Astronauts Repair the Hubble Space Telescope Summary

  • Last updated on November 10, 2022

After space shuttle astronauts undertook repairs of the Hubble Space Telescope in Earth orbit, the telescope produced images of unprecedented detail and clarity.

Summary of Event

One of the earliest envisioned scientific applications of space exploration was the placement of a large telescope beyond the optical distortions imposed by Earth’s atmosphere. The Hubble Space Telescope (HST), the first realization of this dream, experienced many setbacks before finally becoming operational. The launch of the telescope was delayed by two years after the explosion of the space shuttle Challenger Challenger (space shuttle) accident in 1986. Then, when the telescope was launched in April, 1990, astronomers discovered to their horror that its optics contained a major defect. Hubble Space Telescope;repairs Astronomy;telescopes Telescopes Astronauts and cosmonauts [kw]Astronauts Repair the Hubble Space Telescope (Dec. 2-13, 1993) [kw]Hubble Space Telescope, Astronauts Repair the (Dec. 2-13, 1993) [kw]Space Telescope, Astronauts Repair the Hubble (Dec. 2-13, 1993) [kw]Telescope, Astronauts Repair the Hubble Space (Dec. 2-13, 1993) Hubble Space Telescope;repairs Astronomy;telescopes Telescopes Astronauts and cosmonauts [g]North America;Dec. 2-13, 1993: Astronauts Repair the Hubble Space Telescope[08770] [g]United States;Dec. 2-13, 1993: Astronauts Repair the Hubble Space Telescope[08770] [c]Science and technology;Dec. 2-13, 1993: Astronauts Repair the Hubble Space Telescope[08770] [c]Spaceflight and aviation;Dec. 2-13, 1993: Astronauts Repair the Hubble Space Telescope[08770] [c]Astronomy;Dec. 2-13, 1993: Astronauts Repair the Hubble Space Telescope[08770] Akers, Thomas D. Thornton, Kathryn C. Bowersox, Kenneth D. Covey, Richard O. Hoffman, Jeffrey A. Musgrave, F. Story Nicollier, Claude

Like most large astronomical telescopes, the HST uses a large concave mirror, the primary mirror, to focus incoming light to an image. The primary mirror of the HST is 2.4 meters (94.5 inches) in diameter. Secondary mirrors are used to reflect the image to its final viewing location. The secondary mirrors often are curved to assist in focusing the image. The primary and secondary mirrors must be fabricated to exactly the right shape and their curvatures precisely coordinated. The primary mirror in the HST was ground to the wrong shape, resulting in an optical defect known as spherical aberration. Light reflected off different parts of the primary mirror focused in different locations, making it impossible to focus the telescope perfectly.

Ironically, it was the very magnitude of the error that prevented its detection. Telescope mirrors are given their basic shape through grinding; they are then polished to dimensions accurate to within a small fraction of a wavelength of light, or within the width of a few hundred atoms. The optical instrument used in testing the primary mirror during grinding had a lens about a millimeter out of position, causing the primary mirror to be slightly too flat. The later optical tests applied to the HST mirror were designed to monitor the perfection of the mirror surface; it never occurred to any of the builders of the telescope that a gross error in the basic shape of the mirror was possible.

During STS-61, astronaut Kathryn C. Thornton hovers near equipment used to service the Hubble Space Telescope.


Severe as it was, the error in the HST’s primary mirror was only 0.002 millimeters (less than 0.00008 inches). Nevertheless, the error was 100,000 times as great as any surface irregularities in the mirror. At the level of accuracy routinely required in the design of astronomical telescopes, a flaw such as that in the Hubble Space Telescope might be likened to building a bridge across the wrong river. Scientist Robert Shannon Shannon, Robert called it “the single largest mistake that’s ever been made in optics.” A crude optical test of the telescope did reveal the problem, but the test results initially were thought to be a result of the relative crudeness of the test, not a flaw in the telescope.

Many popular media accounts of the problem created the impression that the telescope was useless. In reality, the effects of the spherical aberration could often be largely removed through computer processing of the images. For bright objects, such as planets in our solar system, the HST returned extremely detailed images, even with flawed optics. The problem was most acute for faint objects. Because spherical aberration spreads light out over a larger area than a perfect image, it took about five times longer to make observations of faint objects than would have been the case with the originally designed system. The original design of the HST called for 70 percent of the light from a star to be concentrated in the central core of the image; the flawed HST achieved only 15 percent. It was impossible to collect enough light to observe the faintest objects, such as galaxies at the edge of the observable universe, and observing such faint objects had been one of the principal reasons for launching the telescope.

Replacing the flawed primary mirror was out of the question, but many of the observing instruments aboard the HST were made to be easily replaced. Mirrors in replacement instruments could be shaped to compensate for the flaw in the primary mirror. However, tests showed that the replacement mirrors had to be aligned with great precision or other optical errors would be introduced, making the HST’s images even worse. Two of the imaging cameras on the HST were redesigned to correct the HST’s optical problems.

Flawed optics were not the only problem aboard the HST. The telescope’s large solar panels vibrated every time they expanded and contracted as the HST passed in and out of Earth’s shadow. Ground controllers had developed ways of compensating for the vibration, but the compensations taxed the HST’s onboard computer. Also, three of the HST’s six gyroscopes had failed, leaving the telescope with the bare minimum needed for operation. If another failed, the telescope would be completely inoperable. Replacement of the gyroscopes was a higher priority than even repair of the optics.

On December 2, 1993, the space shuttle Endeavour Endeavour (space shuttle) lifted off on flight STS-61, one of the most complex space missions undertaken up to that time. The crew, six men and one woman, comprised six U.S. astronauts and a Swiss astronaut from the European Space Agency. Beginning on December 5, shuttle astronauts made six-hour space walks Space walks on five successive days to replace two imaging cameras on the HST with corrected optics, replace the solar cell panels, replace the failed gyroscopes and their electronics, and add a processor to the HST’s computer. Endeavour landed on December 13 after eleven days in orbit. The mission proceeded so flawlessly that author R. T. Fienberg, writing about the mission, referred readers to an article written before the mission that described the planned activities and told them simply to “change all the verbs to past tense.”


The M100 galactic nucleus shot with first- and second-generation cameras on the Hubble Space Telescope.


Images returned by the refurbished HST showed that the telescope’s problems were completely fixed and that image quality equaled or exceeded original specifications. In the solar system, the HST succeeded in clearly revealing Pluto and its moon Charon and returning the most detailed images ever of the large asteroid Vesta. It also showed that the atmospheric storm patterns on Neptune Neptune (planet) had changed greatly in the few years since the Voyager encounter in 1989. One unsuccessful observation dramatically illustrated the quality of the telescope. Astronomers imaged a globular star cluster, hoping to find large numbers of extremely small faint stars. As seen from earth, globular star clusters are so tightly packed with stars that the images overlap, creating a solid mass of light. Although the HST did not find the hoped-for faint stars, its images were so sharp that they peered through the cluster to reveal faint galaxies far beyond.

Another widely published image showed a distant cluster of galaxies surrounded by thin, sharp arcs of light. These arcs are the distorted images of more remote galaxies, an example of gravitational lensing and dramatic support for Albert Einstein’s general theory of relativity. In gravitational lensing, Gravitational lensing massive objects such as clusters of galaxies bend light much like a crude lens, creating the distorted images that the HST showed with unprecedented clarity. In another confirmation of general relativity, observations with the HST in May, 1994, indicated the presence of a giant black hole at the center of the galaxy M87.

The repair of the HST ended a particularly difficult period for the U.S. space program, which had been plagued by a seemingly endless string of problems and mission failures since the loss of the space shuttle Challenger in 1986. The almost perfect execution of an extraordinarily complex spaceflight and the equally perfect performance of the repaired Hubble Space Telescope greatly aided in restoring public confidence in the space program. Hubble Space Telescope;repairs Astronomy;telescopes Telescopes Astronauts and cosmonauts

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Fienberg, Richard Tresch. “Endeavour’s Excellent Adventure.” Sky and Telescope 87 (April, 1994): 20-23. Presents a photographic history of the repair of the HST in space.
  • citation-type="booksimple"

    xlink:type="simple">“Gravity’s Lens at Work.” Sky and Telescope 90 (June, 1995): 11. Includes a spectacular image of gravitational lensing caused by distant galaxies.
  • citation-type="booksimple"

    xlink:type="simple">Hoffman, Jeffrey A. “How We’ll Fix the Hubble Space Telescope.” Sky and Telescope 86 (November, 1993): 23-29. One of the participating astronauts presents a schedule of the repairs to be made to the HST. The mission proceeded almost exactly as projected in this article.
  • citation-type="booksimple"

    xlink:type="simple">“Hubble’s Image Restored.” Sky and Telescope 87 (April, 1994): 24-27. Article is devoted primarily to a gallery of images made with the repaired Hubble telescope.
  • citation-type="booksimple"

    xlink:type="simple">“Hubble’s Road to Recovery.” Sky and Telescope 86 (November, 1993): 16-22. Presents a history of the problems with the HST and describes the development of solutions. Aimed at readers who are familiar with telescopes and imaging terminology.
  • citation-type="booksimple"

    xlink:type="simple">Leverington, David. New Cosmic Horizons: Space Astronomy from the V2 to the Hubble Space Telescope. New York: Cambridge University Press, 2000. Presents the history of space-based astronomy since World War II, devoting the final chapter to discussion of the HST. Includes illustrations, glossary, bibliography, and indexes.
  • citation-type="booksimple"

    xlink:type="simple">Petersen, Carolyn Collins, and John C. Brandt. Hubble Vision: Further Adventures with the Hubble Space Telescope. 2d ed. New York: Cambridge University Press, 1998. Comprehensive discussion of the astronomical discoveries made possible by the HST. Includes many illustrations, glossary, bibliography, and index.
  • citation-type="booksimple"

    xlink:type="simple">Smith, Robert W. The Space Telescope: A Study of NASA, Science, Technology, and Politics. New York: Cambridge University Press, 1993. Provides a detailed chronological account of the construction of the HST from its inception to launch preparation. One of the most complete works available on the subject; includes historical background as well as discussion of the involvement of NASA, industry, and the scientific community.

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Categories: History