Hubble Space Telescope

Above the turbulent atmosphere

The design

Hubble’s blurry vision

Endeavor to the rescue

Other repair missions

Daily operations

Resources

The Hubble Space Telescope is a general-purpose orbiting observatory. Orbiting approximately 380 mi (612 km) above Earth, the 12.5-ton Hubble Space Telescope (Hubble, or HST) has peered farther into the universe than any telescope before it. The Hubble, which was launched on April 24, 1990, has produced images with unprecedented resolution at visible, near-ultraviolet, and near-infrared wavelengths since its originally faulty optics were corrected in 1993. Although ground-based technology is finally starting to catch up—the European Southern Observatory’s Very Large Telescope atop Cerro Paranal, Chile, can now produce narrow-field images even sharper than Hubble’s—the Hubble continues to produce a stream of unique observations. During the 1990s and now into the 2000s, the Hubble has revolutionized the science of astronomy, becoming one, if not the most, important instruments ever used in astronomy.

The Hubble was the first of the four great observatories planned by the U. S. National Aeronautics and Space Administration (NASA). This series of orbital telescopes also includes the Compton Gamma Ray Observatory (Compton, launched 1991), the Chandra X-Ray Observatory (Chandra, formerly Advanced X-ray Astrophysics Facility, launched 1999), and the Spitzer Space Telescope (Spitzer, formerly the Space Infrared Telescope Facility, launched in 2003). Together, the light-sensing abilities of the Great Observatories span much of the electromagnetic spectrum. They are designed to do so because each part of the spectrum conveys different astronomical information. Specifically, Compton, named after American physicist Arthur Holly Compton (1892–1962), who won the Nobel Prize (1927) for his work with gamma ray physics and for his work with what is now known as the Compton effect, will view objects in the gamma ray range of the spectrum. Chandra, named after Indian-American physicist, mathematician, and astrophysicist Subrahmanyan Chandrasekhar (1910–1995) who determined the mass limit for white dwarf stars becoming neutron stars and was awarded the Nobel Prize in 1983, works within the x-ray spectrum. Spitzer, named after American theoretical physicist Lyman Spitzer, Jr. (1914–1997) who first proposed placing telescopes in space in the mid-1940s, will produce images and spectra from the infrared range. Hubble, named after American astronomer Edwin Hubble (1889–1953) for his discovery of galaxies outside the Milky Way galaxy, observes in the visible, near-ultraviolet, and near-infrared parts of the electromagnetic spectrum.

Above the turbulent atmosphere

The twinkling of stars is a barrier between astronomers and the information they wish to gather. In reality, stars do not twinkle but burn steadily; they only appear to ground observers to twinkle because atmospheric turbulence distorts their light waves en route to Earth. Although telescopes on Earth’s surface incorporate enormous mirrors to gather starlight and sophisticated instruments to minimize atmospheric distortion, the images gathered still suffer from some image degradation. Recently, much progress has been made in the use of adaptive optical systems. These systems aim lasers along a telescope’s line of sight to measure atmospheric turbulence. This information is fed to computers, which calculate and apply an ever-changing counter-warp to the surface of the telescope’s mirror (or mirrors) to undo the effect of the turbulence in real time. Adaptive optics is starting to overcome some of the problems caused by atmospheric turbulence. However, the fact that the Earth’s atmosphere absorbs much of the electromagnetic spectrum cannot be overcome from the ground; only space-based telescopes can make observations at certain wavelengths (e.g., the infrared).

Scientists first conceived of an orbital telescope in the 1940s. The observatory proposed at that time was called, optimistically, the Large Space Telescope. By the 1970s, the concept had coalesced into an actual design, being less large in size due to the political backlash against the huge space-exploration budgets of the 1960s. In 1990, after a decade of development and years of delay caused by the Challenger space shuttle disaster of 1986, the space shuttle Discovery deployed the Hubble Space Telescope into an orbit approximately 380 mi (612 km) above Earth. The way astronomers see the universe was about to be changed—but not for another three years, due to a design flaw in the main mirror.

The design

The Hubble Space Telescope is a large cylinder sporting long, rectangular solar panels on either side like the winding stems of a giant toy. Almost 43 ft (13 m) long and more than 14 ft (4.2 m) in diameter, this cylinder houses a large mirror to gather light and a host of instruments designed to analyze the light thus gathered.

The telescope itself is a Ritchey-Chretien Cassegrain type that consists of a concave primary mirror 8 ft (2.4 m) in diameter and a smaller, convex secondary mirror 1 ft (0.3 m) in diameter that is mounted facing the primary. This pair of mirrors is mounted deep within the long tube of the Hubble’s housing, which prevents unwanted light from degrading the image.

Light follows a Z-shaped path through the telescope. First, light from the target travels straight down the tube to the primary mirror. This reflects the light, focusing it on the secondary mirror. The secondary mirror reflects the light again and further focuses it, aiming it through a small hole in the center of the primary at the telescope’s focal plane, which is located behind the primary. The focal plane is where the light gathered by the telescope is formed into a sharp image. Here, the focused light is directed to one of the observatory’s many instruments for analysis. All data collected by the Hubble is radioed to the Earth in digital form.

The Hubble’s original complement of instruments, since replaced by a series of space-shuttle service missions, included the Wide Field/Planetary Camera (WF/PC1), the Faint Object Spectrograph (FOS), the Faint Object Camera (FOC), the High Resolution Spectrograph (HRS), and the High Speed Photometer (HSP). WF/PC1 was designed to capture spectacular photos from space. The FOS, operating from ultraviolet to near-infrared wavelengths, did not create images, but analyzed light from stars and galaxies spectroscopically, that is, by breaking it into constituent wavelengths. The FOS contained image intensifiers that amplify light, allowing it to view very faint, far away objects. The HRS also analyzed light spectroscopically, but was limited to ultraviolet wavelengths. Although it could not study very faint stars as the FOS could, the HRS operated at comparatively high precision. The HSP provided quantitative data on the amount of light emanating from different celestial objects.

Every aspect of the Hubble had to be designed for operation in space. For example, the Hubble is designed to function under radical temperature extremes. Although the vacuum of space itself has no temperature, at Earth’s distance from the sun, an object in deep shadow cools to a temperature of -250°F (-155°C) while an object in direct sunlight can be heated to hundreds of Fahrenheit degrees above zero. The Hubble itself orbits the Earth every 97 minutes, spending 25 minutes of that time in the Earth’s shadow and the rest in direct sunlight. It thus passes, in effect, from an extreme deep freeze to an oven and back again about 15 times a day, and must be effectively insulated to keep its instruments and mirrors stable.

Another aspect of the Hubble that had to be specially designed for its orbit situation is its pointing system. Because astronomical observations often require minutes or hours of cumulative, precisely-aimed viewing of the target, the Hubble—which rotates with respect to the fixed stars an average of once every 97 minutes—must turn itself while making observations. Hubble must do this in order to keep its target in view and unblurred. Ground-based telescopes must cope with a similar problem, but rotate with respect to the fixed stars at much slower paces. Turning by Hubble keeps it aligned while it is observing a target, checking for movement 40 times per second.

Another problem for any space vehicle is the supply of electrical power. In the Hubble’s case, a pair of 40 ft x 8 ft (12 m x 2.4 m) solar arrays provide power for the observatory, generating up to 2,400 watts of electricity. Batteries supply power while the telescope is in the Earth’s shadow.

Hubble’s blurry vision

After the Hubble’s launch in 1990, astronomers eagerly awaited its first observations. When they saw the test images, however, it quickly became clear that something was seriously wrong: the Hubble had defective vision. Scientists soon realized that the primary mirror of the space telescope suffered from a spherical aberration, an error in its shape that caused it to focus light in a thin slab of space rather than at a sharply defined focal plane. In the focal plane, therefore, a star’s image appeared as a blurred disk instead of a sharp point.

The fabrication of a large astronomical mirror such as the Hubble’s primary is a painstaking task. The mirror is first cast in the rough and must be ground and polished down to its precise final shape. The computer-controlled tools used for this process remove glass from the rough cast one micron at a time. After each grinding or polishing step, the mirror is re-measured to determine how closely it approximates the desired shape. With these measurements in hand, engineers can tell the computer how much glass to remove in the next grinding or polishing pass and where the glass must be removed. This cycle of grind, polish, measure, and re-grind, a single round of which can take weeks, must be repeated dozens of times before the mirror’s final shape is achieved.

During the metrology (measuring) step, a repeated or systematic error caused the manufacturers to produce a mirror with a shape that was slightly more flat around the edges than specified. The error was small— the thickness of extra glass removed was a fraction of the width of a human hair—but it was enough to produce a significant spherical aberration. Although useful science could still be performed with the telescope’s spectroscopic instruments, the Hubble was unable to perform its imaging mission.

Endeavor to the rescue

The design and manufacture of a space telescope like the Hubble is a large project that takes many years; of necessity, the design must be finalized early on. As a result, by the time the observatory reaches orbit its scientific instruments rarely represent the state of the art. Having this constraint in mind, the telescope engineers designed the Hubble’s instruments as modular units that could be easily swapped out for improved designs. The Hubble was thus, engineered for periodic servicing missions by space shuttle crews over the course of its planned 15-year lifetime (since extended to 20 years). Its housing or outer shell is studded with a host of handholds and places for astronauts to secure themselves, bolt heads are large-sized for easy manipulation by astronauts wearing clumsy gloves, and more than 90 components are designed to be replaced in orbit. The Hubble’s housing also includes a fixture that enables the shuttle’s robot arm to seize it and draw the Hubble and shuttle together. The shuttle’s cargo bay includes a servicing platform to hold the telescope while the bay doors are open, and astronauts can affect repairs while standing on small platforms nearby.

One benefit of the primary mirror’s precision fabrication was that despite the error imparted by the systematic metrology error, the mirror’s shape— error and all—was precisely known. Its surface is so smooth that if the mirror were the width of the United States, its largest variation in surface height would be less than 3 ft (1 m). Once scientists understood what was wrong, therefore, they knew the exact correction required. Replacing the primary mirror would have required bringing the Hubble back to the Earth, rebuilding it, and re-launching it, much too expensive to be feasible. Instead, designers developed an add-on optics module to compensate for the focusing error. This module would correct the vision of the telescope to the level originally designed for, much as a pair of glasses corrects for defective eyesight.

This module—the Corrective Optics Space Telescope Axial Replacement (COSTAR)—contained five mirrors that would refocus light gathered by the primary and secondary mirrors and relay it to the instruments. The challenge was to build the module to fit into the compact interior of a telescope that was, and would remain, in orbit, and which had never been designed for such a fix. Engineers also produced an improved version of the Wide Field/Planetary Camera, the WF/ PC2, that included its own corrective optics to allow it to capture images of the clarity that astronomers had originally hoped for.

In addition to the flaw in its optics, the observatory was experiencing difficulties with its pointing stability and with its solar arrays, which turned out to be prone to wobbling due to thermal stress created during the transition from sunlight to shadow. This wobbling further degraded observation quality. NASA planned an ambitious repair mission that would attempt to correct all the Hubble’s problems at once.

In December, 1993, the space shuttle Endeavor took off to rendezvous with the Hubble Space Telescope. During the course of the mission, astronauts performed five space walks. They captured the Hubble with the shuttle’s robotic arm, repaired some of the pointing gyroscopes, replaced the wobbling solar arrays, and installed the WF/PC2 and COSTAR.

The mission was a success; the contrast between the images taken before and after the repairs was stunning. Suddenly the Hubble was dazzling the world, and astronomers were lining up for observing time. Since the 1993 repair, the Hubble’s available observing time has invariably been booked for years in advance; in fact, it is so over-subscribed that only one out of every ten proposals for observing time can be accepted.

Other repair missions

In February 1997, the crew of the Discovery space shuttle (STS-82) replaced the Hubble’s Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS) with the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). They also replaced an Engineering and Science Tape Recorder with a new Solid State Recorder, repaired thermal insulation, and boosted the Hubble into a higher orbit.

Unlike the older instrument, the STIS collects light from hundreds of points over a target area instead of just one point. The NICMOS allows the telescope to gather images and spectroscopic data in the infrared spectral region (0.8 and 2.5 micrometers), which in effect allows the Hubble to see through interstellar clouds of gas and dust that block visible light.

The crew also made repairs to the telescope’s electrical, data storage, computer, and pointing systems, as well as to its battered thermal insulation blanket, which had been severely damaged by collisions with small bits of space debris. The final task of the repair mission was to nudge the observatory to an orbit six miles higher than previously, to enhance its longevity and stability. Altitude affects longevity because the orbit of any near-earth object, including the Hubble, is degrading all the time due to friction with outlying traces of the Earth’s atmosphere. Therefore, unless it is boosted out of Earth orbit, the Hubble will eventually burn up in the atmosphere. Because the Hubble is so massive, it would not vaporize entirely on reentry, but would shower some part of the Earth’s surface with chunks of metal and glass.

In December 1999, the crew of the Discovery space shuttle (STS-103) replaced all six gyroscopes, replaced a Fine Guidance Sensor and its computer, installed the Voltage/temperature Improvement Kit, and replaced thermal insulation blankets.

In March 2002, the crew of the Columbia space shuttle (STS-109) installed the Advanced Camera for Surveys instrument (which replaced the Faint Object Camera (FOC) and refilled the coolant within the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). The solar arrays were replaced with smaller but more efficient ones (30% more power and two-thirds the size of the older ones), and the Power Distribution Unit was replaced after the old one was temporarily shut down due to faulty parts.

Daily operations

Making observations with an orbital telescope is not a simple process. The telescope must be instructed where to point to acquire a new target, how to move in order to avoid light contamination from the sun and moon, how long to observe and with what instruments, what data format to use for transmission of result, how to orient its radio antennas to send and receive future commands, and so forth. All commands must be written in computer code and relayed to the Hubble by radio during a point in its orbit where it can communicate with antennas on the ground.

How does the Hubble know where to find a given target object? Like a person trying to find his or her way in unfamiliar territory, the telescope searches for stellar landmarks termed guide stars. The position of any star, planet, or galaxy can be specified in terms of particular guide stars—bright, easily found stars located near the object of interest. (The guide stars are not literally close to the objects they locate, but appear to be near them in the sky.) Sky surveys performed by ground-based telescopes have mapped many of these stars, so the Hubble merely points itself to the appropriate coordinates, then uses the guide stars to maintain its position.

The near-term future of Hubble is uncertain. With sixteen years of service in space, Hubble will not be able to remain in orbit for very many more years; and, it needs new parts to replace ailing parts, such as its stabilizing gyroscopes. In addition, in 2004, the power system of the STIS failed after redundant electronics had gone bad in 2001. After the destruction of the Columbia space shuttle during re-entry in February 2003, NASA decided that a repair mission to Hubble was too dangerous. Since that initial statement, NASA has reconsidered its position, and now plans one more repair mission. Without a repair mission to Hubble, it is expected to fail before 2009. NASA officials have indicated that if no repairs are made to Hubble, then the agency plans to send a robotic mission to Hubble to deorbit it by the year 2013 so that it will fall to Earth away from land and any populated areas.

The Hubble Space Telescope has revolutionized astronomy by bringing a whole new understanding of the Universe to mankind. The following list highlights a few of the Hubble’s achievements:

• Imaged Comet Shoemaker-Levy 9 crashing into Jupiter in 1994, along with detailed pictures of Jupiter and its atmosphere.
• Showed that protoplanetary dust disks (proplyds) are common around young stars.

KEY TERMS

Guide star —Bright star used as landmark to identify other stellar objects.

Metrology —The process of measuring mirrors and lenses precisely during the fabrication process

Spectrograph —Instrument for dispersing light into its spectrum of wavelengths, and then photographing that spectrum.

Spectroscopy —A technique in which light is spread out into its constituent wavelengths (colors, for visible light). The presence of energy at certain wavelengths in the light emitted by a star or galaxy indicates the presence of certain elements or processes in that star or galaxy.

Spherical aberration —A distortion in the curvature of a lens or mirror. When spherical aberration is present in a mirror, light from different radial sections of the mirror focuses at different distances rather than all at the same point. The image produced is thus blurred, or aberrated.

• Proved that Jupiter-size planets are uncommon in globular clusters.
• Shown that quasars reside in galaxies, many of which are colliding with each other.
• Provided first concrete evidence of the existence of a black hole, which occurred around the center of the galaxy M87.
• Shown that supermassive black holes reside at the centers of many galaxies.
• Permitted more accurate measurement of the universe’s rate of expansion than ever before (by measuring distances to Cepheid variable stars more accurately—from 50% error rate to 10%).
• Observed distant supernovae which give evidence that the expansion of the universe is actually accelerating, prompting a major revision of cosmological thought.
• Imaged large numbers of very distant galaxies distances with its Deep Field study, greatly increasing estimate of how many galaxies there are in the universe.
• Found evidence for the presence of extrasolar planets around stars similar to the sun.
• Discovered gamma-ray bursts.
• Discoveries made with Hubble have generated thousands of scientific papers and thousands of speeches at conferences.

The Hubble will eventually be decommissioned, whether it is repaired in the future or not. Work is already under way on its replacement, the James Webb Space Telescope (JWST, named for a former NASA administrator), possibly due for launch in 2013. Unlike the Hubble, which travels around Earth in a moderately low orbit, the JWST will be located some 930,000 mi (1.5 million km) away, to avoid glare from Earth. The JWST will make observations only at near- and mid-infrared wavelengths, seeking to study the early history of the universe. Optical and ultraviolet wavelengths will not be observed by the new telescope.

See also Space probe; Spectral classification of stars.

Resources

BOOKS

Kanipe, Jeff. Chasing Hubble’s Shadow: The Search for Galaxies at the Edge of Time. New York: Hill and Wang, 2006.

Kerrod, Robin. Hubble: The Mirror on the Universe. Toronto, Canada, and Buffalo, NY: Firefly Books, 2003.

PERIODICALS

Lawler, Andrew, “Glimpsing the Post-Hubble Universe.” Science (February 22, 2002): 1448-1451.

Leary, Warren, “NASA Starts Planning Hubble’s Going-Away Party.” New York Times. September 17, 2002.

OTHER

National Aeronautics and Space Administration. “The Hubble Space Telescope.” <http://hubble.nasa.gov/index.php> (accessed October 12, 2006).

Kristin Lewotsky

Hubble Space Telescope

Floating in orbit approximately 380 miles (612 km) above the earth , the 12.5-ton Hubble Space Telescope has peered farther into the Universe than any telescope before it. The Hubble, which was launched in 1990, has produced images with unprecedented resolution at visible, near-ultraviolet, and near-infrared wavelengths since its originally faulty optics were corrected in 1993. Although ground-based technology is finally starting to catch up—the European Southern Observatory's Very Large Telescope atop Cerro Paranal, Chile, can now produce narrow-field images even sharper than Hubble's—the Hubble continues to produce a stream of unique observations. Over the last decade, the Hubble has revolutionized astronomy .

The Hubble was the first of the four great observatories planned by the United States. National Aeronautics and Space Administration (NASA). This series of orbital telescopes also includes the Compton Gamma Ray Observatory (launched 1991), the Chandra X-Ray Observatory (launched 1999), and the Space Infrared Telescope Facility (scheduled for launch in 2003). Together, the light-sensing abilities of the Great Observatories span much of the electromagnetic spectrum . They are designed to do so because each part of the spectrum conveys different astronomical information.

Above the turbulent atmosphere

The twinkling of stars is a barrier between astronomers and the information they wish to gather. In reality, stars do not twinkle but burn steadily; they only appear to ground observers to twinkle because atmospheric turbulence distorts their light waves en route to us. Although telescopes on Earth's surface incorporate enormous mirrors to gather starlight and sophisticated instruments to minimize atmospheric distortion, the images gathered still suffer from some image degradation. Recently much progress has been made in the use of adaptive optical systems. These systems aim lasers along a telescope's line of sight to measure atmospheric turbulence. This information is fed to computers, which calculate and apply an ever-changing counter-warp to the surface of the telescope's mirror (or mirrors) to undo the effect of the turbulence in real time . Adaptive optics are starting to overcome some of the problems caused by atmospheric turbulence. However, the fact that the Earth's atmosphere absorbs much of the electromagnetic spectrum cannot be overcome from the ground; only space-based telescopes can make observations at certain wavelengths (e.g., the infrared).

Scientists first conceived of an orbital telescope in the 1940s. The observatory proposed at that time was called, optimistically, the Large Space Telescope. By the 1970s, the concept had coalesced into an actual design, less "large" thanks to political backlash against the huge space-exploration budgets of the 1960s. In 1990, after a decade of development and years of delay caused by the Challenger shuttle disaster of 1986, the space shuttle Discovery deployed the Hubble Space Telescope into an orbit approximately 380 mi (612 km) above Earth. The way we see the universe was about to be changed—but not for another three years, due to a design flaw in the main mirror.

The design

The Hubble Space Telescope is a large cylinder sporting long, rectangular solar panels on either side like the winding stems of a giant toy. Almost 43 ft (13 m) long and more than 14 ft (4.2 m) in diameter, this cylinder houses a large mirror to gather light and a host of instruments designed to analyze the light thus gathered.

The telescope itself is a Ritchey-Chretien Casse-grain type that consists of a concave primary mirror 8 feet (2.4 m) in diameter and a smaller, convex secondary mirror 1 foot (.3 m) in diameter that is mounted facing the primary. This pair of mirrors is mounted deep within the long tube of the Hubble's housing, which prevents unwanted light from degrading the image.

Light follows a Z-shaped path through the telescope. First, light from the target travels straight down the tube to the primary mirror. This reflects the light, focusing it on the secondary mirror. The secondary mirror reflects the light again and further focuses it, aiming it through a small hole in the center of the primary at the telescope's focal plane , which is located behind the primary. The focal plane is where the light gathered by the telescope is formed into a sharp image. Here, the focused light is directed to one of the observatory's many instruments for analysis. All data collected by the Hubble is radioed to Earth in digital form.

The Hubble's original complement of instruments, since replaced by a series of space-shuttle service missions, included the Wide Field/Planetary Camera (WF/PC1), the Faint Object Spectrograph (FOS), the High Resolution Spectrograph (HRS), and the High Speed Photometer (HSP). WF/PC1 was designed to capture spectacular photos from space. The FOS, operating from ultraviolet to near-infrared wavelengths, did not create images, but analyzed light from stars and galaxies spectroscopically, that is, by breaking it into constituent wavelengths. The FOS contained image intensifiers that amplify light, allowing it to view very faint, far away objects. The HRS also analyzed light spectroscopically, but was limited to ultraviolet wavelengths. Although it could not study very faint stars as the FOS could, the HRS operated at comparatively high precision. The HSP provided quantitative data on the amount of light emanating from different celestial objects.

Every aspect of the Hubble had to be designed for operation in space. For example, the Hubble is designed to function under radical temperature extremes. Although the vacuum of space itself has no temperature, at the Earth's distance from the Sun , an object in deep shadow cools to a temperature of —250°F ( —155°C) while an object in direct Sunlight can be heated to hundreds of Fahrenheit degrees above zero . The Hubble itself orbits the Earth every 97 minutes, spending 25 minutes of that time in Earth's shadow and the rest in direct sunlight. It thus passes, in effect, from an extreme deep freeze to an oven and back again about 15 times a day, and must be effectively insulated to keep its instruments and mirrors stable.

Another aspect of the Hubble that had to be specially designed for its orbit situation is its pointing system. Because astronomical observations often require minutes or hours of cumulative, precisely-aimed viewing of the target, the Hubble—which rotates with respect to the fixed stars an average of once every 97 minutes—must turn itself while making observations in order to keep its target in view and unblurred; ground-based telescopes must cope with a similar problem, but rotate with respect to the fixed stars at much slower keeps the Hubble aligned while it is observing a target, checking for movement 40 times per second.

Another problem for any space vehicle is the supply of electrical power. In the Hubble's case, a pair of 40 ft X 8 ft (12 m X 2.4 m) solar arrays provide power for the observatory, generating up to 2400 W of electricity . Batteries supply power while the telescope is in the earth's shadow.

Hubble's blurry vision

After the Hubble's launch in 1990, astronomers eagerly awaited its first observations. When they saw the test images, however, it quickly became clear that something was seriously wrong: the Hubble had defective vision . Scientists soon realized that the primary mirror of the space telescope suffered from a spherical aberration, an error in its shape that caused it to focus light in a thin slab of space rather than at a sharply defined focal plane. In the focal plane, therefore, a star's image appeared as a blurred disk instead of a sharp point.

The fabrication of a large astronomical mirror such as the Hubble's primary is a painstaking task. The mirror is first cast in the rough and must be ground and polished down to its precise final shape. The computer-controlled tools used for this process remove glass from the rough cast one micron at a time. After each grinding or polishing step, the mirror is re-measured to determine how closely it approximates the desired shape. With these measurements in hand, engineers can tell the computer how much glass to remove in the next grinding or polishing pass and where the glass must be removed. This cycle of grind, polish, measure, and re-grind, a single round of which can take weeks, must be repeated dozens of times before the mirror's final shape is achieved.

During the metrology (measuring) step, a repeated or systematic error caused the manufacturers to produce a mirror with a shape that was slightly more flat around the edges than specified. The error was small—the thickness of extra glass removed was a fraction of the width of a human hair—but it was enough to produce a significant spherical aberration. Although useful science could still be performed with the telescope's spectroscopic instruments, the Hubble was unable to perform its imaging mission.

Endeavor to the rescue

The design and manufacture of a space telescope like the Hubble is a large project that takes many years; of necessity, the design must be finalized early on. As a result, by the time the observatory reaches orbit its scientific instruments rarely represent the state of the art. Having this constraint in mind, the telescope engineers designed the Hubble's instruments as modular units that could be easily swapped out for improved designs. The Hubble was thus, engineered for periodic servicing missions by space shuttle crews over the course of its planned 15-year lifetime (since extended to 20 years). Its housing or outer shell is studded with a host of handholds and places for astronauts to secure themselves, bolt heads are large-sized for easy manipulation by astronauts wearing clumsy gloves, and more than 90 components are designed to be replaced in orbit. The Hubble's housing also includes a fixture that enables the shuttle's robot arm to seize it and draw the Hubble and shuttle together. The shuttle's cargo bay includes a servicing platform to hold the telescope while the bay doors are open, and astronauts can affect repairs while standing on small platforms nearby.

One benefit of the primary mirror's precision fabrication was that despite the error imparted by the systematic metrology error, the mirror's shape—error and all—was precisely known. Its surface is so smooth that if the mirror were the width of the United States, its largest variation in surface height would be less than 3 ft (1 m). Once scientists understood what was wrong, therefore, they knew the exact correction required. Replacing the primary mirror would have required bringing the Hubble back to Earth, re-building it, and re-launching it, much too expensive to be feasible; instead, designers developed an add-on optics module to compensate for the focusing error. This module would correct the "vision" of the telescope to the level originally designed for, much as a pair of glasses corrects for defective eyesight.

This module—the Corrective Optics Space Telescope Axial Replacement (COSTAR)—contained five mirrors that would refocus light gathered by the primary and secondary mirrors and relay it to the instruments. The challenge was to build the module to fit into the compact interior of a telescope that was, and would remain, in orbit, and which had never been designed for such a fix. Engineers also produced an improved version of the Wide Field/Planetary Camera, the WF/PC2, that included its own corrective optics to allow it to capture images of the clarity that astronomers had originally hoped for.

In addition to the flaw in its optics, the observatory was experiencing difficulties with its pointing stability and with its solar arrays, which turned out to be prone to wobbling due to thermal stress created during the transition from sun to shadow. This wobbling further degraded observation quality. NASA planned an ambitious repair mission that would attempt to correct all the Hubble's problems at once.

In December, 1993, the space shuttle Endeavor took off to rendezvous with the Hubble Space Telescope. During the course of the mission, astronauts performed a total of five space walks. They captured the Hubble with the shuttle arm, repaired some of the pointing gyroscopes, replaced the wobbling solar arrays, and installed the WF/PC2 and COSTAR.

The mission was a success; the contrast between the images taken before and after the repairs was stunning. Suddenly the Hubble was dazzling the world and astronomers were lining up for observing time. Since the 1993 repair, the Hubble's available observing time has invariably been booked for years in advance; in fact, it is so over-subscribed that only one out of every ten proposals for observing time can be accepted.

Daily operations

Making observations with an orbital telescope is not a simple process. The telescope must be instructed where to point to acquire a new target, how to move in order to avoid light contamination from the Sun and Moon , how long to observe and with what instruments, what data format to use for transmission of result, how to orient its radio antennas to send and receive future commands, and so forth. All commands must be written in computer code and relayed to the Hubble by radio during a point in its orbit where it can communicate with antennas on the ground.

How does the Hubble know where to find a given target object? Like a person trying to find his or her way in unfamiliar territory, the telescope searches for stellar landmarks termed guide stars. The position of any star , planet , or galaxy can be specified in terms of particular guide stars—bright, easily found stars located near the object of interest. (The guide stars are not literally close to the objects they are used to locate, but appear to be near them in the sky.) Sky surveys performed by ground-based telescopes have mapped many of these stars, so the Hubble merely points itself to the appropriate coordinates, then uses the guide stars to maintain its position.

In early 1997, astronauts aboard the space shuttle Discovery performed another servicing mission, this time to swap out instruments. The HRS was replaced by the Space Telescope Imaging Spectrograph (STIS). Unlike the older instrument, the STIS collects light from hundreds of points over a target area instead of just one point. The servicing crew removed the FOS and in its place installed the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which allows the telescope to gather images and spectroscopic data in the infrared spectral region (0.8 and 2.5 micrometers), which in effect allows the Hubble to see through interstellar clouds of gas and dust that block visible light.

The crew also made repairs to the telescope's electrical, data storage, computer, and pointing systems, as well as to its battered thermal insulation blanket, which had been severely damaged by collisions with small bits of space debris. The final task of the repair mission was to nudge the observatory to an orbit six miles higher than previously, to enhance its longevity and stability. Altitude affects longevity because the orbit of any near-Earth object, including the Hubble, is degrading all the time due to friction with outlying traces of the Earth's atmosphere. Therefore, unless it is boosted out of Earth orbit or brought back to Earth by a space shuttle, the Hubble will eventually burn in the atmosphere. Because the Hubble is so massive, it would not vaporize entirely on reentry, but would shower some part of the Earth's surface with chunks of metal and glass. NASA is presently debating whether to (a) retrieve the Hubble intact after it is scheduled to go out of service in 2010, (b) guide it to a chosen crash zone on Earth, or (c) push it right out of Earth orbit with a specially-built rocket.

The Hubble Space Telescope has revolutionized astronomy by bringing a whole new understanding of the Universe to mankind. The following list highlights a few of the Hubble's achievements:

• Imaged comet Shoemaker-Levy 9 crashing into Jupiter in 1994.
• Showed that protoplanetary dust disks are common around young stars.
• Proved that Jupiter-size planets are uncommon in globular clusters.
• Shown that quasars reside in galaxies, many of which are colliding with each other.
• Shown that supermassive black holes reside at the centers of many galaxies.
• Permitted more accurate measurement of the Universe's rate of expansion than ever before.
• Observed distant supernovae which give evidence that the expansion of the Universe is actually accelerating, prompting a major revision of cosmological thought.
• Imaged large numbers of very distant galaxies distances with its Deep Field study, greatly increasing our estimate of how many galaxies there are in the Universe.

The Hubble will eventually be decommissioned. Work is already under way on its replacement, the James Webb Space Telescope (JWST, named for a former NASA administrator), due for launch in 2010. Unlike the Hubble, which travels around Earth in a moderately low orbit, the JWST will be located some 930,000 mi (1.5 million km) away, to avoid glare from the Earth. The JWST will make observations only at near- and mid-infrared wavelengths, seeking to study the early history of the Universe. Optical and ultraviolet wavelengths will not be observed by the new telescope.

See also Space probe; Spectral classification of stars.

Resources

periodicals

Lawler, Andrew, "Glimpsing the Post-Hubble Universe." Science (February 22, 2002): 1448–1451.

Leary, Warren, "NASA Starts Planning Hubble's Going-Away Party." New York Times. September 17, 2002.

other

National Aeronautics and Space Administration. "Hubble's Parts." August 8, 2002. [cited November 23, 2002]. <http://hubble.nasa.gov/technology/parts.html#optics>.

Kristin Lewotsky

KEY TERMS

Guide star

—Bright star used as landmark to identify other stellar objects.

Metrology

—The process of measuring mirrors and lenses precisely during the fabrication process

Spectrograph

—Instrument for dispersing light into its spectrum of wavelengths then photographing that spectrum.

Spectroscopy

—A technique in which light is spread out into its constituent wavelengths (colors, for visible light). The presence of energy at certain wavelengths in the light emitted by a star or galaxy indicates the presence of certain elements or processes in that star or galaxy.

Spherical aberration

—A distortion in the curvature of a lens or mirror. When spherical aberration is present in a mirror, light from different radial sections of the mirror focuses at different distances rather than all at the same point. The image produced is thus blurred, or aberrated.

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 astrometry—the 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 apart—in 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/>.

Hubble Space Telescope

April 25, 1990

"We do not know why we are born into the world, but we can try to find out what sort of a world it is at least in its physical aspects."

Edwin P. Hubble

The Hubble Space Telescope (HST) is the most significant advance in astronomy since Italian astronomer Galileo Galilei (1564–1642) perfected the telescope in the seventeenth century. (An astronomer is a scientist who studies bodies in space.) Named for twentieth-century astronomer Edwin P. Hubble (1889–1953), the HST orbits Earth in outer space, taking pictures of stars, galaxies, planets, and vast regions previously unknown to humans. (A galaxy is a large group of stars and associated matter.) Since it is positioned beyond Earth's atmosphere, the space observatory receives images that are brighter and more detailed than those captured by telescopes based on land. Launched by the National Aeronautics and Space Administration (NASA) in 1990, the HST has taken spectacular pictures of the universe. In 2004, a year after the Columbia space shuttle disaster (see box in Challenger Crew entry), NASA canceled the final service mission to the HST. Supporters of the orbiting observatory began seeking ways to prolong the life of the largest, most successful astronomy project in history.

How the HST works

The HST is an observatory (a structure that houses a telescope for astronomical viewing) approximately the size of a school bus that orbits Earth at a speed of 5 miles (about 8 kilo-meters) per second. The cylinder-shaped body of the spacecraft holds a reflecting telescope and scientific instruments. The telescope contains a primary mirror and a secondary mirror, which operate in conjunction with five main recording instruments: a faint-object camera, a wide-field planetary camera, a faint-object spectrograph (an instrument that sends out radiation), a high-resolution (rendering of detail) spectrograph, and a high-speed photometer (an instrument that measures light intensity). The reflecting telescope collects light from objects in space. The primary mirror, which measures 94 inches (2.4 meters), and the smaller secondary mirror then direct the light into the two cameras and the two spectrographs.

The wide-field planetary camera can record images with a resolution that is ten times greater than that of any telescope based on Earth. It can take pictures of wide expanses of space, or it can take more focused pictures of objects such as planets and bodies inside and outside of galaxies. The faint-object camera can detect objects that are fifty times fainter than those that can be observed by any telescope on Earth. The high-resolution spectrograph receives ultraviolet light (light at the violet end of the light spectrum) that cannot reach Earth because it is absorbed by the atmospheric layer. The faint-object spectrograph records data about the chemical composition of an object.

The telescope is pointed in the right direction by six gyroscopes (wheels or disks that spin horizontally and perpendicularly), which also keep it stable. Attached to the HST are two solar arrays (panels that capture light from the Sun) that resemble wings. Each measuring 8 feet (2.4 meters) wide and 40 feet (12 meters) long, the arrays supply the spacecraft with electrical power. A solar cell blanket (layer) on each array converts sunlight into energy and charges six nickel-hydrogen batteries during the sunlit part of orbit. The batteries then provide power when the HST is in Earth's shadow.

Telescopes make revolutionary discoveries

Since ancient times, astronomers have been developing theories about the universe. Prior to the invention of the telescope, they had to depend entirely on the naked eye to make their observations. Although they produced knowledge about stars and planets, their theories about the relation of Earth to other celestial bodies were wrong. By the third century b.c.e. astronomers were asserting that Earth was a sphere at rest at the center of the universe, with twenty-seven concentric (having a common center) spheres rotating around it. This theory was not questioned until the late sixteenth century c.e., when Polish astronomer Nicolaus Copernicus (1473–1543) published an opposite view.

Using the naked-eye method and complex mathematical formulas, Copernicus proposed that the Sun is at the center of the solar system and that the planets—including Earth—orbit around the Sun. He also came to believe that Earth is a relatively small and unimportant component of the universe. Copernicus's theory was proven in the early seventeenth century by Galileo, when he perfected his telescope. Made with two lenses (pieces of glass ground and polished to magnify objects), the telescope magnified objects thirty-two times their original size and was strong enough for astronomical viewing.

From Galileo's time onward, astronomers all over the world developed larger and more powerful telescopes. They were able to collect records of stars, planets, and other bodies that previously could not be seen by the naked eye. Although the Sun-centered universe was now an accepted theory, astronomers still believed that there was only one galaxy—the Milky Way, which contains Earth and its known solar system. According to this theory, nothing existed outside the Milky Way except a vast, empty space.

In 1924 Edwin Hubble (see box on page 98) made a revolutionary discovery: Using the 100-inch (254 centimeters) telescope at Mount Wilson near Los Angeles, California, he observed billions of other galaxies. Moreover, Hubble found that the galaxies were moving away from one another, an indication that the universe is expanding. As a result of Hubble's discovery, astronomers engaged in speculation about the beginning and end of the universe, producing such ideas as the big-bang theory. The big-bang theory states that the universe was formed ten to twenty billion years ago when a highly condensed form of energy and matter exploded. According to this view, effects of the huge explosion are still taking place with the expansion of the universe.

History of the HST

Despite the benefits of advanced technology, modern astronomers were unable to receive precise images from their telescopes. Like the naked-eye observers before them, they encountered a limitation—in this case Earth's atmosphere (air surrounding Earth). Even the most powerful telescopes situated atop the highest mountains could not penetrate the thick layer of dust and gases that make up the atmosphere. As early as 1923 the German rocket scientist Hermann Oberth (1894–1989; see entry) had come upon a solution to the problem. He envisioned attaching a telescope to a rocket and sending it into Earth orbit. In 1946 American astrophysicist Lyman Spitzer Jr. (1914–1997) expanded this idea, proposing a space-based observatory that would orbit above Earth's atmosphere.

Edwin P. Hubble

The Hubble Space Telescope was named for astronomer Edwin P. Hubble (1889–1953), who added important basic knowledge to the field of astronomy. Among his contributions was the discovery of other galaxies, which proves that the universe is constantly expanding. In simplified terms, he basically discovered the universe.

At a time when scientists did not believe any other galaxies existed outside the Milky Way, Hubble proved otherwise. He also showed that the universe is expanding, and he developed a mathematical model known as Hubble's law for observing this expansion. Although he was not the first to suggest that the universe is expanding, he was the first to recognize the existence of other galaxies and to form a clear theory along with a law to prove it. Generally speaking, Hubble's law states that the farther away a galaxy exists from Earth's galaxy (the Milky Way), the faster it is moving away from Earth. This concept later became part of the big-bang theory

of the creation of the universe. In 1990 NASA launched the Hubble Space Telescope in honor of the astronomer. Orbiting 370 miles (595 kilometers) above Earth's surface, the device was designed to collect data that would build upon Hubble's earlier findings.

In the late 1950s NASA launched two Orbital Astronomical Observatories (OAOs) into Earth orbit. The OAOs led to the construction of the HST over twenty-five years later. The first step was the Large Space Telescope (LST) project, which was initiated in 1969. An immediate result of the LST was the introduction of the space shuttle, a reusable vehicle that would launch the LST into orbit. Five space shuttles—Columbia, Challenger, Atlantis, Discovery, and Endeavour—have been built since establishment of the program. (Enterprise was the first shuttle to be built; however, it never went into orbit and was used primarily for "captured flights" involving takeoff and re-entry exercises.) The space shuttle represented a major shift in direction for the U.S. manned space flight program. During the 1960s NASA had concentrated its efforts on reaching the Moon by developing Project Mercury, Project Gemini, and Project Apollo (see Buzz Aldrin [1930–], Neil Armstrong [1930–], John Glenn [1921–], and Christopher Kraft [1924–] entries). Interest in further Moon exploration had steadily declined in the early 1970s, and NASA's attention was refocused on space research that could be conducted by the LST.

Technical issues and lack of funding caused a series of delays before the LST project was finally approved by the U.S. Congress in 1977. Collaborating with the European Space Agency, NASA began building the telescope, which was first renamed the Space Telescope and then the Hubble Space Telescope. The Hubble was assembled and ready for launch in 1985, but the Challenger (see entry) disaster forced a two-year delay. During that time NASA improved the solar panels, made modifications for easier instrument replacement and servicing, and upgraded computer systems. On April 24, 1990, the HST was lifted into space during a five-day mission by the space shuttle Discovery. The five crew members were commander Loren J. Shriver (1944–), pilot Charles F. Bolden Jr. (1946–), and mission specialists Steven A. Hawley (1951–), Bruce McCandless II (1937–), and Kathryn D. Sullivan (1951–). They released the HST into orbit on April 25.

HST fulfills mission

The HST had been orbiting for about a month when NASA scientists became concerned about fuzzy images it was sending back to Earth. The scientists determined that the primary mirror had a defect called spherical aberration. This means the manufacturer had made the mirror the wrong shape, which prevented it from reflecting sharp, clear images. In addition, there were problems with the gyroscopes and solar panels. On December 2, 1993, NASA launched the space shuttle Endeavour on an eleven-day mission to the HST. (The Endeavour was the newest shuttle in the NASA fleet, having been built to replace the Challenger.) The five crew members were commander Kenneth D. Cameron (1949–), pilot Stephen S. Oswald (1951–), and mission specialists C. Michael Foale (1957–), Kenneth D. Cockrell (1950–), and Ellen Ochoa (1958–; see entry), who was the first Hispanic woman to travel in space.

To correct the problem with the primary mirror, the crew installed a device containing ten small mirrors that redirect the light paths from the primary mirror to the spectrographs and the photometer. Making a total of five spacewalks, the crew also replaced the wide-field planetary camera and changed a bent solar panel. (The panel could not be taken back to Earth, so it was released into space. It reentered Earth's atmosphere nearly five years later, in October 1998.)

The HST worked perfectly after the repair mission, capturing spectacular images and making many astronomical advances and discoveries. The orbiting observatory has provided the sharpest view of Mars ever obtained by a telescope, showing icy white clouds and orange dust storms that swirl above the rust-red planet. The HST also confirmed the existence of black holes. (A black hole is a region with intense gravitational force caused by the collapse of a star.) The HST revealed black holes to be at the center of most galaxies. Moreover, it proved that quasars (bright, distant objects that resemble stars) are nuclei (centers) of galaxies and that they are powered by black holes. Another important discovery is that gamma rays (photons emitted by a radioactive substance) originated from distant galaxies in the early universe. Other observations include the expansion of the universe by an unknown force, the birth and death of stars, and the collisions of comets. Images sent back to Earth by the HST reveal a vast blue-black space full of dazzling light, brilliant colors, swirling clouds and dust, and endless galaxies. The telescope is still making new discoveries about the universe, which astronomers once thought to be nothing but a deep, dark void beyond the Milky Way.

The future of the HST

Although the HST has reached expectations, it was designed to have a limited life span. After the observatory was launched in 1990, astronauts were to make periodic visits to do maintenance work and install new equipment. By 2003 three service missions were completed, and the fourth and final mission was scheduled for 2006. It was canceled after the Columbia disaster (see box in Challenger Crew entry). In

February 2003 the Columbia broke apart over the western United States while returning to Earth from a mission to the International Space Station (ISS; see entry). All seven crew members were killed. Built earlier than other existing shuttles, the Columbia had been used primarily for scientific missions and for servicing the HST. The day after the accident NASA administrator Sean O'Keefe (1956–) appointed the Columbia Accident Investigation Board (CAIB). In August the CAIB issued a final report, which stated that shuttle flights were becoming increasingly dangerous and that NASA should fly a minimum number only when necessary. The following January, as a result of new safety guidelines stated in the report, O'Keefe canceled the final service mission to the HST. In April of that year, O'Keefe reported to a House subcommittee that preliminary experiments indicated that robots (devices designed to perform human activities) possibly could be used to service the HST.

The HST continued to operate normally, but it could not be expected to last indefinitely. Its original mission was expected to last fifteen years, and that had been extended to twenty years, or until 2010. Without servicing and repair, the components of the observatory will eventually wear out. The HST was built to dock with a space shuttle, so another type of spacecraft could not be used for a service mission. Concern over the fate of the HST prompted O'Keefe to ask the National Academy of Science (NAS) to study possible ways to prolong its life. In 2004 NAS appointed a committee of former astronauts, professors, scientists, and engineers to explore alternatives.

For More Information

Books

Goodwin, Simon. Hubble's Universe: A Portrait of Our Cosmos. New York: Viking Penguin, 1997.

Kerrod, Robin. Hubble: The Mirror on the Universe. Buffalo, NY: Firefly Books, 2003.

Periodicals

"HST, Keck Find a Galaxy from the 'Dark Ages.'" (May 2004): p. 30.

"Hubble's Gifts." Kids Discover (May 2004): pp. 10–11.

Muir, Hazel. "I Spy with My Little Eye." New Scientist (April 3, 2004): pp. 40+.

Reddy, Francis. "Swirling Echoes of Light." Astronomy (June 2004): p. 22.

Reichhardt, Tony. "NASA Seeks Robotic Rescuers to Give Hubble Extra Lease on Life." Nature (March 25, 2004): p. 353.

Web Sites

"The Hubble Project." NASA.http://hubble.nasa.gov (accessed on June 25, 2004).

HubbleSite.http://hubblesite.org (accessed on June 25, 2004).

Other Sources

The Big Bang. World Almanac Video, 1999.

Exploding Stars and Black Holes. PBS Home Video, 1997.

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 storage—at 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