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Hubble Space Telescope

Hubble Space Telescope

Overview
The Hubble Space Telescope (HST) is a space telescope
Space observatory
A space observatory is any instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects...

 that was carried into orbit by a Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 in 1990 and remains in operation. A 2.4 meter (7.9 ft) aperture telescope in low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared. The telescope is named after the astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

 Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...

.

Hubble's orbit outside the distortion of Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 allows it to take extremely sharp images with almost no background
Background (astronomy)
In astronomy, background commonly refers to the incoming light from an apparently empty part of the night sky.Even if no visible astronomical objects are present in given part of the sky, there always is some low luminosity present, due mostly to light diffusion from the atmosphere...

 light.
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Encyclopedia
The Hubble Space Telescope (HST) is a space telescope
Space observatory
A space observatory is any instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects...

 that was carried into orbit by a Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 in 1990 and remains in operation. A 2.4 meter (7.9 ft) aperture telescope in low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared. The telescope is named after the astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

 Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...

.

Hubble's orbit outside the distortion of Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 allows it to take extremely sharp images with almost no background
Background (astronomy)
In astronomy, background commonly refers to the incoming light from an apparently empty part of the night sky.Even if no visible astronomical objects are present in given part of the sky, there always is some low luminosity present, due mostly to light diffusion from the atmosphere...

 light. Hubble's Ultra Deep Field
Hubble Ultra Deep Field
The Hubble Ultra-Deep Field is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 24, 2003, through to January 16, 2004...

 image, for instance, is the most detailed visible-light
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

 image ever made of the universe's most distant objects. Many Hubble observations have led to breakthroughs in astrophysics
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...

, 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
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

. The HST was built by the United States space agency NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

, with contributions from the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

, and is operated by the Space Telescope Science Institute
Space Telescope Science Institute
The Space Telescope Science Institute is the science operations center for the Hubble Space Telescope and for the James Webb Space Telescope...

. The HST is one of NASA's Great Observatories
Great Observatories program
NASA's series of Great Observatories satellites are four large, powerful space-based telescopes. Each of the Great Observatories has had a similar size and cost at program outset, and each has made a substantial contribution to astronomy...

, along with the Compton Gamma Ray Observatory
Compton Gamma Ray Observatory
The Compton Gamma Ray Observatory was a space observatory detecting light from 20 KeV to 30 GeV in Earth orbit from 1991 to 2000. It featured four main telescopes in one spacecraft covering x-rays and gamma-rays, including various specialized sub-instruments and detectors...

, the Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...

, and the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

.

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
Space Shuttle Challenger disaster
The Space Shuttle Challenger disaster occurred on January 28, 1986, when Space Shuttle Challenger broke apart 73 seconds into its flight, leading to the deaths of its seven crew members. The spacecraft disintegrated over the Atlantic Ocean, off the coast of central Florida at 11:38 am EST...

. 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
Space Shuttle Columbia disaster
The Space Shuttle Columbia disaster occurred on February 1, 2003, when shortly before it was scheduled to conclude its 28th mission, STS-107, the Space Shuttle Columbia disintegrated over Texas and Louisiana during re-entry into the Earth's atmosphere, resulting in the death of all seven crew members...

. However, after spirited public discussion, NASA administrator Mike Griffin
Michael D. Griffin
Michael Douglas Griffin is an American physicist and aerospace engineer. From April 13, 2005 to January 20, 2009 he served as Administrator of NASA, the space agency of the United States...

 approved one final servicing mission
STS-125
STS-125, or HST-SM4 , was the fifth and final space shuttle servicing mission to the Hubble Space Telescope .Launch occurred on 11 May 2009 at 2:01 pm EDT...

, completed in 2009. The telescope is now expected to function until at least 2014. Its scientific successor, the James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...

 (JWST), is to be launched in 2018 or possibly later.

Proposals and precursors


In 1923, Hermann Oberth
Hermann Oberth
Hermann Julius Oberth was an Austro-Hungarian-born German physicist and engineer. He is considered one of the founding fathers of rocketry and astronautics.- Early life :...

—considered along with Robert H. Goddard
Robert H. Goddard
Robert Hutchings Goddard was an American professor, physicist and inventor who is credited with creating and building the world's first liquid-fueled rocket, which he successfully launched on March 16, 1926...

 and Konstantin Tsiolkovsky
Konstantin Tsiolkovsky
Konstantin Eduardovich Tsiolkovsky was an Imperial Russian and Soviet rocket scientist and pioneer of the astronautic theory. Along with his followers the German Hermann Oberth and the American Robert H. Goddard, he is considered to be one of the founding fathers of rocketry and astronautics...

 fathers of modern rocketry—published ("The Rocket into Planetary Space"), which mentioned how a telescope could be propelled into Earth orbit by a rocket.

The history of the Hubble Space Telescope can be traced back as far as 1946, when the astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

 Lyman Spitzer
Lyman Spitzer
Lyman Strong Spitzer, Jr. was an American theoretical physicist and astronomer best known for his research in star formation, plasma physics, and in 1946, for conceiving the idea of telescopes operating in outer space...

 wrote the paper "Astronomical advantages of an extraterrestrial observatory". In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes. First, the angular resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...

 (smallest separation at which objects can be clearly distinguished) would be limited only by diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

, rather than by the turbulence in the atmosphere, which causes stars to twinkle and is known to astronomers as seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...

. 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 0.05 arcsec for a telescope with a mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

 2.5 m in diameter. Second, a space-based telescope could observe infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 and ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 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
United States National Academy of Sciences
The National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...

 recommended the development of a space telescope as part of the space program
Human spaceflight
Human spaceflight is spaceflight with humans on the spacecraft. When a spacecraft is manned, it can be piloted directly, as opposed to machine or robotic space probes and remotely-controlled satellites....

, 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, as scientists made use of developments that had taken place in rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

 technology. The first ultraviolet spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

 of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 was obtained in 1946, and NASA launched the Orbiting Solar Observatory
Orbiting Solar Observatory
The Orbiting Solar Observatory Program was the name of a series of nine American science satellites primarily intended to study the Sun, though they also included important non-solar experiments. Eight were launched successfully by NASA between 1962 and 1975 using Delta rockets...

 to obtain UV, X-ray, and gamma-ray spectra in 1962. An orbiting solar telescope
Ariel 1
Ariel 1, also known as UK-1 and S-55, was the first British satellite, and the first satellite in the Ariel programme. Its launch in 1962 made the United Kingdom the third country to operate a satellite, after the Soviet Union and the USA...

 was launched in 1962 by the United Kingdom as part of the Ariel space program, and in 1966 National Aeronautics and Space Administration
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 (NASA) launched the first Orbiting Astronomical Observatory
Orbiting Astronomical Observatory
The Orbiting Astronomical Observatory satellites were a series of four American space observatories launched by NASA between 1966 and 1972, which provided the first high-quality observations of many objects in ultraviolet light...

 (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 star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s and galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 from its launch in 1968 until 1972, well beyond its original planned lifetime of one year.

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
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...

 with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope (LST), with a launch slated for 1979. These plans emphasized the need for manned maintenance missions to the telescope to ensure such a costly program had a lengthy working life, and the concurrent development of plans for the reusable space shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 indicated that the technology to allow this was soon to become available.

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
United States Congress
The United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....

 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
Gerald Ford
Gerald Rudolph "Jerry" Ford, Jr. was the 38th President of the United States, serving from 1974 to 1977, and the 40th Vice President of the United States serving from 1973 to 1974...

 led to Congress cutting all funding for the telescope project.

In response to this, a nationwide lobbying effort was coordinated among astronomers. Many astronomers met congressmen
United States House of Representatives
The United States House of Representatives is one of the two Houses of the United States Congress, the bicameral legislature which also includes the Senate.The composition and powers of the House are established in Article One of the Constitution...

 and senators
United States Senate
The United States Senate is the upper house of the bicameral legislature of the United States, and together with the United States House of Representatives comprises the United States Congress. The composition and powers of the Senate are established in Article One of the U.S. Constitution. Each...

 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.

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
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

. ESA agreed to provide funding and supply one of the first generation instruments for the telescope, as well as the solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....

s 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. Congress eventually approved funding of US$
United States dollar
The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....

36,000,000 for 1978, and the design of the LST began in earnest, aiming for a launch date of 1983. In 1983 the telescope was named after Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...

, who made one of the greatest scientific breakthroughs of the 20th century when he discovered that the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 is expanding.

Construction and engineering


Once the Space Telescope project had been given the go-ahead, work on the program was divided among many institutions. Marshall Space Flight Center
Marshall Space Flight Center
The George C. Marshall Space Flight Center is the U.S. government's civilian rocketry and spacecraft propulsion research center. The largest center of NASA, MSFC's first mission was developing the Saturn launch vehicles for the Apollo moon program...

 (MSFC) was given responsibility for the design, development, and construction of the telescope, while the Goddard Space Flight Center
Goddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...

 was given overall control of the scientific instruments and ground-control center for the mission. MSFC commissioned the optics company Perkin-Elmer
PerkinElmer
PerkinElmer, Inc. is an American multinational technology corporation, focused in the business areas of human and environmental health, including environmental analysis, food and consumer product safety, medical imaging, drug discovery, diagnostics, biotechnology, industrial applications, and life...

 to design and build the Optical Telescope Assembly (OTA) and Fine Guidance Sensors for the space telescope. Lockheed
Lockheed Corporation
The Lockheed Corporation was an American aerospace company. Lockheed was founded in 1912 and later merged with Martin Marietta to form Lockheed Martin in 1995.-Origins:...

 was commissioned to construct and integrate the spacecraft in which the telescope would be housed.

Optical Telescope Assembly (OTA)


Optically, the HST is a Cassegrain reflector
Cassegrain reflector
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas....

 of Ritchey-Chrétien design
Ritchey-Chrétien telescope
A Ritchey–Chrétien telescope is a specialized Cassegrain telescope designed to eliminate coma, thus providing a large field of view compared to a more conventional configuration. An RCT has a hyperbolic primary and a hyperbolic secondary mirror. It was invented in the early 1910s by American...

, 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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 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 nanometers
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

, or about 1/65 of the wavelength of red light. On the long wavelength end, the OTA was not designed with optimum IR performance in mind — for example, the mirrors are kept at stable (and warm, about 15 °C) temperatures by heaters. This limits Hubble's performance as an infrared telescope.

Perkin-Elmer intended to use custom-built and extremely sophisticated computer-controlled polishing machines
Computer-aided manufacturing
Computer-aided manufacturing is the use of computer software to control machine tools and related machinery in the manufacturing of workpieces. This is not the only definition for CAM, but it is the most common; CAM may also refer to the use of a computer to assist in all operations of a...

 to grind the mirror to the required shape. 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. (The team of Kodak and Itek
Itek
Itek Corporation was a US defense contractor that initially specialized in the field of camera systems for spy satellites. In the early 1960s they built a conglomerate in a fashion similar to LTV or Litton, during which time they developed the first CAD system and explored optical disk technology...

 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.) The Kodak mirror is now on permanent display at the Smithsonian Institution
Smithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...

. An Itek mirror built as part of the effort is now used in the 2.4 m telescope at the Magdalena Ridge Observatory
Magdalena Ridge Observatory
Magdalena Ridge Observatory is an astronomical observatory under construction in Socorro County, New Mexico about 20 miles west of the town of Socorro off the exit for Water Canyon US 60. MRO is an entity of New Mexico Tech's Office of Research and Economic Development...

.

Construction of the Perkin-Elmer mirror began in 1979, starting with a blank manufactured by Corning from their ultra-low expansion glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

. To keep the mirror's weight to a minimum it consisted of inch-thick top and bottom plates sandwiching a honeycomb
Honeycomb
A honeycomb is a mass of hexagonal waxcells built by honey bees in their nests to contain their larvae and stores of honey and pollen.Beekeepers may remove the entire honeycomb to harvest honey...

 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. 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
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 65 nm-thick and a protective coating of magnesium fluoride
Magnesium fluoride
Magnesium fluoride is an inorganic compound with the formula MgF2. The compound is a white crystalline salt and is transparent over a wide range of wavelengths, with commercial uses in optics.-Production and structure:...

 25 nm-thick.
Doubts continued to be expressed about Perkin-Elmer's competence on a project of this importance, as their budget and timescale for producing the rest of the OTA continued to inflate. In response to a schedule described as "unsettled and changing daily", NASA postponed the launch date of the telescope until April 1985. Perkin-Elmer's schedules continued to slip at a rate of about one month per quarter, and at times delays reached one day for each day of work. NASA was forced to postpone the launch date until first March and then September 1986. By this time the total project budget had risen to US$1.175 billion.

Spacecraft systems


The spacecraft in which the telescope and instruments were to be housed was another major engineering challenge. It would have to adequately withstand frequent passages from direct sunlight into the darkness of Earth's shadow
Shadow
A shadow is an area where direct light from a light source cannot reach due to obstruction by an object. It occupies all of the space behind an opaque object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the...

, 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
Multi-layer insulation
Multi-layer insulation, or MLI, is thermal insulation composed of multiple layers of thin sheets often used on spacecraft. It one of the main items of the spacecraft thermal design, mainly intended to reduce heat loss by thermal radiation. In its basic form, it does not appreciably insulate...

 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. 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.
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.

Initial instruments


When launched, the HST carried five scientific instruments: the Wide Field and Planetary Camera (WF/PC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS). WF/PC was a high-resolution imaging device primarily intended for optical observations. It was built by NASA's Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

, and incorporated a set of 48 filters
Filter (optics)
Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....

 isolating spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

s of particular astrophysical interest. The instrument contained eight charge-coupled device
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 (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
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 than the WF chips, giving it a greater magnification.

The GHRS was a spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...

 designed to operate in the ultraviolet. It was built by the Goddard Space Flight Center and could achieve a spectral resolution
Spectral resolution
The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum...

 of 90,000. 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
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

-counting digicons as their detectors. The FOC was constructed by ESA, while the University of California, San Diego
University of California, San Diego
The University of California, San Diego, commonly known as UCSD or UC San Diego, is a public research university located in the La Jolla neighborhood of San Diego, California, United States...

 and the Martin Marietta
Martin Marietta
Martin Marietta Corporation was an American company founded in 1961 through the merger of The Martin Company and American-Marietta Corporation. The combined company became a leader in chemicals, aerospace, and electronics. In 1995, it merged with Lockheed Corporation to form Lockheed Martin. The...

 corporation built the FOS.

The final instrument was the HSP, designed and built at the University of Wisconsin–Madison
University of Wisconsin–Madison
The University of Wisconsin–Madison is a public research university located in Madison, Wisconsin, United States. Founded in 1848, UW–Madison is the flagship campus of the University of Wisconsin System. It became a land-grant institution in 1866...

. It was optimized for visible and ultraviolet light observations of variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

s and other astronomical objects varying in brightness. It could take up to 100,000 measurements per second with a photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...

 accuracy of about 2% or better.

HST's guidance system can also be used as a scientific instrument. Its three Fine Guidance Sensor
Fine Guidance Sensor
The Fine Guidance Sensor is an interferometric instrument on board the Hubble Space Telescope that provides high-precision pointing information as input to the observatory's attitude control systems....

s (FGS) are primarily used to keep the telescope accurately pointed during an observation, but can also be used to carry out extremely accurate astrometry
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...

; measurements accurate to within 0.0003 arcseconds have been achieved.

Ground support


The Space Telescope Science Institute
Space Telescope Science Institute
The Space Telescope Science Institute is the science operations center for the Hubble Space Telescope and for the James Webb Space Telescope...

 (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
Association of Universities for Research in Astronomy
The Association of Universities for Research in Astronomy is a consortium of universities and other institutions that operates astronomical observatories and telescopes...

 (AURA) and is physically located in Baltimore, Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...

 on the Homewood campus of Johns Hopkins University
Johns Hopkins University
The Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university based in Baltimore, Maryland, United States...

, one of the 33 US universities and 7 international affiliates that make up the AURA consortium. STScI was established in 1981 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
Academia
Academia is the community of students and scholars engaged in higher education and research.-Etymology:The word comes from the akademeia in ancient Greece. Outside the city walls of Athens, the gymnasium was made famous by Plato as a center of learning...

 establishment. The Space Telescope European Coordinating Facility
Space Telescope European Coordinating Facility
right|The Space Telescope - European Coordinating Facility is an institution which provides a number of support and service functions primarily for European observers of the NASA/ESA Hubble Space Telescope...

 (ST-ECF), established at Garching bei München
Garching bei München
Garching bei München or Garching is a city in Bavaria, Germany near Munich. It is the home of several research institutes and university departments. It became a city on 14 September 1990.-Location:...

 near Munich
Munich
Munich The city's motto is "" . Before 2006, it was "Weltstadt mit Herz" . Its native name, , is derived from the Old High German Munichen, meaning "by the monks' place". The city's name derives from the monks of the Benedictine order who founded the city; hence the monk depicted on the city's coat...

 in 1984, provides similar support for European astronomers.

One rather complex task that falls to STScI is scheduling observations for the telescope. 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
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

 by the Earth for slightly less than half of each orbit. Observations cannot take place when the telescope passes through the South Atlantic Anomaly
South Atlantic Anomaly
The South Atlantic Anomaly is an area where the Earth's inner Van Allen radiation belt comes closest to the Earth's surface. This leads to an increased flux of energetic particles in this region and exposes orbiting satellites to higher than usual levels of radiation...

 due to elevated radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 levels, and there are also sizable exclusion zones around the Sun (precluding observations of Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

), 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 degrees to the plane of Hubble's orbit, in which targets are not occulted for long periods. Due to the precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...

 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 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.

Engineering support for HST is provided by NASA and contractor personnel at the Goddard Space Flight Center in Greenbelt, Maryland
Greenbelt, Maryland
Greenbelt is a city in Prince George's County, Maryland, United States. Contained within today's City of Greenbelt is the historic planned community now known locally as "Old Greenbelt" and designated as the Greenbelt Historic District...

, 48 km 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.

Challenger disaster, delays, and eventual launch


By early 1986, the planned launch date of October that year looked feasible, but the Challenger accident
Space Shuttle Challenger disaster
The Space Shuttle Challenger disaster occurred on January 28, 1986, when Space Shuttle Challenger broke apart 73 seconds into its flight, leading to the deaths of its seven crew members. The spacecraft disintegrated over the Atlantic Ocean, off the coast of central Florida at 11:38 am EST...

 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. Furthermore, the ground software needed to control Hubble was not ready in 1986, and in fact was barely ready by the 1990 launch.
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
STS-31
STS-31 was the thirty-fifth mission of the American Space Shuttle program, which launched the Hubble Space Telescope astronomical observatory into Earth orbit...

 saw Discovery launch the telescope successfully into its planned orbit.

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).

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 source
Point source
A point source is a localised, relatively small source of something.Point source may also refer to:*Point source , a localised source of pollution**Point source water pollution, water pollution with a localized source...

s spread out over a radius of more than one arcsecond, instead of having a point spread function
Point spread function
The point spread function describes the response of an imaging system to a point source or point object. A more general term for the PSF is a system's impulse response, the PSF being the impulse response of a focused optical system. The PSF in many contexts can be thought of as the extended blob...

 (PSF) concentrated within a circle 0.1 arcsec in diameter as had been specified in the design criteria. The detailed performance is shown in graphs from STScI illustrating the mis-figured
Figuring
Figuring is the process of final polishing of an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.-Types of figuring:...

 PSFs compared to post-correction and ground-based PSFs.

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, it was too flat at the edges by about 2200 nanometers (2.2 microns). This difference was catastrophic, introducing severe spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...

, a flaw in which light reflecting off the edge of a mirror focuses
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

 on a different point from the light reflecting off its center.
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. NASA and the telescope became the butt of many jokes, and the project was popularly regarded as a white elephant
White elephant
A white elephant is an idiom for a valuable but burdensome possession of which its owner cannot dispose and whose cost is out of proportion to its usefulness or worth...

. (For instance, in the 1991 comedy The Naked Gun 2½: The Smell of Fear
The Naked Gun 2½: The Smell of Fear
The Naked Gun 2½: The Smell of Fear is a 1991 comedy film starring Leslie Nielsen as the comically bumbling Police Lt. Frank Drebin of Police Squad!. Priscilla Presley plays the role of Jane, with O.J. Simpson as Nordberg and George Kennedy as police captain Ed Hocken...

, the Hubble was pictured with the Titanic, the Hindenburg
LZ 129 Hindenburg
LZ 129 Hindenburg was a large German commercial passenger-carrying rigid airship, the lead ship of the Hindenburg class, the longest class of flying machine and the largest airship by envelope volume...

, and the Edsel
Edsel
The Edsel was an automobile manufactured by the Ford Motor Company during the 1958, 1959, and 1960 model years. The Edsel never gained popularity with contemporary American car buyers and sold poorly. Consequently, the Ford Motor Company lost millions of dollars on the Edsel's development,...

). 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
Image processing
In electrical engineering and computer science, image processing is any form of signal processing for which the input is an image, such as a photograph or video frame; the output of image processing may be either an image or, a set of characteristics or parameters related to the image...

 techniques such as deconvolution
Deconvolution
In mathematics, deconvolution is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing...

.

Origin of the problem



A commission headed by Lew Allen
Lew Allen
Lew Allen, Jr. was a United States Air Force four-star General who served as the tenth Chief of Staff of the U.S. Air Force...

, director of the Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

, was established to determine how the error could have arisen. The Allen Commission found that the main null corrector
Null corrector
A null corrector is an optical device used in the testing of large aspheric mirrors. A spherical mirror of any size can be tested relatively easily using standard optical components such as laser, mirrors, beamsplitters, and converging lenses. One method of doing this using a Shack cube is shown...

, a device used to measure the exact shape of the mirror, had been incorrectly assembled—one lens was wrongly spaced by 1.3 mm. 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
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...

. 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.

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.

Design of a solution


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
Glasses
Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

" to correct the spherical aberration.

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
Conic constant
In geometry, the conic constant is a quantity describing conic sections, and is represented by the letter K...

 of the mirror as built was −1.01390±0.0002, instead of the intended −1.00230. The same number was also derived by analyzing the null corrector
Null corrector
A null corrector is an optical device used in the testing of large aspheric mirrors. A spherical mirror of any size can be tested relatively easily using standard optical components such as laser, mirrors, beamsplitters, and converging lenses. One method of doing this using a Shack cube is shown...

 used by Perkin-Elmer to figure the mirror, as well as by analyzing interferograms obtained during ground testing of the mirror.

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
Wide Field and Planetary Camera 2
The Wide Field and Planetary Camera 2 is a baby grand piano sized camera built by the Jet Propulsion Laboratory and formerly installed on the Hubble Space Telescope. It was installed by servicing mission 1 in 1993, replacing the telescope's original Wide Field and Planetary Camera...

, already planned to replace the existing WF/PC, included relay mirrors to direct light onto the eight separate CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 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.

The Corrective Optics Space Telescope Axial Replacement
Corrective Optics Space Telescope Axial Replacement
The Corrective Optics Space Telescope Axial Replacement is the instrument designed to correct Hubble Space Telescope's spherical aberration for light focused at the FOC, FOS and GHRS instruments. Built by Ball Aerospace Corp., it replaced the High Speed Photometer during the first Hubble...

 (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. To fit the COSTAR system onto the telescope, one of the other instruments had to be removed, and astronomers selected the High Speed Photometer
High Speed Photometer
The High Speed Photometer was a scientific instrument installed on the Hubble Space Telescope. The HSP was designed to measure the brightness and polarity of rapidly varying celestial objects. It could observe in ultraviolet, visible light, and near infrared at a rate of one measurement per 10...

 to be sacrificed. By 2002 all of the original instruments requiring COSTAR had been replaced by instruments with their own corrective optics. 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
Cosmic Origins Spectrograph
The Cosmic Origins Spectrograph is a science instrument that was installed on the Hubble Space Telescope during Servicing Mission 4 in May 2009. It is designed for ultraviolet spectroscopy of faint point sources with a resolving power of ≈1,550–24,000...

.

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 more specialized tools that would be needed. The mission STS-61
STS-61
STS-61 was the first Hubble Space Telescope servicing mission, and the fifth flight of the Space Shuttle Endeavour. The mission launched on 2 December 1993 from Kennedy Space Center in Florida. The mission restored the spaceborne observatory's vision, marred by spherical aberration, with the...

 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
High Speed Photometer
The High Speed Photometer was a scientific instrument installed on the Hubble Space Telescope. The HSP was designed to measure the brightness and polarity of rapidly varying celestial objects. It could observe in ultraviolet, visible light, and near infrared at a rate of one measurement per 10...

 was replaced with the COSTAR
Corrective Optics Space Telescope Axial Replacement
The Corrective Optics Space Telescope Axial Replacement is the instrument designed to correct Hubble Space Telescope's spherical aberration for light focused at the FOC, FOS and GHRS instruments. Built by Ball Aerospace Corp., it replaced the High Speed Photometer during the first Hubble...

 corrective optics package, and WFPC was replaced with the Wide Field and Planetary Camera 2
Wide Field and Planetary Camera 2
The Wide Field and Planetary Camera 2 is a baby grand piano sized camera built by the Jet Propulsion Laboratory and formerly installed on the Hubble Space Telescope. It was installed by servicing mission 1 in 1993, replacing the telescope's original Wide Field and Planetary Camera...

 (WFPC2) with its internal optical correction system. In addition, the solar arrays
Photovoltaic module
A solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...

 and their drive electronics were replaced, as well as four of the gyroscope
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...

s 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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 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. At the time, the mission had been one of the most complex ever undertaken, involving five lengthy periods of extra-vehicular activity
Extra-vehicular activity
Extra-vehicular activity is work done by an astronaut away from the Earth, and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth , but also applies to an EVA made on the surface of the Moon...

, and its resounding success was an enormous boon for NASA, as well as for the astronomers who now had a fully capable space telescope.

Servicing Mission 2


Servicing Mission 2, flown by Discovery (STS-82
STS-82
STS-82 was a Hubble Space Telescope servicing mission by Space Shuttle Discovery. The mission launched from Kennedy Space Center, Florida, on 11 February 1997 and returned to earth on 21 February 1997 at Kennedy Space Center.-Crew:...

) in February 1997, replaced the GHRS and the FOS with the Space Telescope Imaging Spectrograph
Space Telescope Imaging Spectrograph
The Space Telescope Imaging Spectrograph is a spectrograph, also with a camera mode, installed on the Hubble Space Telescope. It operated continuously from 1997 until a power supply failure in 2004. After repairs, it began operating again in 2009...

 (STIS) and the Near Infrared Camera and Multi-Object Spectrometer
Near Infrared Camera and Multi-Object Spectrometer
The Near Infrared Camera and Multi-Object Spectrometer is a scientific instrument for infrared astronomy, installed on the Hubble Space Telescope , operating from 1997 to 1999, and from 2002 to 2008...

 (NICMOS), replaced an Engineering and Science Tape Recorder with a new Solid State Recorder, repaired thermal insulation and again boosted Hubble's orbit. NICMOS contained a heat sink
Heat sink
A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator in a car...

 of solid nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 to reduce the thermal noise from the instrument, but shortly after it was installed, an unexpected thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

 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


Servicing Mission 3A flown by Discovery (STS-103
STS-103
STS-103 was a Hubble Space Telescope servicing mission by Space Shuttle Discovery. The mission launched from Kennedy Space Center, Florida, on 19 December 1999 and returned on 27 December 1999.-Crew:-Mission parameters:*Mass:...

), 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
Fine Guidance Sensor
The Fine Guidance Sensor is an interferometric instrument on board the Hubble Space Telescope that provides high-precision pointing information as input to the observatory's attitude control systems....

 and the computer, installed a Voltage/temperature Improvement Kit (VIK) to prevent battery overcharging, and replaced thermal insulation blankets. Although the new computer is hardly a powerhouse (a 25 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
DF-224
The DF-224 is a space-qualified computer used in space missions from the 1980s. It was built by Rockwell Autonetics. As with many spacecraft computers, the design is very redundant, since servicing in space is at best difficult and often impossible. The configuration had three CPUs, one active...

 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.

Servicing Mission 3B


Servicing Mission 3B flown by Columbia (STS-109
STS-109
STS-109 was a Space Shuttle mission that launched from the Kennedy Space Center on 1 March 2002. It was the 108th mission of the Space Shuttle program, the 27th flight of the orbiter Columbia and the fourth servicing of the Hubble Space Telescope...

) 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
Advanced Camera for Surveys
The Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...

 (ACS). This meant that COSTAR was no longer required, since all new instruments had correction for the main mirror aberration built in.

The mission also saw the revival of NICMOS, which had run out of coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

 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. ACS in particular enhanced Hubble's capabilities; it and the revived NICMOS together imaged the Hubble Ultra Deep Field
Hubble Ultra Deep Field
The Hubble Ultra-Deep Field is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 24, 2003, through to January 16, 2004...

.

The mission replaced the solar arrays for the second time. The new arrays were derived from those built for the Iridium
Iridium (satellite)
Iridium Communications Inc. is a company, based in McLean, VA, United States which operates the Iridium satellite constellation, a system of 66 active satellites used for worldwide voice and data communication from hand-held satellite phones and other transceiver units...

 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.

Servicing Mission 4



Servicing Mission 4 (SM4), which took place in May 2009, was the last scheduled shuttle mission (STS-125
STS-125
STS-125, or HST-SM4 , was the fifth and final space shuttle servicing mission to the Hubble Space Telescope .Launch occurred on 11 May 2009 at 2:01 pm EDT...

) for the Hubble Space Telescope. The mission was first planned for October 14, 2008. However on September 27, 2008, the Science Instrument Command and Data Handling (SI C&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. Therefore on September 29, 2008, NASA announced that the launch of SM4 would be postponed until 2009 so the SI C&DH unit could be replaced as well. SM4, with a replacement SI C&DH unit, was launched aboard on May 11, 2009.

On SM4, astronauts, over the course of five spacewalks, installed two new instruments, Wide Field Camera 3
Wide Field Camera 3
The Wide Field Camera 3 is the Hubble Space Telescope's last and most technologically advanced instrument to take images in the visible spectrum...

 (WFC3), and the Cosmic Origins Spectrograph
Cosmic Origins Spectrograph
The Cosmic Origins Spectrograph is a science instrument that was installed on the Hubble Space Telescope during Servicing Mission 4 in May 2009. It is designed for ultraviolet spectroscopy of faint point sources with a resolving power of ≈1,550–24,000...

 (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
Faint Object Camera
The Faint Object Camera was a camera installed on the Hubble Space Telescope from launch in 1990 until 2002. It was replaced by the Advanced Camera for Surveys.The camera was built by Dornier GmbH and was funded by the European Space Agency...

 in 2002, the last original instrument without the necessary correction built in.) 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
Advanced Camera for Surveys
The Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...

 (ACS) and the Space Telescope Imaging Spectrograph
Space Telescope Imaging Spectrograph
The Space Telescope Imaging Spectrograph is a spectrograph, also with a camera mode, installed on the Hubble Space Telescope. It operated continuously from 1997 until a power supply failure in 2004. After repairs, it began operating again in 2009...

 (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&DH unit; all six of the 125 pounds (56.7 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.

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. These efforts are expected to keep the telescope fully functioning at least into 2014, and perhaps longer.

Hubble was originally designed to be returned to earth on board a shuttle. With the retirement of the shuttle fleet
Space Shuttle retirement
The retirement of NASA's Space Shuttle fleet took place from March to July 2011. Discovery was the first of the three active space shuttles to be retired, completing its final mission on March 9, 2011; Space Shuttle Endeavour did so on June 1...

, 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
Space rendezvous
A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance . Rendezvous requires a precise match of the orbital velocities of the two spacecraft, allowing them to remain at a constant...

, capture, and safe disposal of Hubble by either a crew
Crew
A crew is a body or a class of people who work at a common activity, generally in a structured or hierarchical organization. A location in which a crew works is called a crewyard or a workyard...

ed or robot
Robot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...

ic mission. The next mission will be to deorbit Hubble at the end of its service life.

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; (2) a medium deep survey using the Wide Field Camera to take data whenever one of the other instruments was being used 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.

Important discoveries


The Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required new theories
Scientific theory
A scientific theory comprises a collection of concepts, including abstractions of observable phenomena expressed as quantifiable properties, together with rules that express relationships between observations of such concepts...

 to explain them. Among its primary mission targets was to measure distances to Cepheid variable
Cepheid variable
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...

 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
Errors and residuals in statistics
In statistics and optimization, statistical errors and residuals are two closely related and easily confused measures of the deviation of a sample from its "theoretical value"...

 of up to 50%, but Hubble measurements of Cepheid variables in the Virgo Cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...

 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 estimates of the age of the universe, it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team
High-z Supernova Search Team
The High-z Supernova Search Team was an international cosmology collaboration which used Type Ia Supernovae to chart the expansion of the Universe. The team was formed in 1994 by Brian P. Schmidt, then a post-doctoral research associate at Harvard University, and Nicholas B. Suntzeff, a staff...

 and the Supernova Cosmology Project
Supernova Cosmology Project
The Supernova Cosmology Project is one of two research teams that determined the likelihood of an accelerating universe and therefore a positive Cosmological constant, using data from the redshift of Type Ia supernovae...

 used the telescope to observe distant supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

e 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; the most common cause attributed is dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

.

The high-resolution spectra and images provided by the HST have been especially well-suited to establishing the prevalence of black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

s 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. 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.
The collision of Comet Shoemaker-Levy 9
Comet Shoemaker-Levy 9
Comet Shoemaker–Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of solar system objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by...

 with Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 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
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 were sharper than any taken since the passage of Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 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
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...

; evidence for the presence of extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

s around sun-like stars; and the optical counterparts of the still-mysterious gamma ray burst
Gamma ray burst
Gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...

s. HST has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 and Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

.
A unique legacy of Hubble are the Hubble Deep Field
Hubble Deep Field
The Hubble Deep Field is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area 2.5 arcminutes across, two parts in a million of the whole sky, which is equivalent in angular size to a 65 mm tennis...

 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
SCP 06F6
SCP 06F6 is an astronomical object of unknown type, discovered on 21 February 2006 in the constellation Boötesduring a survey of galaxy cluster CL 1432.5+3332.8 with the Hubble Space Telescope's Advanced Camera for Surveys Wide Field Channel....

 was discovered by the Hubble Space Telescope (HST) in February 2006.

Impact on astronomy


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, and countless more have appeared in conference proceedings
Proceedings
In academia, proceedings are the collection of academic papers that are published in the context of an academic conference. They are usually distributed as printed books either before the conference opens or after the conference has closed. Proceedings contain the contributions made by researchers...

. Looking at papers several years after their publication, about one-third of all astronomy papers have no citation
Citation
Broadly, a citation is a reference to a published or unpublished source . More precisely, a citation is an abbreviated alphanumeric expression Broadly, a citation is a reference to a published or unpublished source (not always the original source). More precisely, a citation is an abbreviated...

s, 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.

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 m ground-based telescope such as the William Herschel Telescope
William Herschel Telescope
The William Herschel Telescope is a optical/near-infrared reflecting telescope located at the Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands, Spain. The telescope, which is named after William Herschel, is part of the Isaac Newton Group of Telescopes...

, the HST costs about 100 times as much to build and maintain.

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
Aperture masking interferometry
Aperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...

 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. Since then, advances in adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

 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.

Usage



Anyone can apply for time on the telescope; there are no restrictions on nationality or academic affiliation. 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.

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.

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.

Amateur observations


The first director of STScI, Riccardo Giacconi
Riccardo Giacconi
Riccardo Giacconi is an Italian/American Nobel Prize-winning astrophysicist who laid the foundations of X-ray astronomy. He is currently a professor at the Johns Hopkins University.- Biography :...

, announced in 1986 that he intended to devote some of his Director Discretionary time to allowing amateur astronomers
Amateur astronomy
Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...

 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 review
Peer review
Peer review is a process of self-regulation by a profession or a process of evaluation involving qualified individuals within the relevant field. Peer review methods are employed to maintain standards, improve performance and provide credibility...

ed 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 — 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, 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.

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.

Transmission to Earth


Hubble data was initially stored on the spacecraft. When launched, the storage facilities were old-fashioned reel-to-reel tape recorder
Tape recorder
An audio tape recorder, tape deck, reel-to-reel tape deck, cassette deck or tape machine is an audio storage device that records and plays back sounds, including articulated voices, usually using magnetic tape, either wound on a reel or in a cassette, for storage...

s, but these were replaced by solid state
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...

 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
Tracking and Data Relay Satellite System
The Tracking and Data Relay Satellite System is a network of American communications satellites and ground stations used by NASA for space communications. The system was designed to replace an existing network of ground stations that had supported all of NASA's manned flight missions...

 (TDRSS). 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
White Sands Test Facility
White Sands Test Facility is a U.S. government rocket engine test facility and a resource for testing and evaluating potentially hazardous materials, space flight components, and rocket propulsion systems. NASA established WSTF on the White Sands Missile Range in 1963...

 in White Sands
White Sands, New Mexico
White Sands is a census-designated place in Doña Ana County, New Mexico, United States. The population was 1,323 at the 2000 census. It is part of the Las Cruces Metropolitan Statistical Area...

, New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

. From there they are sent to the Goddard Space Flight Center and finally to the Space Telescope Science Institute for archiving.

These data are then transmitted to the Space Telescope Operations Control Center (STOCC)
Goddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...

 located in Greenbelt, Maryland
Greenbelt, Maryland
Greenbelt is a city in Prince George's County, Maryland, United States. Contained within today's City of Greenbelt is the historic planned community now known locally as "Old Greenbelt" and designated as the Greenbelt Historic District...

.

Archives


All Hubble data is eventually made available via the archive
Archive
An archive is a collection of historical records, or the physical place they are located. Archives contain primary source documents that have accumulated over the course of an individual or organization's lifetime, and are kept to show the function of an organization...

s at STScI, CADC and ESO
ESO
ESO, as a three-letter abbreviation, may stand for:* European Southern Observatory* Ensemble Studios Online* English Symphony Orchestra* Edmonton Symphony Orchestra* Executive Stock Options...

. Data is usually proprietary—available only to the principal investigator
Principal investigator
A principal investigator is the lead scientist or engineer for a particular well-defined science project, such as a laboratory study or clinical trial....

 (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 is in the FITS
FITS
Flexible Image Transport System is a digital file format used to store, transmit, and manipulate scientific and other images. FITS is the most commonly used digital file format in astronomy...

 format, which is suitable for astronomical analysis but not for public use. The Hubble Heritage Project
Hubble Heritage Project
The Hubble Heritage Project was founded by a group of astronomers in 1998. The team releases on an almost monthly basis pictures of celestial objects like planets, stars, galaxies and galaxy clusters....

 processes and releases to the public a small selection of the most striking images in JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....

 and TIFF
Tagged Image File Format
TIFF is a file format for storing images, popular among graphic artists, the publishing industry, and both amateur and professional photographers in general. As of 2009, it is under the control of Adobe Systems...

 formats.

Pipeline reduction


Astronomical data taken with CCDs
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 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.

Data analysis


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
IRAF
IRAF is a collection of software written at the National Optical Astronomy Observatory geared towards the reduction of astronomical images in pixel array form. This is primarily data taken from imaging array detectors such as CCDs...

, a popular astronomical data reduction program.

Outreach activities




It has always been important for the Space Telescope to capture the public's imagination, given the considerable contribution of taxpayers to its construction and operational costs. After the difficult early years when the faulty mirror severely dented Hubble's reputation with the public, the first servicing mission allowed its rehabilitation as the corrected optics produced numerous remarkable images.

Several initiatives have helped to keep the public informed about Hubble activities. The Hubble Heritage Project
Hubble Heritage Project
The Hubble Heritage Project was founded by a group of astronomers in 1998. The team releases on an almost monthly basis pictures of celestial objects like planets, stars, galaxies and galaxy clusters....

 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.

In addition, STScI maintains several comprehensive websites for the general public containing Hubble images and information about the observatory. The outreach efforts are coordinated by the Office for Public Outreach, which was established in 2000 to ensure that US taxpayers saw the benefits of their investment in the space telescope program.

Since 1999, the leading Hubble outreach activities group in Europe has been the Hubble European Space Agency Information Centre
Hubble European Space Agency Information Centre
The Hubble European Space Agency Information Centre is a science communication office, established at the Space Telescope - European Coordinating Facility in Munich, Germany late in 1999...

 (HEIC). 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
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 (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
Space Foundation
The Space Foundation is a nonprofit organization that supports the global space industry through information and education programs. It is a resource for the entire space community - industry, national security organizations, civil space agencies, private space companies and the military around the...

 for its outreach activities, in 2001 and 2010.

There is a replica of the Hubble Telescope on the courthouse lawn in Marshfield, Missouri
Marshfield, Missouri
Marshfield is a city in Webster County, Missouri, United States. The population was 6,633 at the 2010 census. It is the county seat and part of the Springfield, Missouri Metropolitan Statistical Area.- History :...

, the hometown of namesake Edwin P. Hubble.

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
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...

.
The episodes are available in several formats and resolutions for different platforms.

Equipment failure


Past servicing missions have exchanged old instruments for new ones, both avoiding failure and making possible new types of science. Without servicing missions, all of the instruments will eventually fail. In August 2004, the power system of the Space Telescope Imaging Spectrograph
Space Telescope Imaging Spectrograph
The Space Telescope Imaging Spectrograph is a spectrograph, also with a camera mode, installed on the Hubble Space Telescope. It operated continuously from 1997 until a power supply failure in 2004. After repairs, it began operating again in 2009...

 (STIS) failed, rendering the instrument inoperable. The electronics had originally been fully redundant, but the first set of electronics failed in May 2001. This power supply was fixed during servicing mission 4 in May 2009. Similarly, the main camera (the ACS
Advanced Camera for Surveys
The Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...

) primary electronics failed in June 2006, and the power supply for the backup electronics failed on January 27, 2007. 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. As of late May 2009, tests of both repaired instruments are still ongoing.

HST uses gyroscope
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...

s 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. There are further contingency plans for science with just one gyro, 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. One more gyro failed in 2007. 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.

In addition to predicted gyroscope failure, Hubble eventually required a change of nickel hydrogen batteries
Nickel hydrogen battery
A nickel–hydrogen battery is a rechargeable electrochemical power source based on nickel and hydrogen. It differs from a nickel–metal hydride battery by the use of hydrogen in a pressurized cell at up to 1200 psi pressure.The cathode is made up of a dry sintered porous nickel plaque, which...

. 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. In the end, however, the batteries were simply replaced during service mission 4.

Orbital decay


Hubble orbits the Earth in the extremely tenuous upper atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, and over time its orbit decays
Orbital decay
Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit.This can be due to drag produced by an atmosphere due to frequent collisions between the satellite and surrounding air molecules. The drag experienced by the object is larger in the case of increased solar...

 due to drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

. 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). With the success of STS-125
STS-125
STS-125, or HST-SM4 , was the fifth and final space shuttle servicing mission to the Hubble Space Telescope .Launch occurred on 11 May 2009 at 2:01 pm EDT...

, 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
Smithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...

. This is no longer possible since the space shuttle has been retired
Space Shuttle retirement
The retirement of NASA's Space Shuttle fleet took place from March to July 2011. Discovery was the first of the three active space shuttles to be retired, completing its final mission on March 9, 2011; Space Shuttle Endeavour did so on June 1...

, 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. The final decision was not to attach a de-orbit module on STS-125
STS-125
STS-125, or HST-SM4 , was the fifth and final space shuttle servicing mission to the Hubble Space Telescope .Launch occurred on 11 May 2009 at 2:01 pm EDT...

, but to add a grapple fixture so a robotic mission could more easily attach such a module later.

Debate over final servicing mission


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
Wide Field and Planetary Camera 2
The Wide Field and Planetary Camera 2 is a baby grand piano sized camera built by the Jet Propulsion Laboratory and formerly installed on the Hubble Space Telescope. It was installed by servicing mission 1 in 1993, replacing the telescope's original Wide Field and Planetary Camera...

 with a new Wide Field Camera 3
Wide Field Camera 3
The Wide Field Camera 3 is the Hubble Space Telescope's last and most technologically advanced instrument to take images in the visible spectrum...

 and installing the Cosmic Origins Spectrograph
Cosmic Origins Spectrograph
The Cosmic Origins Spectrograph is a science instrument that was installed on the Hubble Space Telescope during Servicing Mission 4 in May 2009. It is designed for ultraviolet spectroscopy of faint point sources with a resolving power of ≈1,550–24,000...

 (COS). However, then-NASA Administrator Sean O'Keefe
Sean O'Keefe
Sean O'Keefe is the CEO of EADS North America, a subsidiary of the European aerospace firm EADS, a former Administrator of NASA, and former chancellor of Louisiana State University . O'Keefe is also a former member of the board of directors of DuPont...

 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
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...

 (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.

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
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...

 (JWST), will not be ready until well after the 2010 scheduled retirement of the space shuttle. While Hubble can image in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 and visible wavelengths, JWST is limited to the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

. 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. The consideration that the JWST will not be located in low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

, 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
United States National Academy of Sciences
The National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...

 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 robot
Robot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...

ic service mission. These plans were later canceled, the robotic mission being described as "not feasible". 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.

The arrival in April 2005 of the new NASA Administrator, Michael D. Griffin
Michael D. Griffin
Michael Douglas Griffin is an American physicist and aerospace engineer. From April 13, 2005 to January 20, 2009 he served as Administrator of NASA, the space agency of the United States...

, 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
STS-125
STS-125, or HST-SM4 , was the fifth and final space shuttle servicing mission to the Hubble Space Telescope .Launch occurred on 11 May 2009 at 2:01 pm EDT...

 mission by Atlantis was scheduled for launch in October 2008. 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. 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 and completed all the long planned replacements as well as additional repairs, including replacing the main data-handling unit.

Successors


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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 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
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...

 (JWST), the formally planned successor of Hubble. Very different from a scaled up Hubble, it is designed to operate colder and farther away from the Earth at the L2 Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

, 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. 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
Timeline of the Big Bang
This timeline of the Big Bang describes the history of the universe according to the prevailing scientific theory of how the universe came into being, using the cosmological time parameter of comoving coordinates...

 approximately 280 million years older than stars HST now detects. The telescope is an international collaboration between NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

, the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 and the Canadian Space Agency since 1996, and is planned for launch on a Ariane 5
Ariane 5
Ariane 5 is, as a part of Ariane rocket family, an expendable launch system used to deliver payloads into geostationary transfer orbit or low Earth orbit . Ariane 5 rockets are manufactured under the authority of the European Space Agency and the Centre National d'Etudes Spatiales...

 rocket. Although JWST is primarily an infrared instrument, its wavelength coverage extends down to 600 nm wavelength light, or roughly orange in the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

. A typical human eye can see to about 750 nm wavelength light, so there is some overlap with the longest wavelength visible bands, including orange and red light.
Color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...

Wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

violet
Violet (color)
As the name of a color, violet is synonymous with a bluish purple, when the word "purple" is used in the common English language sense of any color between blue and red, not including either blue or red...

380–450 nm
blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

450–475 nm
cyan
Cyan
Cyan from , transliterated: kýanos, meaning "dark blue substance") may be used as the name of any of a number of colors in the blue/green range of the spectrum. In reference to the visible spectrum cyan is used to refer to the color obtained by mixing equal amounts of green and blue light or the...

476–495 nm
green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...

495–570 nm
yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...

570–590 nm
orange 590–620 nm
red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

620–750 nm

Selected space telescopes & instruments
Name Year Wavelength Aperture
Human Eye - 0.39-0.75 μm 0.01 m
Spitzer
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

 
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
Herschel Space Observatory
The Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of in diameter....

 
2009 60-672 μm 3.5 m
JWST
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...

 
Planned 0.6-28.5 μm 6.5 m


A complementary telescope, looking at even longer wavelengths then Hubble or JWST, is the European Space Agency's Herschel Space Observatory
Herschel Space Observatory
The Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of in diameter....

, 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
Advanced Technology Large-Aperture Space Telescope
The Advanced Technology Large-Aperture Space Telescope is an 8 to 16.8-meter UV-optical-NIR space telescope proposed by Space Telescope Science Institute, the science operations center for the Hubble Space Telescope...

 (AT-LAST) A conceptualized 8 to 16-meter (320 to 640-inch) optical space telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 that if approved, built, and launched (perhaps using the Space Launch System
Space Launch System
The Space Launch System, or SLS, is a Space Shuttle-derived heavy launch vehicle being designed by NASA, following the cancellation of the Constellation Program, to replace the retired Space Shuttle. The NASA Authorization Act of 2010 envisions the transformation of the Ares I and Ares V vehicle...

), 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
Infrared astronomy
Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers...

, but with substantially better resolution than Hubble or Spitzer
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

. This effort is being planned for the 2025-2035 time frame.
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
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

 (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 – Lucky Cam
Lucky imaging
Lucky imaging is one form of speckle imaging used for astronomical photography. Speckle imaging techniques use a high-speed camera with exposure times short enough so that the changes in the Earth's atmosphere during the exposure are minimal.With lucky imaging, those optimum exposures least...

, 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
Airglow
Airglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist...

 that washes out faint, low-contrast astronomical objects.

See also




External links