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Hubble Space Telescope

Hubble Space Telescope

The National Aeronautics and Space Administration's Hubble Space Telescope (HST) is the first major infrared -optical-ultraviolet telescope to be placed into orbit around Earth. The telescope is named after American astronomer Edwin P. Hubble, who found galaxies beyond the Milky Way in the 1920s, and discovered that the universe is uniformly expanding.

Located high above Earth's obscuring atmosphere, at an altitude of 580 kilometers (360 miles), the HST has provided the clearest views of the universe yet obtained in optical astronomy. Hubble's crystal-clear vision has fostered a revolution in optical astronomy. It has revealed a whole new level of detail and complexity in a variety of celestial phenomena, from nearby stars to galaxies near the limits of the observable universe. This has provided key insights into the structure and evolution of the universe across a broad scale. Its location outside of Earth's atmosphere has also provided Hubble with the ability to view astronomical objects across a wide swath of the electromagnetic spectrum , from ultraviolet light through visible and on to near-infrared wavelengths .

The heart of the telescope is the primary mirror, which is 94.5 inches (2.4 meters) in diameter. It is the smoothest optical mirror ever polished, with surface tolerance of one-millionth of an inch. It is made of fused silica glass and weighs about 670 kilograms (1,800 pounds).

Outside the blurring effects of Earth's turbulent atmosphere, the telescope can resolve astronomical objects ten times more clearly than can be seen with even larger ground-based optical telescopes. Hubble can see objects less than one-billionth as bright as what can be seen with the human eye. Hubble can detect objects as faint as thirty-first magnitude, which is comparable to the sensitivity of much larger Earth-based telescopes.

Hubble images have exceptional contrast, which allows astronomers to discern faint objects near bright objects. This enables scientists to study the environments around stars and to search for broad circumstellar disks of dust that may be forming into planets.

Launch and Servicing Missions

The HST was launched by the space shuttle Discovery on April 24, 1990. Hubble initially was equipped with five science instruments: the Wide-Field Planetary Camera, the Faint Object Camera, the Faint Object Spectrograph , the High-Resolution Spectrograph, and the High-Speed Photometer . In addition, Hubble was fitted with three fine guidance sensors used for pointing the telescope and for doing precision astrometrythe measurement of small angles on the sky.

After Hubble was launched, scientists discovered that its primary mirror was misshapen because of a fabrication error. This resulted in spherical aberration: the blurring of starlight because the telescope could not bring all the light to a single focus. Using image-processing techniques to reduce the blurring in HST images, scientists were able to do significant research with Hubble until an optical repair could be developed.

In December 1993, the first HST servicing mission carried replacement instruments and supplemental optics aboard the space shuttle Endeavour to restore the telescope to full optical performance. A deployable optical device, called the Corrective Optics Space Telescope Axial Replacement (COSTAR), was installed to improve the sharpness of the first-generation instruments. The COSTAR was outfitted with pairs of small mirrors that intercepted the incoming light from the primary mirror and reconstructed the beam so that it was in crisp focus. In addition, the original Wide-Field Planetary Camera was replaced with a second camera, the Wide-Field Planetary Camera 2, which has a built-in correction for the aberration in the primary mirror.

In March 1997 the space shuttle Discovery returned to the HST for a second servicing mission. Two advanced instruments, the Near Infrared Camera and Multi-Object Spectrometer and the Space Telescope Imaging Spectrograph were installed to replace two first-generation instruments. Astronauts also replaced or enhanced several electronic subsystems and patched unexpected tears in Hubble's shiny, aluminized, thermal insulation blankets, which give the telescope its distinctive foil-wrapped appearance and protect it from the heat and cold of space.

In December 1999 a third servicing mission replaced a number of subsystems but added no new instruments. About a month before the mission a critical gyroscope had failed, leaving Hubble with only two operational gyros out of a total of six onboard. This had left the telescope incapable of precision pointing. The December mission restored Hubble to six fully functioning gyroscopes. The telescope's main computer was upgraded from a 1960s computer with 48 kilobytes of memory, to an Intel 486 microprocessor.

In March 2002, the next and most ambitious serving mission in the series, involving five exhausting six-hour space walks by pairs of astronauts, took place. They installed a high-efficiency camera called the advanced camera for surveys. The mission also performed "heart surgery" by replacing a complex power control unit, which required completely shutting off the telescope's electrical power. The telescope also got stubby new solar panels that increased the power enough for all of the instruments to operate simultaneously.

In 2004, the last servicing mission will install the wide-field planetary camera 3 and the cosmic origins spectrograph. Hubble will be on its own until 2010, when NASA stops the observing program and must decide whether to retrieve Hubble and install a rocket propulsion system that will put it into a safe higher orbit or let it reenter the atmosphere and largely burn up over the ocean.

How Hubble Operates

Hubble is controlled at the Goddard Space Flight Center in Greenbelt, Maryland. The Space Telescope Science Institute (STSI), located at the Johns Hopkins University in Baltimore, Maryland, directs the science mission. Space telescope research and funding engages a significant fraction of the worldwide community of professional astronomers. Astronomers compete annually for observation time on Hubble.

Observing proposals are submitted to peer review committees of astronomers. The STSI director makes the final decision and can use his or her own discretionary time for special programs. Accepted proposals must be meticulously planned and scheduled by experts at STSI to maximize the telescope's efficiency.

The space telescope is not pointed by "real-time" remote control but instead automatically carries out a series of preprogrammed commands over the course of a day. This is necessary because the telescope is in a low Earth orbit , which prevents any one ground station from staying directly in contact with it. Instead, controllers schedule intermittent daily linkups with the space observatory via a series of satellites in geosynchronous orbit .

A date "pipeline," assembled and maintained by STSI, ensures that all observations are stored on optical disk for archival research. The data are sent to research astronomers for analysis, and then made available to astronomers worldwide one year after the observation.

By the turn of the twenty-first century, Hubble had looked at over 13,000 celestial targets and stored over 6 gigabytes of data onto large optical disks. The telescope had made nearly one quarter million exposures, approximately half of these were of astronomical targets and the rest were calibration exposures.

Hubble Provides New Insights

The HST has made dramatic inroads into a broad range of astronomical frontiers. Astronomers have used Hubble to look out into the universe over distances exceeding 12 billion light-years. Because the starlight harvested from remote objects began its journey toward Earth billions of years ago, the HST looks further back into time the farther away it looks into space (as do all large telescopes). Hubble has seen back to a time when the universe was only about 5 percent of its present age.

The Hubble Deep Field.

Hubble's deepest views of the universe, made with its visible and infrared cameras, are collectively called the Hubble Deep Field. These "long exposures" of the universe reveal galaxies that existed when the universe was less than 1 billion years old. The Hubble Deep Field also uncovered hundreds of galaxies at various stages of evolution, strung along a corridor of billions of light years. The high resolution of Hubble images enables astronomers to actually see the shapes of galaxies in the distant past and to study how they have evolved over time.

Expansion and Age of the Universe.

Another key project for the HST has been to make precise distance measurements for calculating the rate of expansion of the universe. This was achieved by measuring distances to galaxies much farther out than had previously been accomplished in decades of observing.

Determining the exact value of this rate is fundamental to calculating the age of the universe. In 1998, a team of astronomers triumphantly announced that they had accurately measured the universe's expansion rate to within an accuracy of 10 percent. This brought closure to a three-decade-long debate over whether the universe is 10 or 20 billion years old. The final age appears to be between 13 and 15 billion years, but this estimate is also affected by other parameters of the universe.

The HST was also used to find out if the universe was expanding at a faster rate long ago. This was done by using Hubble to peer halfway across the universe to find ancient exploding stars called supernovae. These stars can be used to calculate vast astronomical distances because they are so bright and shine at a predictable luminosity, which is a fundamental requirement for measuring distances.

Hubble observations, as well as other observations done with ground-based telescopes, show that the universe has not decelerated. In fact, to the surprise of astronomers, the expansion of the universe is accelerating, and therefore will likely expand forever. This realization offers compelling evidence that there is a mysterious repulsive force in space, first theorized by German-born American physicist Albert Einstein (1879-1955), which is pushing the galaxies apartin addition to the original impetus of the Big Bang .

This idea was bolstered in 2000 when Hubble astronomers accidentally discovered a supernova so far away, it exploded when the universe was actually decelerating. This supernova happened about 7 billion years ago, just before dark energy began accelerating the universe, like a car accelerating through a traffic light that has just turned green.

Black Holes.

The HST has provided convincing evidence of the existence of supermassive black holes that are millions or even a billion times more massive than the Sun. Hubble's exquisite vision allows astronomers to zoom in on the environment around a black hole and make critical measurement of the motion of stars and gas around the hole, to precisely measure its mass. The measurements show that there is far more mass at the core of galaxies than can be accounted for by starlight. This unseen mass is locked away inside black holes.

HST observations of both quiescent and active galaxies, the latter of which pours out prodigious amounts of energy, have shown that supermassive black holes are commonly found at the hub of a galaxy . A Hubble census of black holes also showed that the mass of a black hole corresponds to the mass of the central bulge of a galaxy. Therefore, galaxies with large bulges have more massive black holes than galaxies with smaller bulges. This suggests that supermassive black holes may be intimately linked to a galaxy's birth and evolution.

Quasars.

Hubble's keen ability to discern faint objects near bright objects allowed for definitive observations that showed the true nature of quasars, which are compact powerhouses of light that resemble stars and that reside largely at the outer reaches of the universe. HST observations conclusively showed that quasars dwell in the cores of galaxies, which means they are powered by supermassive black holes that are swallowing material at a furious rate.

Gamma-Ray Bursts.

Hubble played a key role in helping astronomers resolve questions regarding the nature of mysterious gamma-ray bursts. Gamma-ray bursts are powerful blasts that come from random directions in the universe about once per day. Hubble observations found host galaxies associated with some of these blasts. This places the bursts at cosmological distances rather then being localized phenomena within our galaxy. Hubble also showed that the blasts occur among the young stars in the spiral arms of a host galaxy. This favors neutron star collisions or neutron star-black hole collisions as the source of the bursts.

Stellar Environments.

The HST has unveiled a wide variety of shapes, structures, and fireworks that accompany the birth and death of stars. HST images have provided a clear look at pancake-shaped disks of dust and gas swirling around and feeding embryonic stars. Besides helping build the star, the disks are also the prerequisite for condensing planets. Hubble images also show blowtorch-like jets of hot gas streaming from deep within the disks. These jets are an "exhaust product" of star formation.

In dramatic images, HST has shown the effects of very massive young stars on their surrounding nebulae. The astronomical equivalent of a hurricane, the intense flow of visible and ultraviolet radiation from an exceptionally massive young star eats into surrounding clouds of cold hydrogen gas, laced with dust. This helps trigger a firestorm of star birth in the neighborhood around the star.

The HST has produced a dazzling array of images of colorful shells of gas blasted into space by dying stars. These intricate structures are "fossil evidence" showing that the final stages of a star's life are more complex than once thought. An aging star sheds its outer layers of gas through stellar winds. Late in a star's life, these winds become more like a gale, and consequently sculpt strikingly complex shapes as they plow into slower-moving material that was ejected earlier in the star's life.

The most dramatic star-death observation for the HST has been tracking the expanding wave of debris from the explosion of supernova 1987A. HST observations show that debris from the supernova blast is slamming into a ring of material around the dying star. The crash has allowed scientists to probe the structure around the supernova and uncover new clues about the star's final years.

Extrasolar Planets.

Even Hubble's powerful vision is not adequate to see the feeble flicker of a planet near a star. Nevertheless, Hubble was still very useful for conducting the first systematic search for a special type of planet far beyond our stellar neighborhood. For ten consecutive days Hubble peered at the globular cluster 47 Tucane to capture the subtle dimming of a star due to the eclipse-like passage of a Jupiter-sized planet in front of the star. Based on extrasolar planet discoveries in our own stellar neighborhood, astronomers predicted that seventeen planets should have been discovered. However, Hubble did not find any, which means that conditions favoring planet formation may be different elsewhere in the galaxy.

Aiming at a known planet 150 light-years away, Hubble made the first-ever detection of an atmosphere around a planet. When the planet passed in front of its star, Hubble measured how starlight was filtered by skimming through the atmosphere. Hubble measures the presence of sodium in the atmosphere. These techniques could eventually lead to the discovery of oxygen in the atmospheres in inhabited terrestrial extrasolar planets.

see also Astronomy, Kinds of (volume 2); Extrasolar Planets (volume 2); Gyroscopes (volume 3); Hubble, Edwin P. (volume 2); Observatories, Space-Based (volume 2).

Ray Villard

Bibliography

Chaisson, Eric. The Hubble Wars. New York: Harper Collins, 1994.

Smith, Robert W. The Space Telescope. Cambridge, UK: Cambridge University Press,1993.

Internet Resources

The Hubble Space Telescope. Space Telescope Science Institute. <http://hst.stsci.edu/>.

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Hubble Space Telescope (HST)

Hubble Space Telescope (HST)

The Hubble Space Telescope (HST) is a large Earth-orbiting astronomical telescope designed by the United States National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). Hubble observes the heavens from 380 mi (612 km) above the earth, relaying pictures and data captured above the distortions of Earth's atmosphere. The HST is named after American astronomer Edwin P. Hubble (1889-1953), who early in the twentieth century provided evidence of an expanding universe consisting of many galaxies beyond our Milky Way galaxy. The HST has provided scientists with the clearest views yet obtained of the universe. Moreover, stunning images and spectrographic data sent from the HST provide scientists with critical data relevant to studies regarding the birth of galaxies, the existence of black holes, and the workings of planetary systems around stars.

Deployed from the space shuttle Discovery on April 25, 1990, the Hubble Space Telescope was the culmination of a 20-year scientific effort to construct one of the largest and most complex satellites ever built. Astronomers first proposed the idea of building an orbiting observatory in the 1940s. The $1.5 billion project to build the Hubble Space Telescope began in earnest in 1977 after the United States Congress passed a resolution granting approval for the HST construction. By 1985, the HST was completed and ready for launch. The explosion of the space shuttle Challenger and loss of its crew in January 1986 delayed the Hubble's launch four years. As NASA officials re-evaluated the space shuttle program, the HST was relegated to storageat a maintenance cost of up to one million dollars a month.

The HST is roughly the size of a school bus, and is modular in design to facilitate in-orbit servicing. Like any reflecting telescope, the Hubble uses a system of mirrors to magnify and focus light. The primary mirror is concave, and a smaller convex secondary mirror is placed in front of the primary mirror to boost the telescope's total effective focal length. The telescope receives its main power from a pair of flexible, lightweight solar arrays. Each array is a large (40 ft by 8 ft, or 12.2 m by 2.4 m) rectangle of light-collecting solar cells. Exterior thermal blanketing protects the HST from the extreme temperature changes encountered during each 95-minute orbit of the earth.

Shortly after the 1990 launch of the HST, scientists found the telescope was unable to adequately focus light to provide desired resolutions. Fuzzy halos appeared around objects observed by the HST. The culprit was found to be a defect in the primary mirror. As a result of an incorrect adjustment to a testing device, the mirror was precisely, but inaccurately, ground to a curvature that was too flat at its edge. Although the error measured less than a micron (one ten-thousandth of an inch), the defect caused a spherical aberration when light reflected by the mirror focused across a wider area than necessary for a sharp image. The problem was corrected in December 1993 when, following an orbital rendezvous between the space shuttle Endeavor and the HST, the crew of Endeavor completed the first Hubble servicing mission. During the eleven-day operation, the Corrective Optics Space Telescope Axial Replacement (COSTAR) was installed. COSTAR corrected the spherical aberration of the HST primary mirror with a series of mirrors designed to act as corrective "eyeglasses" able to focus the blurred uncorrected image.

The Hubble Space Telescope carries a variety of on-board, scientific instruments designed to collect and send data to awaiting scientists. As needed, instruments are replaced or added during Hubble servicing missions. In 1977, two spectrographs were replaced with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS), and the Space Telescope Imaging Spectrograph (STIS). NICMOS allows the telescope to see objects in near-infrared wavelengths. These observations are important in astronomy , as well as in the study of the visible-light-obscuring gas and dust nebular clouds where stars are born. The STIS collects light from hundreds of points across a target and spreads it out into a spectrum, creating an image from which scientists can study individual wavelengths of radiation from a distant source. STIS is especially helpful to scientists studying regions of space where black holes are presumed to exist. In 1993, the HST's original Wide Field Planetary Camera was replaced with an updated version complete with relay mirrors spherically aberrated to correct for the spherical aberration on the Hubble's primary mirror. In 1999, the HST received a new high-speed computer.

Once the Hubble gathers data and pictures from celestial objects, its computers send the digitized information to Earth as radio signals. The HST signal is passed through a series of satellite relays, then to the Goddard Space Flight Center in Maryland before reaching the Space Telescope Science Institute at Johns Hopkins University. Here, the signal is converted back into pictures and data. Scientists at these institutions are responsible for the daily programming and operations of the HST.

Scheduled to serve until the year 2010, the Hubble Space Telescope continues to provide dramatic observations that stretch the boundaries of the known universe. Among its accomplishments so far, the HST has provided evidence of the existence of massive black holes at the centers of galaxies, captured the first detailed image of the surface of Pluto, detected protogalaxies (structures presently thought to have existed close to the time of the origin of the universe), and captured spectacular images of the comet Shoemaker-Levy as its parts collided with Jupiter.

In order to provide continuous and broader astronomical observations, NASA is expected to launch the Hubble's successor (tentatively named the Next Generation Space Telescope) more fully equipped with cameras and spectrographs sensitive to multiple regions of the electromagnetic spectrum prior to the end of the HST's expected service life.

See also Big Bang theory; Cosmology; History of manned space exploration; Quasars; Solar system; Spacecraft, manned; Stellar life cycle

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Hubble Space Telescope

HUBBLE SPACE TELESCOPE

HUBBLE SPACE TELESCOPE. Although astronomer Lyman Spitzer first suggested the idea of a space-based telescope in 1946, it was not until 24 April 1990 that one was placed in orbit around the earth. Named after the pioneering astronomer Edwin P. Hubble, it promised to overcome distortions caused by the earth's atmosphere. The forty-three-foot-long telescope could look seven times farther into space than the most powerful terrestrial observatories.

Computer problems in 1982 thwarted the $2 billion telescope's initial launching. Rescheduled for October 1986, its launch was again delayed by the tragedy in January 1986 that killed the crew of the space shuttle Challenger. Four years later, the Hubble Space Telescope fi-nally was lifted into space. Two months after the telescope was placed in orbit, scientists announced that its 94.5-inch primary mirror, polished to incredible smoothness, was flawed, resulting in blurred images. Ironically, the telescope was myopic. Investigation showed that engineers easily could have detected this problem prior to launch. Scientists had to delay or cancel experiments.

In December 1993 the crew of the space shuttle Endeavour fitted the telescope with corrective optics and


made other repairs. After this $629 million outer-space repair job, the telescope worked perfectly. It took detailed views of nebulae and star clusters. In October 1994 astronomers announced that data from the telescope showed that the universe was between eight billion and twelve billion years old, younger than earlier estimates by nearly half. Astronomers announced in January 1996 that the telescope was detecting hundreds of galaxies never before seen, which they speculated could be the most distant and oldest galaxies ever observed.

BIBLIOGRAPHY

Fischer, Daniel, and Hilmar Duerbeck. The Hubble: A New Window to the Universe. Translated by Helmut Jenkner and Douglas Duncan. New York: Copernicus, 1996.

Peterson, Carolyn Collins, and John C. Brandt. Hubble Vision: Further Adventures with the Hubble Space Telescope. 2d ed. New York: Cambridge University Press, 1998.

BrentSchondelmeyer/a. r.

See alsoChallenger Disaster ; Observatories, Astronomical ; Space Program .

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Hubble Space Telescope

Hubble Space Telescope (HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe from a position some 350 mi (560 km) above the earth. Using a Ritchey-Chrétien design that affords wider and flatter fields of view than traditional Cassegrain systems, the telescope has a 7.9-ft (2.4-m) primary mirror that can observe 24 hours a day (but usually observes less than 20% of the time) in a sky that is always clear and always has perfect seeing. Among the instruments are two high-resolution cameras and two spectrographs. The HST was launched from shuttle Atlantis in 1990. Initial tests taken after its launch showed that the primary mirror was astigmatic, and it was discovered that the mirror had been mistakenly ground to the wrong figure. The telescope was repaired by space shuttle astronauts in Dec., 1993; they replaced critical instruments and added corrective optics while in orbit. Subsequent servicing missions in 1997 and 1999 added capabilities to HST, which observes the universe at ultraviolet, near-ultraviolet, visible, and near-infrared wavelengths. In 2002 astronauts made repairs and improvements designed to enable the observatory to function for another decade, but in 2004 the power supply for the ultraviolet spectrograph failed. A final shuttle servicing mission in 2009 made additional repairs, replacements, and enhancements, including replacing the gyroscopes and the batteries and installing a new wide-field camera and a new ultraviolet spectrograph.

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Hubble Space Telescope

Hubble Space Telescope (HST) Optical telescope that was placed in Earth orbit by the space shuttle in 1990. Images transmitted back to Earth revealed that the telescope's main mirror was incorrectly shaped. A repair team corrected the fault in 1993, and it was again repaired in 1997. Hubble now produces acurate images of bodies that cannot be observed clearly by terrestrial telescopes due to atmospheric distortion.

http://www.stsci.edu

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