbs solar energy during the day and then 
releases it at night, producing a faint—but nevertheless discernible—airglow that washes out faint, low-contrast astronomical objects.[160]&nbsp; &nbsp; &nbsp; &nbsp;

&nbsp;&nbsp;




fonte do texto: wikipedia &nbsp; &nbsp; &nbsp;http://en.wikipedia.org/wiki/Hubble_Space_Telescope