Hubble Space Telescope
Hubble Space Telescope
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.
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 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.
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.
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.
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.
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.
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.
Kanipe, Jeff. Chasing Hubble’s Shadow: The Search for Galaxies at the Edge of Time. New York: Hill and Wang, 2006.
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.
National Aeronautics and Space Administration. “The Hubble Space Telescope.” <http://hubble.nasa.gov/index.php> (accessed October 12, 2006).
"Hubble Space Telescope." The Gale Encyclopedia of Science. . Encyclopedia.com. (August 14, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/hubble-space-telescope
"Hubble Space Telescope." The Gale Encyclopedia of Science. . Retrieved August 14, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/hubble-space-telescope