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Nebula

Nebula

: The Nebula Awards, annually given out by the Science Fiction and Fantasy Writers of America, are named after nebulae. Science Fiction and Fantasy Writers of America, 2.7 million light-years from Earth. This nebula is a region in which stars are forming.]] Science Fiction and Fantasy Writers of America away. This image was taken with the Wide Field and Planetary Camera2 aboard NASA's Hubble Space Telescope.]] A nebula (Latin for "mist"; pl. nebulae) is an interstellar cloud of dust, gas and plasma. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy is sometimes referred to as the Andromeda Nebula).

Classification of Nebulae

Nebulae can be classified by how they are illuminated:
- Diffuse nebulae are illuminated nebulae
- Emission nebulae are internally illuminated clouds of ionized gas. Two of the most common types of emission nebula are H II regions and Planetary nebulae
- Reflection nebulae are illuminated by reflections from nearby stars. An example is the nebulosity NGC 1435 surrounding the Pleiades star cluster.
- Planetary nebulae are compact shells of gas around a dead star or an intermittantly active star. See Nova.
- Supernova remnants are generally moving away from their parent star at high speed, and are heated by colliding with (relatively) slow moving galactic dust and gas.
- Dark nebulae are unilluminated. They can be detected when they obscure stars or other nebulae. Famous examples include the Horsehead nebula in Orion, and the Coalsack Nebula in the Southern Cross.

Astrophysics of Nebulae

HII regions are the birthplace of stars. They are formed when very diffuse molecular clouds begin to collapse under their own gravity, often due to the influence of a nearby supernova explosion. The cloud collapses and fragments, forming sometimes hundreds of new stars. The newly-formed stars ionize the surrounding gas to produce an emission nebula. Other nebulae are formed by the death of stars. A star that undergoes the transition to a white dwarf blows off its outer layer to form a planetary nebula. Novae and supernovae can also create nebulae known as nova remnants and supernova remnants respectively.

NASA

] The National Aeronautics and Space Administration (NASA), which was established in 1958, is the agency responsible for the public space program of the United States of America. It is also responsible for long-term civilian and military aerospace research.

Vision and mission

NASA's vision is "to improve life here, extend life to there, and to find life beyond." Its mission is "to understand and protect our home planet; to explore the Universe and search for life; and to inspire the next generation of explorers."

History

Space Race

:For additional background, please see the Space Race article Space Race launch of Redstone rocket and NASA's Mercury 3 capsule Freedom 7 with Alan Shepard Jr. on the United States' first human flight into sub-orbital space. (Atlas rockets were used to launch Mercury's orbital missions.)]] Following the Soviet space program's launch of the world's first man-made satellite (Sputnik 1) on October 4, 1957, the attention of the United States turned toward its own fledgling space efforts. The U.S. Congress, alarmed by the perceived threat to U.S. security and technological leadership, urged immediate and swift action; President Dwight D. Eisenhower and his advisers counseled more deliberate measures. Several months of debate produced agreement that a new federal agency was needed to conduct all nonmilitary activity in space. On July 29, 1958, President Eisenhower signed the National Aeronautics and Space Act of 1958 establishing the National Aeronautics and Space Administration (NASA). When it began operations on October 1, 1958, NASA consisted mainly of the four laboratories and some 8,000 employees of the government's 46-year-old research agency for aeronautics, the National Advisory Committee for Aeronautics (NACA), though the probably most important contribution actually had its roots in the German rocket program led by Wernher von Braun, who is today regarded as the father of the United States space program. NASA's early programs were research into human spaceflight, and were conducted under the pressure of the competition between the USA and the USSR (the Space Race) that existed during the Cold War. The Mercury program, initiated in 1958, started NASA down the path of human space exploration with missions designed to discover simply if man could survive in space. Representatives from the U.S. Army (M.L. Raines, LTC, USA), Navy (P.L. Havenstein, CDR, USN) and Air Force (K.G. Lindell, COL, USAF) were selected/requested to provide assistance to the NASA Space Task Group through coordination with the existing U.S. military research and defense contracting infrastructure, and technical assistance resulting from experimental aircraft (and the associated military test pilot pool) development in the 1950s. On May 5, 1961, astronaut Alan B. Shepard Jr. became the first American in space when he piloted Freedom 7 on a 15-minute suborbital flight. John Glenn became the first American to orbit the Earth on February 20, 1962 during the 5-hour flight of Friendship 7. Once the Mercury project proved that human spaceflight was possible, project Gemini was launched to conduct experiments and work out issues relating to a moon mission. The first Gemini flight with astronauts on board, Gemini III, was flown by Virgil "Gus" Grissom and John W. Young on March 23, 1965. Nine other missions followed, showing that long-duration human space flight was possible, proving that rendezvous and docking with another vehicle in space was possible, and gathering medical data on the effects of weightlessness on humans.

Apollo program

Following the success of the Mercury and Gemini programs, the Apollo program was launched to try to do interesting work in space and possibly put men around (but not on) the Moon. The direction of the Apollo program was radically altered following President John F. Kennedy's announcement on May 25, 1961 that the United States should commit itself to "landing a man on the Moon and returning him safely to the Earth" by 1970. Thus Apollo became a program to land men on the Moon. The Gemini program was started shortly thereafter to provide an interim spacecraft to prove techniques needed for the now much more complicated Apollo missions. Gemini program.]] After eight years of preliminary missions, including NASA's first loss of astronauts with the Apollo 1 launch pad fire, and the first spacecraft to orbit the Moon (Apollo 8) at the end of 1968, the Apollo program achieved its goals with Apollo 11 which landed Neil Armstrong and Buzz Aldrin on the moon's surface on July 20, 1969 and returned them to Earth safely on July 24. Armstrong's first words upon stepping out of the Eagle lander captured the momentousness of the occasion: "That's one small step for [a] man, one giant leap for mankind." Twelve men would set foot on the Moon by the end of the Apollo program in December 1972. NASA had won the moon race, and in some senses this left it without direction, or at the very least without the public attention and interest that was necessary to guarantee large budgets from Congress. After President Lyndon Johnson left office, NASA lost its main political supporter, and rocket scientist Wernher von Braun was moved to a position lobbying in Washington. Plans for ambitious follow-on projects to construct a space station, establish a lunar base and launch a human mission to Mars by 1990 were proposed but with the end to procurement of Saturn and Apollo hardware, there was no capability to support these. The near-disaster of Apollo 13, where an oxygen tank explosion nearly doomed all three astronauts, helped to recapture national attention and concern. Although missions up to Apollo 20 were planned, Apollo 17 was the last mission to fly under the Apollo banner. The program ended because of budget cuts (in part due to the Vietnam War) and the desire to develop a reusable space vehicle.

Other early missions

Although the vast majority of NASA's budget has been spent on human spaceflight, there have been many robotic missions instigated by the space agency. In 1962 the Mariner 2 mission was launched and became the first spacecraft to make a flyby of another planet – in this case Venus. The Ranger, Surveyor, and Lunar Orbiter missions were essential to assessing lunar conditions before attempting Apollo landings with humans on board. Later, the two Viking probes landed on the surface of Mars and sent color images back to Earth, but perhaps more impressive were the Pioneer and particularly Voyager missions that visited Jupiter, Saturn, Uranus and Neptune sending back scientific information and color images. Having lost the moon race, the Soviet Union had, along with the USA, changed its approach. On July 17, 1975 an Apollo craft (finding a new use after the cancelling of planned lunar flights) was docked to the Soviet Soyuz 19 spacecraft, in the Apollo-Soyuz Test Project. Although the Cold War would last many more years, this was a critical point in NASA's history and much of the international co-operation in space exploration that exists today has its genesis with this mission. America's first space station, Skylab, occupied NASA from the end of Apollo until the late 1970s.

Shuttle era

Skylab 1981 ]] The space shuttle became the major focus of NASA in the late 1970s and the 1980s. Planned to be a frequently launchable and mostly reusable vehicle, four space shuttles were built by 1985. The first to launch, Columbia did so on April 12, 1981. The shuttle was not all good news for NASA – flights were much more expensive than initially projected, and even after the 1986 Challenger disaster highlighted the risks of space flight, the public again lost interest as missions appeared to become mundane. Work began on Space Station Freedom as a focus for the manned space programme but within NASA there was argument that these projects came at the expense of more inspiring unmanned missions such as the Voyager probes. The Challenger disaster aside the late 1980s marked a low point for NASA. Nonetheless, the shuttle has been used to launch milestone projects like the Hubble Space Telescope (HST). The HST was created with a relatively small budget of $2 billion but has continued operation since 1990 and has delighted both scientists and the public. Some of the images it has returned have become near-legendary, such as the groundbreaking Hubble Deep Field images. The HST is a joint project between ESA and NASA, and its success has paved the way for greater collaboration between the agencies. In 1995 Russian-American interaction would again be achieved as the Shuttle-Mir missions began, and once more a Russian craft (this time a full-fledged space station) docked with an American vehicle. This cooperation continues to the present day, with Russia and America the two biggest partners in the largest space station ever built – the International Space Station (ISS). The strength of their cooperation on this project was even more evident when NASA began relying on Russian launch vehicles to service the ISS following the 2003 Columbia disaster, which grounded the shuttle fleet for well over two years. Costing over one hundred billion dollars, it has been difficult at times for NASA to justify the ISS. The population at large have historically been hard to impress with details of scientific experiments in space, preferring news of grand projects to exotic locations. Even now, the ISS cannot accommodate as many scientists as planned. During much of the 1990s, NASA was faced with shrinking annual budgets due to Congressional belt-tightening in Washington, DC. In response, NASA's ninth administrator, Daniel S. Goldin, pioneered the "faster, better, cheaper" approach that enabled NASA to cut costs while still delivering a wide variety of aerospace programs (Discovery Program). That method was criticized and re-evaluated following the twin losses of Mars Climate Orbiter and Mars Polar Lander in 1999.

NASA's future

Mars Polar Lander and the planned crew and heavy lift launch vehicles]] NASA's most publicly-inspiring mission of recent years has probably been the Mars Pathfinder mission of 1997. Newspapers around the world carried images of the lander dispatching its own rover, Sojourner, to explore the surface of Mars in a way never done before at any extra-terrestrial location. Less publicly acclaimed but performing science from 1997 to date (2005) has been the Mars Global Surveyor orbiter. Since 2001, the orbiting Mars Odyssey has been searching for evidence of past or present water and volcanic activity on the red planet. NASA expects to continue exploring the Red Planet with more spacecraft such as the Mars Reconnaissance Orbiter, which will reach Mars in 2006. The Space Shuttle Columbia disaster in 2003, which killed the crew of six American and one Israeli astronaut, and caused a 29-month hiatus in space shuttle flights, triggered a serious re-examination of NASA's priorities. The U.S. government, various scientists, and the public all considered the future of the space program. On January 14, 2004, ten days after the landing of Mars Exploration Rover Spirit, President George W. Bush announced a new plan for NASA's future, dubbed the Vision for Space Exploration. According to this plan, humankind will return to the moon by 2020, and set up outposts as a testbed and potential resource for future missions. The space shuttle will be retired in 2010 and the Crew Exploration Vehicle will replace it by 2014, capable of both docking with the ISS and leaving the Earth's orbit. The future of the ISS is somewhat uncertain – construction will be completed, but beyond that is less clear. Although the plan initially met with skepticism from Congress, in late 2004 Congress agreed to provide start-up funds for the first year's worth of the new space vision. Hoping to spur innovation from the private sector, NASA established a series of Centennial Challenges, technology prizes for non-government teams, in 2004. The Challenges include tasks that will be useful for implementing the Vision for Space Exploration, such as building more efficient astronaut gloves.

Criticisms

Some commentators, such as Mark Wade, note that NASA has suffered from a 'stop-start' approach to its human spaceflight programs. The Apollo spacecraft and Saturn family of launch vehicles were abandoned in 1970 after billions of dollars had been spent on their development. In 2004 the U.S. Government proposed eventually replacing the Shuttle with a Crew Exploration Vehicle that would allow the agency to again send astronauts to the Moon. Despite the reduction of its budget following project Apollo, NASA has maintained a top-heavy bureaucracy resulting in inflated costs and compromised hardware. Crew Exploration Vehicle on October 31, 1998.]] Currently, the ISS relies on the Shuttle fleet for all major construction shipments. The Shuttle fleet has lost two spacecraft and fourteen astronauts in two disasters in 1986 and 2003. While the 1986 loss was made up with a Shuttle built from replacement parts, NASA does not plan to build another shuttle to replace the second loss. (But see also CEV.) The ISS, which was intended to have a crew of seven as of 2005, now has a skeleton crew of two, causing many intended research projects to be delayed. Other nations that have invested heavily in the space station's construction, such as the members of the European Space Agency, are fearful that the ISS's fate will soon match the fate of Skylab. As of 2005, however, all of the European and Japanese contributions to the ISS are years behind development schedule themselves.

NASA spaceflight missions

Human spaceflight

- Mercury program
- Gemini program
- Apollo program
- Skylab
- Space Shuttle
- International Space Station (working together with ESA, Rosviakosmos and JAXA)
- Project Constellation

Robotic space missions

- Earth Observing
- Upper Atmosphere Research Satellite
- TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)
- Lunar missions
- Ranger
- Surveyor
- Lunar Orbiter
- Clementine
- Lunar Prospector
- Mercury missions
- Mariner 10
- MESSENGER
- Venus missions
- Mariner 2, 5 and 10
- Pioneer Venus
- Magellan
- Mars missions
- Mariner 4, 6, 7, 8 and 9
- Viking 1 and 2
- Mars Observer
- Mars Pathfinder
- Mars Climate Orbiter
- Mars Polar Lander
- Mars Global Surveyor
- 2001 Mars Odyssey
- Mars Exploration Rovers
- Mars Reconnaissance Orbiter
- Phoenix Lander (Planned for 2007)
- Mars Science Laboratory (Planned for 2009)
- Jupiter missions
- Pioneer 10
- Galileo
- Juno
- Saturn missions
- Cassini-Huygens together with ESA
- Multi-planet missions
- Pioneer 11 – Jupiter and Saturn
- Mariner 10 – Venus and Mercury
- Voyager 1 – Jupiter and Saturn
- Voyager 2 – Jupiter, Saturn, Uranus and Neptune
- New Horizons (Planned for 2006) – Jupiter, Pluto and Kuiper Belt
- Asteroidal/cometary missions
- NEAR Shoemaker
- Deep Space 1
- Stardust
- Deep Impact
- Dawn (Planned for 2006)
- Proposed or canceled planetary-asteroid missions
- JIMO (cancelled)
- CRAF (cancelled)
- NetLanders (cancelled)
- Pluto Kuiper Express (cancelled; New Horizons is replacement)
- Titan Explorer (proposed)
- Neptune Orbiter (proposed)
- Sun observing missions
- SOHO – ESA partnership
- Ulysses – ESA partnership
- Great Observatories for Space Astrophysics
- Hubble Space Telescope – ESA partnership
- Compton Gamma Ray Observatory
- Chandra X-ray Observatory
- Spitzer Space Telescope (formerly known as the Space Infrared Telescope Facility, SIRTF)
- Other observatories
- COBE
- FUSE
- Infrared Astronomical Satellite
- James Webb Space Telescope – ESA partnership
- WMAP

List of NASA administrators

# T. Keith Glennan (1958–1961) # James E. Webb (1961–1968) # Thomas O. Paine (1969–1970) # James C. Fletcher (1971–1977) # Robert A. Frosch (1977–1981) # James M. Beggs (1981–1985) # James C. Fletcher (1986–1989) # Richard H. Truly (1989–1992) # Daniel S. Goldin (1992–2001) # Sean O'Keefe (2001–2005) # Michael Griffin (2005–)

Field installations

In addition to headquarters in Washington, D.C., NASA has field installations at:
- Ames Research Center, Moffett Field, California
- Dryden Flight Research Center, Edwards, California
- John H. Glenn Research Center at Lewis Field, Cleveland, Ohio
- Goddard Space Flight Center, Greenbelt, Maryland
- Goddard Institute for Space Studies, New York, New York
- Independent Verification and Validation Facility, Fairmont, West Virginia
- Wallops Flight Facility, Wallops Island, Virginia
- Jet Propulsion Laboratory, near Pasadena, California
- Deep Space Network stations:
- Goldstone Deep Space Communications Complex, Barstow, California
- Madrid Deep Space Communication Complex, Madrid, Spain
- Canberra Deep Space Communications Complex, Canberra, Australian Capital Territory
- Lyndon B. Johnson Space Center, Houston, Texas
- White Sands Test Facility, Las Cruces, New Mexico
- John F. Kennedy Space Center, Florida
- Langley Research Center, Hampton, Virginia
- George C. Marshall Space Flight Center, Huntsville, Alabama
- Michoud Assembly Facility, New Orleans, Louisiana
- John C. Stennis Space Center, Bay St. Louis, Mississippi

Awards and decorations

NASA presently bestows a number of medals and decorations to astronauts and other NASA personnel. Some awards are authorized for wear on active duty military uniforms. Current NASA awards are as follows:
- Congressional Space Medal of Honor
- NASA Distinguished Public Service Medal
- NASA Distinguished Service Medal
- NASA Equal Employment Opportunity Medal
- NASA Exceptional Achievement Medal
- NASA Exceptional Administrative Achievement Medal
- NASA Exceptional Bravery Medal
- NASA Exceptional Engineering Achievement Medal
- NASA Exceptional Scientific Achievement Medal
- NASA Exceptional Service Medal
- NASA Exceptional Technological Achievement Medal
- NASA Outstanding Leadership Medal
- NASA Public Service Medal
- NASA Space Flight Medal

Related legislation

- 1958 – National Aeronautics and Space Administration PL 85-568 (passed on July 29)
- 1961 – Apollo mission funding PL 87-98 A
- 1970 – National Aeronautics and Space Administration Research and Development Act PL 91-119
- 1984 – National Aeronautics and Space Administration Authorization Act PL 98-361
- 1988 – National Aeronautics and Space Administration Authorization Act PL 100-685
- NASA Budget 1958–2005 in 1996 Constant Year Dollars

External links

General

- [http://www.nasa.gov NASA Home Page]
- [http://www.nasawatch.com NASA Watch]
-

Further research

- [http://history.nasa.gov/series95.html NASA History Series Publications]
- [http://history.nasa.gov/SP-4012/cover.html NASA Historical Data Books (SP-4012)]
- [http://www.hq.nasa.gov/office/pao/History/hhrhist.pdf Research in NASA History: A Guide to the NASA History Program (large PDF – over 1,012 kb)]
- [http://ntrs.nasa.gov/ NTRS: NASA Technical Reports Server]
- [http://www.eventscope.org Eventscope] Category:Independent Agencies of the United States Government ko:미국항공우주국 ja:アメリカ航空宇宙局 simple:NASA th:องค์การนาซา

Hubble Space Telescope

The Hubble Space Telescope is a telescope in orbit around the Earth. Its position outside the Earth's atmosphere allows it to take extremely sharp images, and since its launch in 1990, it has become one of the most important telescopes in the history of astronomy. It has been responsible for many ground-breaking observations and has helped astronomers achieve a better understanding of many fundamental problems in astrophysics. From its original conception in 1946 until its launch, the project to build a space telescope was beset by delays and budget problems. Immediately after its launch, it was found that the main mirror suffered from spherical aberration, severely compromising the telescope's capabilities. However, after a servicing mission in 1993, the telescope was restored to its planned quality and became a vital research tool as well as a public relations boon for astronomy. The future of Hubble is currently uncertain. Though the United States Congress has appropriated funds to repair the telescope in July 2005, it is possible that a servicing mission may be cancelled again. Without intervention it will re-enter the Earth's atmosphere some time after 2010. Its successor telescope, the James Webb Space Telescope, is due to be launched in 2013.

Conception, design and aims

Proposals and precursors

2013 The history of the Hubble Space Telescope can be traced back as far as 1946, when astronomer Lyman Spitzer wrote a paper entitled Astronomical advantages of an extra-terrestrial observatory. In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes: First, the angular resolution (smallest separation at which objects can be clearly distinguished) would be limited only by diffraction, rather than by the turbulence in the atmosphere which causes stars to twinkle and is known to astronomers as seeing. Ground-based telescopes are typically limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.1 arcsec for a telescope with a mirror 2.5 m in diameter. The second major advantage would be that a space-based telescope could observe infrared and ultraviolet light, which are strongly absorbed by the atmosphere. Spitzer devoted much of his career to pushing for a space telescope to be developed. In 1962 a report by the US National Academy of Sciences recommended the development of a space telescope as part of the space program, and in 1965, Spitzer was appointed as head of a committee given the task of defining the scientific objectives for a large space telescope. Space-based astronomy had begun on a very small scale following World War II, as scientists made use of the developments in rocket technology that had taken place. The first ultraviolet spectrum of the Sun was obtained in 1946. An orbiting solar telescope was launched in 1962 by the UK as part of the Ariel space program, and 1966 saw NASA's launch of the first Orbiting Astronomical Observatory (OAO) mission. OAO-1's battery failed after three days, terminating the mission, but OAO-2 carried out ultraviolet observations of stars and galaxies from its launch in 1968 until 1972, well beyond its original planned lifetime of one year. The OAO missions demonstrated the important role space-based observations could play in astronomy, and 1968 saw the development by NASA of firm plans for a space-based reflecting telescope with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope, with a launch slated for 1979. These plans emphasised the need for manned maintenance missions to the telescope to ensure such a costly program had a lengthy working life, and the concurrent development of plans for the reusable Space Shuttle indicated that the technology to allow this was soon to become available .

The quest for funding

The continuing success of the OAO program encouraged increasingly strong consensus within the astronomical community that the LST (Large Space Telescope, the original name) should be a major goal. In 1970 NASA established two committees, one to plan the engineering side of the space telescope project, and the other to determine the science goals of the mission. Once these had been established, the next hurdle for NASA was to obtain funding for the instrument, which would be far more costly than any Earth-based telescope. The US Congress questioned many aspects of the proposed budget for the telescope and forced cuts in the budget for the planning stages, which at the time consisted of very detailed studies of potential instruments and hardware for the telescope. In 1974, public spending cuts instigated by Gerald Ford led to Congress cutting all funding for the telescope project. In response to this, a nationwide lobbying effort was co-ordinated among astronomers. Many astronomers met congressmen and senators in person, and large scale letter-writing campaigns were organised. The National Academy of Sciences published a report emphasising the need for a space telescope, and eventually the Senate agreed to a budget half that originally refused by Congress. The funding issues led to something of a reduction in the scale of the project, with the proposed mirror diameter reduced from 3 m to 2.4 m, both to cut costs and to allow a more compact and effective configuration for the telescope hardware. A proposed precursor 1.5m space telescope to test the systems to be used on the main satellite was dropped, and budgetary concerns also prompted collaboration with the European Space Agency. ESA agreed to supply some of the instruments for the telescope as well as the solar cells which would power it and contribute approximately 15% of the costs, in return for European astronomers being guaranteed at least 15% of observing time on the telescope. Congress eventually approved funding of US$36,000,000 for 1978, and the design of the LST began in earnest, aiming for a launch date of 1983. During the early 1980s, the telescope was named after Edwin Hubble, who made one of the greatest scientific breakthroughs of the 20th century when he discovered that the universe was expanding.

Construction and engineering

universe 1979]] Once the Space Telescope project had been given the go-ahead, work on the program was divided between many institutions. Marshall Space Flight Center was given responsibility for the design, development and construction of the telescope, while the Goddard Space Flight Center was given overall control of the scientific instruments and ground control centre for the mission. Marshall commissioned optics company Perkin-Elmer to design and build the Optical Telescope Assembly (OTA) and Fine Guidance Sensors for the space telescope. Lockheed were commissioned to construct the spacecraft in which the telescope would be housed.

Optical Telescope Assembly (OTA)

The mirror and optical systems of the telescope were the most crucial part, and were designed to exacting specifications. Telescopes typically have mirrors polished to an accuracy of about a tenth of the wavelength of visible light, but because the Space Telescope was to be used for observations ranging from ultraviolet to near-infrared with ten times better resolution than the best previous telescopes, its mirror needed to be polished to an accuracy of 1/20 of the wavelength of visible light, or about 30 nanometres. Perkin-Elmer intended to use extremely sophisticated computer-controlled polishing machines to grind the mirror to the required shape, but in case their cutting-edge technology ran into difficulties, Kodak was commissioned to construct a back-up mirror using traditional mirror-polishing techniques. Construction of the mirror began in 1979, using ultra-low expansion glass. To keep the mirror's weight to a minimum it consisted of inch-thick top and bottom plates sandwiching a honeycomb lattice. Mirror polishing began in 1979 and continued until May 1981. NASA reports at the time questioned Perkin-Elmer's managerial structure, and the polishing began to slip behind schedule and over budget. To save money, NASA halted work on the back-up mirror and put the launch date of the telescope back to October 1984. The mirror was completed by the end of 1981 with the addition of a reflective coating of aluminum 75 nm thick and a protective coating of magnesium fluoride 25 nm thick, which increased the mirror's reflectivity in ultraviolet light. However, doubts continued to be expressed about Perkin-Elmer's competence on a project of this importance as their budget and timescale for producing the rest of the OTA continued to inflate. In response to a schedule described as "unsettled and changing daily," NASA postponed the launch date of the telescope until April 1985. Perkin-Elmer's schedules continued to slip at a rate of about one month per quarter, and at times delays reached one day for each day of work. NASA was forced to postpone the launch date until first March and then September 1986. By this time the total project budget had risen to $1.175 billion .

Spacecraft systems

$ The spacecraft in which the telescope and instruments were to be housed was another major engineering challenge. It would have to adequately withstand frequent passages from direct sunlight into the darkness of Earth's shadow which would generate major changes in temperature, while being stable enough to allow the extremely accurate pointing of the telescope that would be required. A shroud of multi-layered insulation keeps the temperature within the telescope stable, and surrounds a light aluminium shell in which the telescope and instruments sit. Within the shell, a graphite-epoxy frame keeps the working parts of the telescope firmly aligned. While construction of the spacecraft in which the telescope and instruments would be housed proceeded somewhat more smoothly than the construction of the OTA, Lockheed still experienced some budget and schedule slippage, and by the summer of 1985, construction of the spacecraft was 30% over budget and three months behind schedule. An MSFC report said that Lockheed tended to rely on NASA directions rather than take their own initiative in the construction .

Ground support

In 1983, the Space Telescope Science Institute (STScI) was established after something of a power struggle between NASA and the scientific community at large. STScI is operated by the Association of Universities for Research in Astronomy (AURA) and is physically located on the Homewood campus of Johns Hopkins University in Baltimore, which is one of the 32 U.S. universities and 7 international affiliates that comprise the AURA consortium. STScI is responsible for the scientific operation of the telescope and delivery of data products to astronomers, a function which NASA had wanted to keep 'in-house', but which scientists were keen to see based in an academic establishment. Engineering support is provided by NASA and contractor personnel at the Goddard Space Flight Center in Greenbelt, Maryland, 30 miles south of the STScI. Hubble's operation is monitored 24 hours per day by four teams of flight controllers who make up Hubble's Flight Operations Team. The Space Telescope European Coordinating Facility was established at Garching bei München near Munich in 1984 to provide similar support primarily for European astronomers.

Challenger disaster

In early 1986, the planned launch date of October that year looked feasible, but the Challenger disaster brought the US space program to a halt, grounding the Space Shuttle fleet and forcing the launch of Hubble to be postponed for several years. All telescope parts had to be kept in clean rooms until a launch could be rescheduled, a costly situation which pushed the overall costs of the project still higher. Eventually, following the resumption of Shuttle flights in 1988, the launch of the telescope was scheduled for 1990. In preparation for its final launch, dust which had accumulated on the mirror since its completion had to be removed with jets of nitrogen, and all systems were tested extensively to ensure they were fully functional. Finally, on 24 April 1990, shuttle mission STS-31 saw Atlantis launch the telescope successfully into its planned orbit. From its original total cost estimate of 435 million dollars (in FY77 funds), the telescope had by now cost over US$2.5 billion to construct. Hubble's cumulative costs to-date are approximately 14 billion dollars (inflation adjusted to the buying power of FY2005).

Instruments

Atlantis When launched, the HST carried five scientific instruments: the Wide Field and Planetary Camera (WF/PC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS). WF/PC was a high-resolution imaging device primarily intended for optical observations. It was built by NASA's Jet Propulsion Laboratory, and incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest. The instrument contained four CCD chips, three of which were 'wide field' chips while the fourth was the 'planetary camera' (PC). The PC took images at a longer effective focal length than the WF chips, giving it a greater magnification. The GHRS was a spectrograph designed to operate in the ultraviolet. It was built by the Goddard Space Flight Center in conjunction with Ball Aerospace, and could achieve a spectral resolution of 90,000 . Also optimised for ultraviolet observations were the FOC and FOS, both of which were also capable of the highest spatial resolution of any instrument on Hubble. Rather than CCDs these three instruments used photon-counting digicons as their detectors. FOC was constructed by ESA, while the Martin Marietta corporation built the FOS. The final instrument was the HSP, designed and built at the University of Wisconsin. It was optimised for visible and ultraviolet light observations of variable stars and other astronomical objects varying in brightness. It could take up to 100,000 measurements per second with a photometric accuracy of about 2% or better .

Flawed mirror

Within weeks of the launch of the telescope, the images returned showed that there was a serious problem with the optical system. Although the first images appeared to be sharper than ground-based images, the telescope failed to achieve a final sharp focus, and the best image quality obtained was drastically lower than expected. Images of point sources spread out over a radius of more than one arcsecond, instead of having a point spread function concentrated within a circle 0.1 arcsec in diameter as had been specified in the design criteria . Analysis of the flawed images showed that the cause of the problem must be that the primary mirror had been ground to the wrong shape. Although it was probably the most accurately figured mirror ever made, with variations from the prescribed curve of no more than 1/20 of the wavelength of light, it was too flat at the edges. The mirror was barely 2 micrometres out from the required shape, but the difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edges of a mirror reaches a different focus to the light reflecting off the centre. The aberration meant that images from the Space Telescope were only marginally better than the best images obtainable from the ground.

Origin of the problem

focus Working backwards from images of point sources, astronomers determined that the conic constant of the mirror was −1.0139, instead of the intended −1.00229. The same number was also derived by analysing the null correctors (instruments which accurately measure the curvature of a polished surface) used by Perkin-Elmer to figure the mirror, as well as by analysing interferograms obtained during ground testing of the mirror. A commission was established to determine how the error could have arisen and was headed by Lew Allen, director of the Jet Propulsion Laboratory. The Allen Commission found that the null corrector used by Perkin-Elmer had been incorrectly calibrated, as a spot on a metering scale where an end cap had worn away was wrongly believed to be a valid scale. The null corrector had then been wrongly spaced by 1.3 mm. During the polishing of the mirror, Perkin-Elmer had analysed its surface with two other null correctors, both of which (correctly) indicated that the mirror was suffering from spherical aberration. These tests were specifically designed to eliminate the possibility of major optical aberrations. Against written quality guidelines the company ignored these test results as it believed that the two null correctors were less accurate than the primary device which was reporting that the mirror was perfectly figured. The commission blamed the failings primarily on Perkin-Elmer. Relations between NASA and the optics company had been severely strained during the telescope construction due to frequent schedule slippage and cost overruns. NASA found that Perkin-Elmer had not regarded the telescope mirror as a crucial part of their business and were also secure in the knowledge that NASA could not take its business elsewhere once the polishing had begun. While the commission heavily criticised Perkin-Elmer for these managerial failings, NASA was also criticised for not picking up on the quality control shortcomings such as relying totally on test results from a single instrument.

Design of a solution

The flaw meant that Hubble could obtain data about as good as that achievable with a large ground-based telescope on a night of good seeing, but at a vastly greater cost. NASA and the telescope became the butt of many jokes, and the project was popularly regarded as a white elephant. However, the design of the telescope had always incorporated servicing missions, and astronomers immediately began to seek potential solutions to the problem which could be applied at the first servicing mission, scheduled for 1993. While Kodak had ground a back-up mirror for Hubble, it would have been impossible to replace the mirror in orbit, or bring the telescope temporarily back to Earth for a refit. Instead, the fact that the mirror had been ground so precisely to the wrong shape led to the design of new optical components with exactly the same error but in the opposite sense, to be added to the telescope at the servicing mission, effectively acting as 'spectacles' to correct the spherical aberration. Because of the way the instruments were designed, two different sets of correctors were required. The design of the Wide Field and Planetary Camera (WFPC) included four relay mirrors to direct light onto the four separate charge-coupled device (CCD) chips making up the camera, and so the relay mirrors on the replacement Wide Field and Planetary Camera 2 could be figured to correct the aberration. However, the other instruments lacked any intermediate surfaces which could be figured in this way, and so required an external correction device.

COSTAR

The system designed to correct the spherical aberration for light focussed at the FOC, FOS and GHRS was called the "Corrective Optics Space Telescope Axial Replacement" (COSTAR) and consisted essentially of two mirrors in the light path, one of which would be figured to correct the aberration . To fit the COSTAR system onto the telescope, one of the other instruments had to be removed, and astronomers selected the High Speed Photometer to be sacrificed. During the first three years of the Hubble mission, before the optical corrections could be fitted, the telescope still carried out a large number of observations. Spectroscopic observations in particular were not too badly affected by the aberration, but many imaging projects were cancelled as the space telescope no longer gave decisive advantages over ground-based observations. Despite the setbacks, the first three years saw numerous scientific advances as astronomers worked to optimise the results obtained using sophisticated image processing techniques.

Servicing missions and new instruments

image processing

Servicing mission 1

The telescope had always been designed so that it could be regularly serviced, but after the problems with the mirror came to light, the first servicing mission assumed a much greater importance, as the astronauts would have to carry out extensive work on the telescope to install the corrective optics. The seven astronauts selected for the mission were trained intensively in the use of the hundred or so specialised tools which would need to be used. The mission (STS-61) took place in December 1993, and involved installation of several instruments and other equipment over a total of 10 days. Most importantly, the High Speed Photometer was replaced with the COSTAR corrective optics package, and WFPC was replaced with the Wide Field and Planetary Camera 2 (WFPC2), with its internal optical correction system. In addition, the solar arrays and their drive electronics were replaced, as well as four of the gyroscopes used in the telescope pointing system, two electrical control units and other electrical components, and two magnetometers. The onboard computers were upgraded, and finally, the telescope's orbit was boosted, having been slowly decaying for three years due to drag in the tenuous upper atmosphere. drag On January 13, 1994, NASA declared the mission a complete success and showed the first of many much sharper images . The mission had been one of the most complex ever undertaken, involving five lengthy periods of extravehicular activity, and its resounding success was an enormous boon for NASA, as well as for the astronomers who now had a fully capable space telescope.

Subsequent servicing missions

Subsequent servicing missions were less dramatic, but each gave the space telescope new capabilities. Servicing Mission 2 (STS-82) in February 1997 replaced the GHRS and the FOS with the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), replaced an Engineering and Science Tape Recorder with a new Solid State Recorder, repaired thermal insulation and again boosted Hubble's orbit. NICMOS contained a heat sink of solid nitrogen to reduce the thermal noise from the instrument, but shortly after it was installed, an unexpected thermal expansion resulted in part of the heat sink coming into contact with an optical baffle. This led to an increased warming rate for the instrument and reduced its original expected lifetime of 4.5 years to about 2 years. Servicing Mission 3A (STS-103) took place in December 1999, replaced all six gyroscopes (one had failed and rendered the telescope unusable just weeks before the mission), replaced a Fine Guidance Sensor and the computer, installed a Voltage/temperature Improvement Kit (VIK) to prevent battery overcharging, and replaced thermal insulation blankets. The new computer was based on a space-qualified Intel 486 and permits some computing tasks that were previously performed by computers on the ground to be handled on board the spacecraft. Servicing Mission 3B (STS-109) in March 2002 saw the installation of a new instrument, with the FOC being replaced with the Advanced Camera for Surveys (ACS), and also saw the revival of NICMOS, which had run out of coolant in 1999. A new cooling system was installed which reduced the instrument's temperature enough for it to be usable again, although it was not as cold as its original design called for. The mission replaced the solar arrays for a third time, with the new arrays being smaller but generating more power. The new arrays were derived from those built for the Iridium comsat system and were only two-thirds the size of the old arrays, resulting in less drag against the tenuous reaches of the upper atmosphere, while providing 30% more power. The additional power allowed all instruments on board the Hubble to be run simultaneously, and reduced a vibration problem that occurred when the old, less rigid arrays entered and left direct sunlight. Hubble's Power Distribution Unit was also replaced in order to correct a problem with sticky relays, a procedure that required the complete electrical power down of the spacecraft for the first time since it was launched. The completion of this servicing mission considerably enhanced Hubble's capabilities. The two instruments primarily affected by the mission, ACS and NICMOS, together imaged the Hubble Ultra Deep Field in 2003 to 2004.

Scientific results

Important discoveries

Hubble Ultra Deep Field] Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required whole new theories to explain them. Among its primary mission targets was to measure distances to Cepheid variable stars more accurately than ever before, and thus constrain the value of the Hubble constant, the measure of the rate at which the universe is expanding, which is also related to its age. Before the launch of Hubble, estimates of the Hubble constant typically had errors of up to 50%, but Hubble measurements of Cepheid variables in the Virgo cluster and other distant galaxy clusters provided a measured value with an accuracy of 10%, which is consistent with other accurate measurements made since Hubble's launch using other techniques. While Hubble helped to refine the age of the universe, it also threw doubt on its future. Astronomers using the telescope to observe distant supernovae uncovered evidence that far from decelerating under the influence of gravity, the universe may in fact be accelerating. This acceleration was later confirmed by other ground-based and space-based telescopes, but the cause of this acceleration is currently very poorly understood. The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was very fortuitously timed for astronomers, coming just a few months after Servicing Mission 1 had restored Hubble's optical performance. Hubble images of the planet were sharper than any taken since the passage of Voyager 2 in 1979, and were crucial in studying the dynamics of the collision of a comet with Jupiter, an event believed to occur once every few centuries. Other major discoveries made using Hubble data include proto-planetary disks (proplyds) in the Orion Nebula; evidence for the presence of extrasolar planets around sun-like stars; and the optical counterparts of the still-mysterious gamma-ray bursts.

Impact on astronomy

gamma-ray burst Many objective measures show the enormous impact of Hubble data on astronomy. Over 4,000 papers based on Hubble data have been published in peer-reviewed journals, and countless more have appeared in conference proceedings. Looking at papers several years after their publication, about one-third of all astronomy papers have no citations, while only 2% of papers based on Hubble data have no citations. On average, a paper based on Hubble data receives about twice as many citations as papers based on non-Hubble data. Of the 200 papers published each year which receive the most citations, about 10% are based on Hubble data . Although the HST has clearly had a significant impact on astronomical research, the financial cost of this impact has been very large. A study on the relative impacts on astronomy of different sizes of telescopes found that while papers based on HST data generate 15 times as many citations as a 4 m ground-based telescope such as the William Herschel Telescope, the HST cost about 100 times as much to build and maintain . The development of adaptive optics in recent years means that ground-based telescopes can take images approaching the sharpness of Hubble images, at much lower cost, and this has been a key consideration in the debate about the future of space telescopes (see below).

Using the telescope

Anyone can apply for time on the telescope; there are no restrictions on nationality or academic affiliation. Competition for time on the telescope is extremely intense, and the ratio of time requested to time available (the oversubscription ratio) typically ranges between 6 and 9. Calls for proposals are issued roughly annually, with time allocated for a 'cycle' lasting approximately one year. Proposals are divided into several categories; 'general observer' proposals are the most common, covering routine observations. 'Snapshot observations' are those in which targets require only 45 minutes or less of telescope time, including the overheads of acquiring the target and so on; snapshot observations are used to fill in gaps in the telescope schedule which cannot be filled by regular GO programs. Astronomers may make 'Target of Opportunity' proposals, in which observations are scheduled if a transient event covered by the proposal occurs during the scheduling cycle. In addition, up to 10% of the telescope time is designated Director's Discretionary (DD) Time. Astronomers can apply to use DD time at any time of year, and it is typically awarded for study of unexpected transient phenomena such as supernovae. Other uses of DD time have included the observations that led to the production of the Hubble Deep Field and Hubble Ultra Deep Field, and in the first four cycles of telescope time, observations carried out by amateur astronomers (discussed below).

Observation scheduling

Hubble Ultra Deep Field Scheduling observations for Hubble is not a simple matter. It is situated in a low-Earth orbit so that it can be reached by the Space Shuttle for servicing missions, but this means that most astronomical targets are occulted by the Earth for slightly less than half of each orbit. Observations cannot take place when the telescope passes through the South Atlantic Anomaly due to elevated radiation levels, and there is a also a sizable exclusion zone around the Sun, and for some instruments around the Moon and Earth, which cannot be observed. However, there is a so-called continuous viewing zone (CVZ), at roughly 90 degrees to the plane of Hubble's orbit, in which targets are not occulted for long periods. Due to the precession of the orbit, the location of the CVZ moves slowly over a period of eight weeks. Because the limb of the Earth is always within about 30° of regions within the CVZ, the brightness of scattered earthshine may be elevated for long periods during CVZ observations. Because Hubble orbits in the upper atmosphere, its orbit changes over time in a way that is not accurately predictable. The density of the upper atmosphere varies according to many factors, and this means that Hubble's predicted position for six week's time could be in error by up to 4,000 km. Observation schedules are typically finalised only a few days in advance, as a longer lead time would mean there was a chance that the target would be unobservable by the time it was due to be observed .

Amateur observations

The first director of the STScI, Riccardo Giacconi, announced in 1986 that he intended to devote some of his DD time to allowing amateur astronomers to use the telescope. The total time to be allocated was only a few hours per cycle, but excited great interest among amateur astronomers. Proposals for amateur time were stringently peer reviewed by a committee of leading amateur astronomers, and time was awarded only to proposals with genuine scientific merit which did not duplicate proposals made by professionals and which required the unique capabilities of the space telescope. In total, 13 amateur astronomers were awarded time on the telescope, with observations being carried out between 1990 and 1997. After that time, however, budget reductions at STScI made the support of work by amateur astronomers untenable, and no further amateur programs have been carried out .

Hubble data

Transmission to Earth

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

Pipeline reduction

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

Data analysis

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

Outreach activities

Mac OS X It has always been important for the Space Telescope to capture the public's imagination, given the considerable contribution of taxpayers to its construction and operational costs. After the difficult early years when the faulty mirror severely dented Hubble's reputation with the public, the first servicing mission allowed its rehabilitation as the corrected optics produced numerous remarkable images. Several initiatives have helped to keep the public informed about Hubble activities. The Hubble Heritage Project was established to produce high-quality images for public consumption of the most interesting and striking objects observed. The Heritage Team is composed of amateur as well as professional astronomers as well as people with backgrounds outside astronomy and emphasises the artistic nature of Hubble images. Hubble has also been used to photograph the Apollo 15 and 17 landing sites in the hope that parts of the lunar landing modules would be visible. In addition, STScI maintains several comprehensive websites for the general public containing Hubble images and information about the observatory. The outreach efforts are coordinated by the Office for Public Outreach, which was established in 2000 to ensure that US taxpayers saw the benefits of their investment in the space telescope program. The Heritage Project is granted a small amount of time to observe objects which, for scientific reasons, may not have images taken at enough wavelengths to construct a full colour image. In 2001, to celebrate the 11th anniversary of the launch of Hubble, NASA polled internet users to find out what they would most like Hubble to observe, and they overwhelmingly selected the Horsehead Nebula [http://heritage.stsci.edu/2001/12/caption.html]. A Heritage Project image of the nebula was released on 24 April 2001, the 11th anniversary of the launch.

Future

Equipment failure

2001]] Past servicing missions have exchanged old instruments for new ones, both avoiding failure and making possible new types of science. Without servicing missions, all of the instruments will eventually fail. On August 3, 2004, the power system of the Space Telescope Imaging Spectrograph (STIS) failed, rendering the instrument inoperable. The electronics had originally been fully redundant, but the first set of electronics failed in May 2001. It seems unlikely that any science functionality can be salvaged without a servicing mission. Hubble uses gyroscopes to stabilize itself in orbit and point accurately and steadily at astronomical targets. Normally, three gyroscopes are required for operation; observations are still possible with two gyros, but the area of sky that can be viewed would be somewhat restricted, and observations requiring very accurate pointing would be more difficult. In 2005, it was decided to switch to two-gyroscope mode for regular telescope operations as a means of extending the lifetime of the mission. The switch to this mode was made on August 31, 2005, leaving Hubble with two gyroscopes in use and two on backup. Estimates of the failure rate of the gyros indicate that Hubble may be down to one gyro by 2008, after which the telescope would be rendered unusable. In addition to predicted gyroscope failure, Hubble will eventually require a change of batteries. A robotic servicing mission including this would be tricky, as it requires many operations, and a failure in any might result in irreparable damage to Hubble. However, the observatory was designed so that during Shuttle servicing missions it would receive power from a connection to the Space Shuttle, and this fact may be utilized by adding an external power source (an additional battery) rather than changing the internal ones [http://news.bbc.co.uk/2/hi/science/nature/3652627.stm].

Orbital decay

Hubble orbits the Earth in the extremely tenuous upper atmosphere, and over time its orbit decays due to drag. If it is not re-boosted by a shuttle or other means, it will re-enter the Earth's atmosphere sometime between 2010 and 2032, with the exact date depending on how active the Sun is and its impact on the upper atmosphere. The state of Hubble's gyros also impacts the re-entry date, as a controllable telescope can be made to minimize atmospheric drag. Not all of the telescope would burn up on re-entry. Parts of the main mirror and its support structure would probably survive, leaving the potential for damage or even human fatalities (estimated at up to a 1 in 700 chance of human fatality for a completely uncontrolled re-entry). Addition of an external propulsion module to allow controlled re-entry is currently being investigated by NASA. It would not have to be executed until the expected natural re-entry date, after Hubble has completed its operational lifetime. One potential model involves a Pac-Man shaped unit entirely enclosing the satellite. Alternatively, instead of being used to control re-entry, the propulsion module could boost the telescope into a much higher orbit, in which it could remain indefinitely. Another possibility for safely de-orbiting Hubble is retrieval by a space shuttle. The Hubble telescope would then most likely be displayed in the Smithsonian Institution. The problems with this method are the cost of a shuttle flight (about US$500 million by some estimates) and risk to a shuttle's crew. In the wake of the Space Shuttle Columbia disaster, NASA's astronaut office is wary of risking a shuttle crew simply to retrieve a museum-bound telescope [http://www.space.com/businesstechnology/technology/hubble_grunsfeld_0306731.html]. Also, this mission would require a rebuild of the cargo space of the space shuttle sent to retrieve Hubble, since the only space shuttle unmodified since Hubble's launch (and therefore able to hold it in its cargo space) was the destroyed Columbia shuttle.

Debate over final servicing mission

The Space Shuttle was originally scheduled to visit Hubble again in February 2005. The tasks of this servicing mission would include adding fresh gyroscopes and replacing the Wide Field and Planetary Camera 2 with a new Wide Field Camera 3. However, then-NASA Administrator Sean O'Keefe decided that, in order to prevent a repeat of the Columbia disaster, all future shuttles must be inspected externally on orbit before re-entry, a task which cannot be done without the facilities of the International Space Station (ISS). The shuttle is incapable of reaching both HST and ISS during the same mission, and so future manned service missions were cancelled. This decision was assailed by numerous astronomers, who felt that the Hubble telescope was valuable enough to merit the risk. In particular, Hubble is one of the few telescopes currently operating which can image in the ultraviolet, and its successor telescope will not be launched until possibly several years after Hubble's demise. However, many astronomers feel strongly that the servicing of Hubble should not take place if the costs of the servicing come from the budget of its more important successor telescope, the JWST, as that could well cripple future space astronomy. The break in space observing capabilities between the decommissioning of Hubble and the commissioning of a successor is of major concern to some astronomers, given the great scientific impact of many space telescope observations. On 29 January 2004, Sean O'Keefe said that that he would review his decision to cancel the final servicing mission of the Hubble Space Telescope due to public outcry and requests from Congress for NASA to look for a way to save the Hubble Space Telescope. On 13 July 2004, an official panel from the National Academy of Sciences made the recommendation that the Hubble telescope be preserved despite the apparent risks. Their report urged "NASA should take no actions that would preclude a space shuttle servicing mission to the Hubble Space Telescope". On August 11, 2004, Sean O'Keefe requested the Goddard Space Flight Center to prepare a detailed proposal for a robotic service mission. It is expected that the proposal will take 12 months to produce—any such mission, likely to cost in excess of $1 billion, will not take place before 2007. The arrival, in April 2005, of the new NASA Administrator, Mike Griffin, has changed the status of both of the manned and unmanned rescue missions. Griffin has stated that he will reconsider the possibility of a manned servicing mission. Soon after his appointment, he authorized NASA's Goddard Space Flight Center to proceed with preparing for a manned Hubble maintenance flight, saying he would make the final decision on this flight after the next two shuttle missions. At the same time, Griffin decided to cancel the plans for a robotic rescue mission, calling it "not feasible." [http://www.washingtonpost.com/wp-dyn/content/article/2005/04/12/AR2005041201646.html]

Solutions

NASA and the ESA are currently investigating building a follow on to the Hubble Space Telescope called the Hubble Origins Probe http://www.pha.jhu.edu/hop/. If approved, it would not be ready for launch until 2010. The probe would very likely use an Atlas V rocket for its ride to orbit. It would also incorporate new technology into its design to reduce its weight in respect to the original. The mission would be a one time five year run and would receive no servicing from the Space Shuttle. The mission is still being debated and is still absent of any funding. Critics argue that the money would be better spent on a modern cost-effective space telescope design like the JWST rather than re-using the outdated design of Hubble. It may never be built.

References

- Benn C.R., Sánchez S.F. (2001), Scientific Impact of Large Telescopes, Publications of the Astronomical Society of the Pacific, v. 113, p.385
- Bless R.C., Walter L.E., White R.L. (1992), High Speed Photometer Instrument Handbook, v 3.0, STSci
- Brandt J.C. et al (1994), The Goddard High Resolution Spectrograph: Instrument, goals, and science results, Publications of the Astronomical Society of the Pacific, v. 106, p. 890-908
- Burrows C.J. et al (1991), The imaging performance of the Hubble Space Telescope, Astrophysical Journal, v.369, p.21
- Dunar A.J., Waring S.P. (1999), Power To Explore -- History of Marshall Space Flight Center 1960-1990, US Government Printing Office, ISBN 0160589924 (Chapter 12, Hubble Space telescope: [http://history.msfc.nasa.gov/book/chpttwelve.pdf]{{{{{{{{{{

Plasma

:This article is about plasma in the sense of an ionized gas. For other uses of the term, such as blood plasma, see plasma (disambiguation). plasma (disambiguation) In physics and chemistry, a plasma is an ionized gas, and is usually considered to be a distinct phase of matter. "Ionized" in this case means that at least one electron has been removed from a significant fraction of the molecules. The free electric charges make the plasma electrically conductive so that it couples strongly to electromagnetic fields. This fourth state of matter was first identified by Sir William Crookes in 1879 and dubbed "plasma" by Irving Langmuir in 1928, because it reminded him of a blood plasma [http://www.plasmacoalition.org/what.htm].

Common plasmas

blood plasma Plasmas are the most common phase of matter. The entire visible universe outside the Solar System is plasma, since all we can see are stars. Since the space between the stars is filled with a plasma, although a very sparse one (see interstellar- and intergalactic medium), essentially the entire volume of the universe is plasma. In the Solar System, the planet Jupiter accounts for most of the non-plasma, only about 0.1% of the mass and 10⁻¹⁵ of the volume within the orbit of Pluto. Alfvén also noted that due to their electric charge, very small grains also behave as ions and form part of a plasma (see dusty plasmas). Commonly encountered forms of plasma include:
- Artificially produced
- Inside fluorescent lamps (low energy lighting), neon signs
- Rocket exhaust
- The area in front of a spacecraft's heat shield during reentry into the atmosphere
- Fusion energy research
- The electric arc in an arc lamp or an arc welder
- Plasma ball (sometimes called a plasma sphere or plasma globe)
- Earth plasmas
- Flames (ie. fire)
- Lightning
- The ionosphere
- The polar aurorae
- Space and astrophysical
- The Sun and other stars (which are plasmas heated by nuclear fusion)
- The solar wind
- The Interplanetary medium (the space between the planets)
- The Interstellar medium (the space between star systems)
- The Intergalactic medium (the space between galaxies)
- The Io-Jupiter flux-tube
- Accretion disks
- Interstellar nebulae

Characteristics

The term plasma is generally reserved for a system of charged particles large enough to behave as one. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e. respond to magnetic fields and be highly electrically conductive). In technical terms, the typical characteristics of a plasma are: # Debye screening lengths that are short compared to the physical size of the plasma. # Large number of particles within a sphere with a radius of the Debye length. # Mean time between collisions usually is long when compared to the period of plasma oscillations.

Plasma scaling

Plasmas and their characteristics exist over a wide range of scales (ie. they are scaleable over many orders of magnitude). The following chart deals only with conventional atomic plasmas and not other exotic phenomena, such as, quark gluon plasmas:

	Typical plasma scaling ranges: orders of magnitude (OOM)
Characteristic	Terrestrial plasmas	Cosmic plasmas
Size in metres (m)	10⁻⁶ m (lab plasmas) to: 10² m (lightning) (~8 OOM)	10⁻⁶ m (spacecraft sheath) to 10²⁵ m (intergalactic nebula) (~31 OOM)
Lifetime in seconds (s)	10⁻¹² s (laser-produced plasma) to: 10⁷ s (fluorescent lights) (~19 OOM)	10¹ s (solar flares) to: 10¹⁷ s (intergalactic plasma) (~17 OOM)
Density in particles per cubic metre	10⁷ to: 10²¹ (inertial confinement plasma)	10³⁰ (stellar core) to: 10⁰ (i.e., 1) (intergalactic medium)
Temperature in kelvins (K)	~0 K (Crystalline non-neutral plasma[http://sdphca.ucsd.edu/]) to: 10⁸ K (magnetic fusion plasma)	10² K (aurora) to: 10⁷ K (Solar core)
Magnetic fields in teslas (T)	10⁻⁴ T (Lab plasma) to: 10³ T (pulsed-power plasma)	10⁻¹² T (intergalactic medium) to: 10⁷ T (Solar core)

Temperatures

plasma scaling characteristic of the gas being excited.]] The defining characteristic of a plasma is ionization. Although ionization can be caused by UV radiation, energetic particles, or strong electric fields, (processes that tend to result in a non-Maxwellian electron distribution function), it is more commonly caused by heating the electrons in such a way that they are close to thermal equilibrium so the electron temperature is relatively well-defined. Because the large mass of the ions relative to the electrons hinders energy transfer, it is possible for the ion temperature to be very different from (usually lower than) the electron temperature. The degree of ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density) in accordance with the Saha equation. If only a small fraction of the gas molecules are ionized (for example 1%), then the plasma is said to be a cold plasma, even though the electron temperature is typically several thousand degrees. The ion temperature in a cold plasma is often near the ambient temperature. Because the plasmas utilized in plasma technology are typically cold, they are sometimes called technological plasmas. They are often created by using a very high electric field to accelerate electrons, which then ionize the atoms. The electric field is either capacitively or inductively coupled into the gas by means of a plasma source, e.g. microwaves. Common applications of cold plasmas include plasma-enhanced chemical vapor deposition, plasma ion doping, and reactive ion etching. A hot plasma, on the other hand, is nearly fully ionized. This is what would commonly be known as the "fourth-state of matter". The Sun is an example of a hot plasma. The electrons and ions are more likely to have equal temperatures in a hot plasma, but there can still be significant differences.

Densities

Next to the temperature, which is of fundamental importance for the very existence of a plasma, the most important property is the density. The word "plasma density" by itself usually refers to the electron density, that is, the number of free electrons per unit volume. The ion density is related to this by the average charge state

\langle Z\rangle

of the ions through

n_e=\langle Z\rangle n_i

. (See quasineutrality below.) The third important quantity is the density of neutrals

n_0

. In a hot plasma this is small, but may still determine important physics. The degree of ionization is

n_i/(n_0+n_i)

Potentials

reactive ion etching Since plasmas are very good conductors, electric potentials play an important role. The potential as it exists on average in the space between charged particles, independent of the question of how it can be measured, is called the plasma potential or the space potential. If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to the development of a Debye sheath. Due to the good electrical conductivity, the electric fields in plasmas tend to be very small, although where double layers are formed, the potential drop can be large enough to accelerate ions to relativistic velocities and produce synchrotron radiation such as x-rays and gamma rays. This results in the important concept of quasineutrality, which says that, on the one hand, it is a very good approximation to assume that the density of negative charges is equal to the density of positive charges (

n_e=\langle Z\rangle n_i

), but that, on the other hand, electric fields can be assumed to exist as needed for the physics at hand. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation,

n_e \propto e^

. Differentiating this relation provides a means to calculate the electric field from the density:

\vec = (k_BT_e/e)(\nabla n_e/n_e)

. It is, of course, possible to produce a plasma that is not quasineutral. An electron beam, for example, has only negative charges. The density of a non-neutral plasma must generally be very low, or it must be very small, otherwise it will be dissipated by the repulsive electrostatic force. In astrophysical plasmas, Debye screening prevents electric fields from directly affecting the plasma over large distances (ie. greater than the Debye length). But the existence of charged particles causes the plasma to generate and be affected by magnetic fields. This can and does cause extremely complex behavior, such as the generation of plasma double layers, an object that separates charge over a few tens of Debye lengths. The dynamics of plasmas interacting with external and self-generated magnetic fields are studied in the academic discipline of magnetohydrodynamics.

In contrast to the gas phase

Plasma is often called the fourth state of matter. It is distinct from the three lower-energy phases of matter; solid, liquid, and gas, although it is closely related to the gas phase in that it also has no definite form or volume. There is still some disagreement as to whether a plasma is a distinct state of matter or simply a type of gas. Most physicists consider a plasma to be more than a gas because of a number of distinct properties including the following:

Property	Gas	Plasma
Electrical Conductivity	Very low	Very high For many purposes the electric field in a plasma may be treated as zero, although when current flows the voltage drop, though small, is finite, and density gradients are usually associated with an electric field according to the Boltzmann relation. The possibility of currents couples the plasma strongly to magnetic fields, which are responsible for a large variety of structures such as filaments, sheets, and jets. Collective phenomena are common because the electric and magnetic forces are both long-range and potentially many orders of magnitude stronger than gravitational forces.
Independently acting species	One	Two or three Electrons, ions, and neutrals can be distinguished by the sign of their charge so that they behave independently in many circumstances, having different velocities or even different temperatures, leading to new types of waves and instabilities, among other things
Velocity distribution	Maxwellian	May be non-Maxwellian Whereas collisional interactions always lead to a Maxwellian velocity distribution, electric fields influence the particle velocities differently. The velocity dependence of the Coulomb collision cross section can amplify these differences, resulting in phenomena like two-temperature distributions and run-away electrons.
Interactions	Binary Two-particle collisions are the rule, three-body collisions extremely rare.	Collective Each particle interacts simultaneously with many others. These collective interactions are about ten times more important than binary collisions.

Complex plasma phenomena

Boltzmann relation. Langmuir coined the name plasma because of its similarity to blood plasma, and Hannes Alfvén noted its cellular nature. Note also the filamentary blue outer shell of X-ray emitting high-speed electrons]] Plasma may exhibit complex behaviour. And just as plasma properties scale over many orders of magnitude (see table above), so do these complex features. Many of these features were first studied in the laboratory, and in more recent years, have been applied to, and recognised throughout the universe. Some of these features include:
- Filamentation, the striations or "stringy things" seen in a "plasma ball", the aurora, lightning, electric arcs, and nebulae. They are caused by larger current densities, and are also called magnetic ropes or plasma cables.
- Double layers, localised char