The Hubble Space Telescope
The Flawed Mirror, the Repair Mission, and the Deep Field
- 20 chapters
- 49m
- Astronomy
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This audiobook follows the telescope through its early development, funding struggles, and complex systems design. Chapters cover the flawed mirror discovery, the planning of Servicing Mission 1, and the installation of COSTAR to correct the optical error. Later chapters describe how the telescope's instruments captured deep field images, made discoveries about dark energy, and continued operating through multiple repair missions.
The book explains how amateur astronomers used Hubble data, how the telescope's computer systems worked, and what happened during its final servicing mission in 2009. Anyone interested in space science or the history of major scientific instruments will find this a compelling listen.
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In 1923, Hermann Oberth, who is considered one of the fathers of modern rocketry alongside Robert H. Goddard and Konstantin Tsiolkovsky, published a book titled Die Rakete zu den Planetenräumen, which discussed how a telescope could be sent into Earth orbit using a rocket.
The story of the Hubble Space Telescope begins in 1946 with a paper by astronomer Lyman Spitzer titled "Astronomical advantages of an extraterrestrial observatory." In it, Spitzer outlined two major benefits a space-based telescope would have over ones on the ground. First, it would avoid the blurring effect of Earth’s atmosphere, known as “seeing,” which limits ground-based telescopes to resolutions of about 0.5 to 1.0 arcseconds, while a space telescope could theoretically reach as sharp as 0.05 arcsec with a mirror 2.5 meters across. Second, it could observe light in the infrared and ultraviolet spectrum that Earth’s atmosphere blocks completely.
Spitzer spent much of his career advocating for a space telescope. In 1962, the U.S. National Academy of Sciences released a report recommending such a telescope as part of the space program. Then, in 1965, Spitzer was named head of a committee tasked with outlining the scientific goals for what would become a large space telescope.
Nancy Grace Roman, known as the “Mother of Hubble,” played a vital role even before the telescope became an official NASA project. She gave public lectures highlighting the scientific potential of the mission. Once approved, she took on the role of program scientist, forming a steering committee to make astronomer needs possible and testifying to Congress throughout the 1970s to keep funding flowing. Her efforts as project scientist helped establish standards for how NASA would manage large scientific projects.
Space-based astronomy began in the years after World War II, as scientists started using new rocket technology to study the universe from above the Earth’s atmosphere. In 1946, researchers captured the first ultraviolet spectrum of the Sun. Then in 1962, NASA launched the Orbiting Solar Observatory, or OSO, to observe UV, X-ray, and gamma-ray radiation. The United Kingdom also joined the effort that same year with an orbiting solar telescope as part of its Ariel programme. By 1966, NASA had launched the first Orbiting Astronomical Observatory mission, known as OAO-1, but its battery failed after just three days. That was followed by OAO-2, which began operating in 1968 and continued ultraviolet observations of stars and galaxies until 1972, far longer than its original one-year plan.
In the late 1960s, NASA began serious planning for a major new space telescope, one with a mirror three meters across, tentatively called the Large Space Telescope or LST. The project was shaped by earlier missions like OSO and OAO, which showed how powerful space-based observations could be. Launch was scheduled for 1979, but planners knew that keeping such an expensive instrument working would require crewed maintenance visits. At the time, NASA was also developing the Space Shuttle, a reusable spacecraft that would soon make those repairs possible.
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The ongoing success of the OAO program helped build strong support among astronomers for the Large Space Telescope as a major goal. In 1970, NASA formed two committees—one to handle the engineering plan and another to define the scientific aims of the mission. After those were set, NASA faced the challenge of securing funding, since the telescope would cost far more than any Earth-based instrument. Congress reviewed the budget carefully and made cuts to the planning phase, which included in-depth studies of possible instruments and hardware. Then, in 1974, public spending reductions caused Congress to eliminate all funding for the project.
In 1977, NASA Administrator James C. Fletcher suggested a $5 million budget line for the Hubble Space Telescope. But then, Noel Hinners, who held the title of NASA Associate Administrator for Space Science, decided to cut all funding for Hubble instead. He took a gamble, hoping that by doing so, he would push the scientific community into action, demanding full support for the project. As Hinners later recalled:
It was clear that year that they wouldn’t be able to get a full start on Hubble. There was opposition on Capitol Hill, largely due to budget issues. Jim Fletcher suggested putting in five million dollars as a placeholder. I didn’t like that idea. In today’s terms, it was a “sop” to the astronomy community. It gave the impression that something was being done, that all was well.
I thought if we left it blank, it would shock the astronomy community into action. That way, they’d realize how serious the situation was and rally together. So I suggested we not include anything at all. I don’t recall the specifics of any discussions, but Jim agreed, and we ended up zeroing it out. From my point of view, it worked as intended—getting the group to push harder on lobbying efforts. I like to believe it was a smart political decision in hindsight, though looking back, I’m not sure I planned it that well. It was more of a spontaneous idea than a calculated strategy.
They needed five million to make it seem like everything was fine, but it wasn’t. So they decided to send a message. My own thinking was to get them excited and push them into action. Zeroing it out would definitely send that message. I don’t think I talked to anyone else about it first—just said, “Let’s go do that.” Voilà, it worked. Don’t know if I’d do it again.
When NASA initially stripped the Hubble Space Telescope of its funding, astronomers across the country mobilized. They lobbied congressmen and senators directly, while also launching widespread letter-writing campaigns. The National Academy of Sciences weighed in with a report underscoring the importance of a space telescope. Eventually, the Senate approved only half of what Congress had originally funded.
The funding problems forced a smaller telescope design, reducing the mirror from three meters to 2.4 meters, cutting costs and improving hardware efficiency. A planned smaller 1.5-meter test telescope was canceled, and to save money, NASA partnered with the European Space Agency. ESA contributed funds, a key instrument, solar panels, and staff, in exchange for at least fifteen percent of observing time for European astronomers. Congress approved $36 million in 1978, and work began in earnest on the Large Solar Telescope, targeting a 1983 launch. That same year, the telescope was named Edwin Hubble, honoring the scientist who confirmed the universe’s expansion—first proposed by Georges Lemaître.
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The Hubble Space Telescope is a Cassegrain reflector of Ritchey–Chrétien design, a type used in most large professional telescopes. This design uses two hyperbolic mirrors to deliver sharp images across a wide field of view, though it's difficult to make and test the mirrors properly. The telescope’s mirror had to be polished to an extremely high standard—only 10 nanometers of accuracy—to match its goal of being diffraction limited. That level of precision is about 1/65th the wavelength of red light. While the telescope was built to observe across visible and ultraviolet light, it wasn’t optimized for infrared use, with mirrors kept at a stable, warm temperature around 15°C by heaters, which limits its infrared performance.
Perkin-Elmer planned to use advanced, computer-controlled machines to shape the mirror for the telescope. But because NASA wanted backup options, they also required that Kodak build a second mirror using older, traditional methods. At the time, a team from Kodak and Itek had actually submitted a bid to do the main polishing work, proposing that both companies verify each other’s results. That approach might have prevented the mistake that later caused major problems. The mirror made by Kodak is now on display at the National Air and Space Museum. A mirror built by Itek for this effort is currently in use at the Magdalena Ridge Observatory.
Construction of the Perkin-Elmer mirror began in 1979, using a blank made by Corning from their ultra-low expansion glass. The mirror was designed to be lightweight, with top and bottom plates each 25 mm thick enclosing a honeycomb lattice. Perkin-Elmer simulated microgravity by supporting it from the back with 130 rods applying varying forces to ensure correct shape in space. Polishing went on until May 1981, but NASA later questioned Perkin-Elmer’s management and noted the project was falling behind schedule and over budget. To cut costs, NASA canceled work on the backup mirror and moved the telescope’s launch date to October 1984. The mirror was finished by the end of 1981, washed with 9,100 liters of hot, deionized water, then coated with 65-nanometer-thick aluminum and a 25-nanometer-thick protective layer of magnesium fluoride.
Doubts about Perkin-Elmer’s ability to handle such a critical project kept growing as their timeline and budget for building the rest of the Optical Telescope Assembly kept expanding. When NASA heard that the schedule was “unsettled and changing daily,” they delayed the telescope’s launch until April 1985. But Perkin-Elmer's delays kept getting worse, with schedules slipping about one month every quarter, sometimes even a day for every day of work. Because of this, NASA had to postpone the launch again, first to March, then to September 1986. By that point, the project’s total cost had climbed to $1.175 billion.
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The spacecraft that would carry the telescope and its instruments was a major engineering feat. It had to handle extreme temperature shifts as it moved in and out of Earth’s shadow, while staying stable enough for precise pointing. A multi-layer insulation shroud helped keep the temperature steady inside, surrounding an aluminum shell where the telescope and its parts were mounted. Inside that shell, a graphite-epoxy frame held everything in alignment. Because graphite composites absorb moisture, there was concern that water vapor trapped in the truss during cleaning at Lockheed might freeze in space, covering the instruments with ice. To prevent this, the frame was purged with nitrogen gas before launch.
The Hubble Space Telescope relies on more than just its power systems to stay on course. Its pointing control system keeps the telescope properly oriented in space using five different kinds of sensors—magnetic sensors, optical sensors, and six gyroscopes. To move and adjust its position, the system uses two main types of tools called actuators: reaction wheels and magnetic torquers. Together, these components ensure that Hubble can lock onto distant stars, galaxies, and other celestial objects with incredible precision.
While the construction of the spacecraft that would house the telescope and its instruments went more smoothly than the building of the Optical Telescope Assembly, Lockheed ran into some budget and schedule issues. By the summer of 1985, the spacecraft work was already 30% over budget and three months behind schedule. An MSFC report noted that Lockheed tended to rely on NASA directions rather than taking their own initiative in the construction.
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The Hubble Space Telescope initially relied on two main computers: the 1.25 MHz DF-224 system from Rockwell Autonetics, which had three redundant CPUs, and two NSSC-1 (NASA Standard Spacecraft Computer, Model 1) systems built by Westinghouse and GSFC using diode–transistor logic. During Servicing Mission 1 in 1993, a co-processor was added to the DF-224, featuring two redundant strings of an Intel-based 80386 processor with an 80387 math co-processor. Then, in Servicing Mission 3A in 1999, the entire system was upgraded to a 25 MHz Intel-based 80486 processor setup, which was twenty times faster and had six times more memory than the old DF-224, improving efficiency by shifting some tasks to the spacecraft and enabling modern programming languages.
The Hubble Space Telescope relied on several embedded microprocessor-based systems to function. The MATs, or Multiple Access Transponders, specifically MAT-1 and MAT-2, used Hughes Aircraft CDP1802CD chips. The Wide Field and Planetary Camera, known as WFPC, utilized an RCA 1802 processor, possibly the earlier 1801 model. In 1993, during Servicing Mission 1, the WFPC was replaced by WFPC-2. Then, in 2009, during Servicing Mission 4, WFPC-2 gave way to the Wide Field Camera 3, or WFC3, which expanded Hubble’s ability to observe deeper into space and capture images across three major regions of the spectrum.
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When it launched, the Hubble Space Telescope carried five scientific instruments: the Wide Field and Planetary Camera, the Goddard High Resolution Spectrograph, the High Speed Photometer, the Faint Object Camera, and the Faint Object Spectrograph. The Wide Field and Planetary Camera was mounted in a radial instrument bay, while the other four instruments each occupied an axial instrument bay.
The WF/PC was a high-resolution imaging device built by NASA's Jet Propulsion Laboratory, designed mainly for optical observations. It used a set of 48 filters to isolate specific spectral lines of interest in astrophysics. The instrument included eight charge-coupled device chips split between two cameras, each with four CCDs. Each CCD had a resolution of 0.64 megapixels. One camera, the wide field camera, captured large angular fields at the cost of resolution, while the planetary camera, or PC, operated at a longer focal length than the WF chips, offering greater magnification.
The Goddard High Resolution Spectrograph, or GHRS, was built by the Goddard Space Flight Center and designed to observe in the ultraviolet spectrum, with a spectral resolution of 90,000. The FOC and FOS were also optimized for ultraviolet work and delivered the highest spatial resolution among all Hubble’s instruments. These three instruments didn’t use CCDs like later models; instead, they relied on photon-counting digicons for detection. The FOC was constructed by ESA, while the FOS came from a collaboration between the University of California, San Diego, and Martin Marietta Corporation.
The final instrument was the HSP, built at the University of Wisconsin–Madison. It was designed to observe variable stars and other objects that change brightness, working best in visible and ultraviolet light. The instrument could make up to 100,000 measurements each second, with a photometric accuracy of about 2% or better.
The Hubble Space Telescope’s guidance system served a dual role, functioning not just as a tool to keep the telescope precisely aimed, but also as a scientific instrument in its own right. Its three Fine Guidance Sensors, or FGS, were primarily designed to maintain accurate pointing during observations, yet they proved capable of performing extremely precise astrometry. Measurements made using these sensors have reached accuracy levels as fine as 0.0003 arcseconds.
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The Space Telescope Science Institute, or STScI, handled the scientific work of the Hubble telescope and made its data available to astronomers. It was set up in 1981 and is based in Baltimore, at Johns Hopkins University, operated by AURA, the association that includes thirty-nine U.S. universities and seven international partners. The institute came into being after a debate between NASA and scientists, with the latter pushing for an academic home. Meanwhile, the Space Telescope European Coordinating Facility, or ST-ECF, was created in 1984 near Munich and supported European astronomers until 2011, when its duties moved to the European Space Astronomy Centre.
At STScI, scheduling observations for Hubble is one of the most complex tasks involved in operating the telescope. Because Hubble orbits low to Earth, most targets are blocked from view for slightly less than half of each orbit. The telescope also must avoid the South Atlantic Anomaly due to high radiation levels. Exclusion zones surround the Sun, Moon, and Earth to protect sensitive instruments—especially the FGSs, which must stay free of bright light. The solar avoidance angle is about 50°, and Earth and Moon avoidance keeps scattered light from entering the telescope’s systems. Early in the mission, Earth observations were used to generate flat-fields for the WFPC1 instrument. There is also a continuous viewing zone within roughly 24° of Hubble’s orbital poles, where targets remain visible for long periods without being occulted.
Hubble orbits about 540 kilometers above Earth, tilted at 28.5 degrees, but its path shifts in ways that are hard to predict. Because of this, engineers can't be sure where it will be six weeks ahead—its position might be off by as much as 4,000 kilometers. The telescope's location along its orbit changes slowly over time, and the Earth’s atmosphere affects its path in unpredictable ways. That means observation schedules are usually set just days in advance. Hubble’s operations are watched around the clock by four teams of flight controllers. Engineering support comes from NASA and contractors at Goddard Space Flight Center in Greenbelt, Maryland, which is 48 kilometers south of the STScI.
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Within weeks of its launch, the Hubble Space Telescope began sending back images that revealed a serious flaw in its optical system. Though the early pictures looked sharper than those from ground-based telescopes, Hubble couldn’t focus properly. The best images it produced were far worse than expected. Stars and other point sources appeared spread out over more than one arcsecond, instead of being concentrated within the designed limit of 0.1 arcseconds in diameter.
The Hubble Space Telescope's primary mirror was flawed from the start, its shape incorrect in a way that would cause major problems. Even though it was thought to be among the most accurate optical mirrors ever crafted, smooth down to just 10 nanometers, the edge of the mirror had been ground too flat by about 2200 nanometers. This small but critical error led to severe spherical aberration, a condition where light bouncing off the mirror’s center and edge focuses on different points, ruining the telescope's ability to produce sharp images.
The mirror's flaw affected observations differently: the core remained sharp enough for high-resolution views of bright objects and spectroscopy wasn't completely ruined, just less sensitive. Extra light spread into a large halo making it hard to study faint objects or do high-contrast imaging, meaning most cosmological goals were impossible since they needed to see extremely dim things. Politicians questioned NASA's ability, scientists regretted the cost that could have gone elsewhere, and comedians made fun of the project. In the 1991 film The Naked Gun 2½: The Smell of Fear, Hubble was shown alongside the Titanic and Hindenburg as a historical disaster. Still, during the first three years before problems were fixed, astronomers managed to do useful work on easier targets. Because the error was stable, they could partly fix it using advanced image processing techniques like deconvolution.
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A commission led by Lew Allen, who directed the Jet Propulsion Laboratory, was formed to figure out what went wrong with the mirror. The Allen Commission discovered that a testing tool called a reflective null corrector had been put together incorrectly. One lens was off by 1.3 millimeters. During the mirror’s creation, Perkin-Elmer had used two regular refractive null correctors early on. But for the final shaping step, they turned to a custom-built reflective null corrector made to exact standards. Because of how it was assembled, the mirror got ground very precisely—but to the wrong shape. A few tests with the older tools had shown signs of spherical aberration, but those results were ignored. The team trusted the new device more, even though it was flawed.
The commission placed most of the blame on Perkin-Elmer for the problems with the mirror. Tensions between NASA and the optics company had grown throughout the telescope’s construction, caused by repeated delays and budget overruns. NASA discovered that Perkin-Elmer hadn’t properly reviewed or supervised the mirror’s creation. They hadn’t assigned their top optical scientists to the project like they had for the prototype, and they failed to include the original optical designers in building and checking the mirror. Though the commission heavily criticized Perkin-Elmer for these management failures, NASA also came under fire for not noticing the weak quality control, especially for depending entirely on results from just one testing instrument.
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Dr. Charles Pellerin, who led the Astrophysics division, assembled a group of experts after many feared Hubble might be left behind. With help from his budget analyst, he shifted funds by canceling or reprioritizing other projects to secure $60 million for repairs. He made a public promise to fix the telescope by 1994. Since the telescope’s original design included servicing missions, astronomers quickly began looking for fixes that could be applied during the 1993 mission. Although Kodak had produced a backup mirror, replacing it in space was not possible, and returning the telescope to Earth would have been too costly and slow. Instead, because the mirror was shaped so precisely but incorrectly, engineers developed new optical parts with the exact same flaw but reversed, which would act like corrective lenses during the repair.
The process of fixing the mirror began with a careful analysis of its exact flaw. Astronomers studied images of point sources, like stars, to trace back the error. They found that the mirror’s conic constant measured −1.01390±0.0002, when it should have been −1.00230. This same value was confirmed by reviewing the null corrector used by Perkin-Elmer during the mirror's grinding phase, as well as by inspecting interferograms collected during the mirror’s ground testing.
Because of how the Hubble Space Telescope’s instruments were built, engineers needed two different fixes to correct the mirror problem. The Wide Field and Planetary Camera 2, which was already planned to replace the old WF/PC, included relay mirrors that could redirect light onto four separate charge-coupled device chips. An intentional error in those mirrors’ surfaces could cancel out the main mirror’s flaw. But the other instruments didn’t have any adjustable intermediate surfaces, so they needed a separate external device for correction.
The Hubble Space Telescope's flawed mirror was corrected with the help of a system called COSTAR, or the Corrective Optics Space Telescope Axial Replacement. It was designed to fix light coming from three instruments: the FOC, FOS, and GHRS. The system used two mirrors in the light path, with one specifically ground to correct the spherical aberration. To install COSTAR, another instrument had to be removed—astronomers chose the High Speed Photometer for that. By 2002, all the original instruments that needed COSTAR had been replaced by newer ones with built-in corrective optics. COSTAR was then taken out of the telescope and returned to Earth in 2009. It is now displayed at a museum in Washington, D.C. The space it once occupied is now home to the Cosmic Origins Spectrograph.
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In 1993, the initial plan for repairing the Hubble Space Telescope was set before the mirror issue came to light. Once the problem became clear, the mission took on new urgency, as astronauts would need to install corrective optics. Without success, Hubble could have been left useless or permanently impaired. Additional failures in the telescope’s components prior to the mission pushed repair costs up to $500 million—excluding shuttle flight expenses. A successful repair would also help validate the concept of building Space Station Alpha.
STS-49 in 1992 revealed just how challenging spacewalks could be. Although the rescue of Intelsat 603 drew praise, the crew had taken what might have been risky risks to finish the task. The mission’s unrelated work on assembling prototype space station components also went off-script, prompting NASA to reexamine its approach to planning and training for future missions—especially the upcoming Hubble repair. In response, the agency selected Story Musgrave, who had been involved in satellite repair efforts since 1976, along with six other veterans, including two who had flown on STS-49. This would be the first mission director since Project Apollo to oversee a team with 16 previous shuttle flights. The crew prepared using around a hundred specialized tools.
The astronauts had faced problems with heat during earlier spacewalks, which happened in sunlight. Hubble needed repairs in shadow, not light. Before the mission, Musgrave found that the spacesuit gloves didn’t protect well enough from the cold of space. After STS-57 confirmed this issue in orbit, NASA quickly adjusted equipment, procedures, and the flight plan. Seven full simulations of the mission took place before launch, the most thorough preparation ever for a shuttle mission. There was no complete mockup of Hubble, so the crew studied several separate models—including one at the Smithsonian—and tried to imagine how they fit together.
In December 1993, the crew aboard Endeavour began a ten-day mission to service the Hubble Space Telescope. They installed new equipment and replaced several key components. Most importantly, they swapped out the High Speed Photometer for the COSTAR corrective optics package, and replaced the WF/PC with the Wide Field and Planetary Camera 2, which had its own internal optical correction. The solar arrays and their drive electronics were also replaced, along with four gyroscopes, two electrical control units, and other electrical parts. Two magnetometers were swapped out as well. The onboard computers received upgrades with added coprocessors, and Hubble’s orbit was boosted.
On January 13, 1994, NASA announced that Servicing Mission 1 had been a complete success and revealed the first sharper images from the Hubble Space Telescope. The mission, which involved five long periods outside the telescope for repairs, was one of the most complex ever performed at that time. Its success brought relief and renewed excitement for NASA and the astronomical community, as the telescope was now functioning with much greater capability than before.
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Plans for a Hubble repair mission in February 2005 were canceled after the Columbia shuttle disaster in 2003, which led NASA Administrator Sean O'Keefe to require all future shuttle flights be capable of reaching the International Space Station for safety. Since no shuttles could reach both HST and the station, servicing missions were postponed. Astronomers criticized the decision, saying Hubble was worth the risk. By 2004, the James Webb Space Telescope wasn’t expected to launch until at least 2011, and it wouldn’t be in low Earth orbit, making repairs impossible if problems arose. NASA officials worried that continuing to service Hubble would take money from JWST, which eventually launched in December 2021.
In late 2004, Senator Barbara Mikulski led Congressional members in public hearings, fighting with broad public support to convince the Bush Administration and NASA to reconsider canceling plans for a Hubble rescue mission. Their efforts followed outcry and pressure from Congress after O'Keefe had decided to cancel the final servicing mission to the Hubble Space Telescope. The National Academy of Sciences responded in July 2004 by convening an official panel that recommended preserving the HST despite risks, urging NASA to avoid actions that would block a space shuttle mission. Later that August, O'Keefe directed Goddard Space Flight Center to draft a proposal for a robotic service mission, which was eventually deemed “not feasible.”
In April 2005, Michael D. Griffin was nominated as NASA’s new Administrator, and he said he would consider a crewed servicing mission to the Hubble Space Telescope. Soon after, Griffin authorized preparations for the mission, saying he’d decide after the next two shuttle flights. In October 2006, he gave final approval, setting a 11-day mission by Atlantis for October 2008. But in September 2008, Hubble’s main data-handling unit failed, stopping all scientific data transmission until its backup came online on October 25. Because the backup unit was also at risk, the service mission was delayed to include a replacement for the primary unit.
Atlantis flew the final planned shuttle mission for Hubble in May 2009, known as Servicing Mission 4. That flight brought in a new data-handling unit, fixed the ACS and STIS systems, and swapped out the nickel–hydrogen batteries and all six gyroscopes. The crew also installed two entirely new instruments: the Wide Field Camera 3 and the Cosmic Origins Spectrograph. A Soft Capture and Rendezvous System was added too, meant to help future missions safely capture and dispose of the telescope. The work restored Hubble’s full functionality, with the exception of the ACS’s High Resolution Channel, which had to be disabled because it could not be repaired.
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The Hubble Space Telescope operates under a system where anyone can submit a proposal for observing time, regardless of their country or academic background. However, financial support for analyzing the data gathered is limited to U.S. institutions only. Each round of proposals sees fierce competition, with only roughly 20 percent of submissions being approved for actual use of the telescope.
Every year, astronomers get a call to submit proposals for time observing with the Hubble Space Telescope. The process lasts about a year, and there are different types of requests. Most proposals are for "general observer" programs, which cover normal telescope use. There’s also a category called "snapshot observations," which are quick tasks that take no more than 45 minutes, including the time needed to aim the telescope at a target. These short jobs help fill in open slots in the schedule that can’t be filled by regular observing programs.
Astronomers can apply for what are called "Target of Opportunity" proposals, which let them observe sudden cosmic events like flares or eclipses if they happen during the telescope's scheduling cycle. Also, up to ten percent of the time is set aside as “director's discretionary” time, or DD time. This special allocation is used for unexpected phenomena such as supernovae, and astronomers can apply for it anytime throughout the year.
DD time was used for observations that led to the Hubble Deep Field and Hubble Ultra Deep Field, and during the first four cycles of telescope time, amateur astronomers also carried out observations.
In 2012, the European Space Agency launched a contest inviting the public to process images from Hubble data. The goal was to inspire people to uncover "hidden treasures" within the raw information the telescope had captured. By encouraging broad participation, the ESA hoped to bring fresh perspectives to the vast archive of space observations. This effort aimed not just at scientific discovery, but also at engaging everyone in exploring the cosmos through Hubble’s eyes.
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In 1986, Riccardo Giacconi, who was the first director of STScI, said he wanted to use some of his director discretionary time to let amateur astronomers operate the telescope. Even though the total time allocated was just a few hours per cycle, it sparked great excitement among amateur astronomers.
A committee of amateur astronomers carefully reviewed proposals for time on the telescope, giving it only to projects that showed real scientific value, didn’t repeat work done by professionals, and needed Hubble’s special abilities. Thirteen amateurs were chosen to use the telescope between 1990 and 1997. One study, “Transition Comets – UV Search for OH,” was carried out. The first proposal, “A Hubble Space Telescope Study of Posteclipse Brightening and Albedo Changes on Io,” was published in Icarus, a journal for solar system research. Another amateur group also had their work published in Icarus. After that, budget cuts at STScI made it impossible to support more amateur projects, so no further programs were conducted.
Amateur astronomers have contributed to Hubble observations since the early days of the telescope, with one of the first examples being the Great White Spot on Saturn in 1990, discovered by S. Wilber and then studied by HST under a proposal from J. Westphal of Caltech. Later collaborations included findings from the Galaxy Zoo project, such as the discovery of Voorwerpjes and Green Pea galaxies. The "Gems of the Galaxies" program was based on a list of objects volunteered by Galaxy Zoo members, which was then narrowed down through an online vote. Hubble also observed minor planets like 2I/Borisov and changes in the atmospheres of Jupiter, Saturn, Uranus, and Neptune. In the pro-am project backyard worlds, HST observed a planetary mass object known as WISE J0830+2837, and the telescope's non-detection helped classify this unusual object.
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In 1994, a comet named Shoemaker-Levy 9 crashed into Jupiter, and astronomers were lucky to catch it with Hubble, which had just returned to full strength after Servicing Mission 1. The images were sharper than any since Voyager 2 passed by in 1979, helping scientists understand how a large comet hitting Jupiter—something thought to happen once every few centuries—played out.
In March 2015, researchers shared findings that came from studying aurorae around Ganymede, a moon of Jupiter. Using the Hubble Space Telescope, they tracked how those lights moved and discovered something surprising: beneath its icy surface lies a vast ocean of saltwater. This ocean is about 100 kilometers deep and sits under a crust of ice that's roughly 150 kilometers thick. The presence of this hidden sea helps explain how Jupiter’s powerful magnetic field interacts with Ganymede’s own, essentially dampening the effects between them.
Hubble has helped astronomers study distant objects in our Solar System, like the dwarf planets Pluto, Eris, and Sedna. In June and July 2012, U.S. astronomers using the telescope discovered Styx, a small fifth moon orbiting Pluto.
From June to August 2015, the Hubble Space Telescope was turned toward the Kuiper Belt to help find a target for the New Horizons Kuiper Belt Extended Mission, or KEM. Ground-based searches had failed to locate a suitable object, so astronomers hoped Hubble could succeed where others had not. The effort led to the discovery of at least five new Kuiper Belt objects, including one that would become the mission’s final target: 486958 Arrokoth. New Horizons went on to perform a close flyby of Arrokoth on January 1, 2019.
In April 2022, NASA revealed that astronomers had used images from the Hubble Space Telescope to measure the size of comet C/2014 UN271, also called Bernardinelli–Bernstein. This comet holds the record for the largest icy nucleus ever observed. The comet’s nucleus was found to have an estimated mass of fifty trillion tons, making it fifty times more massive than other known comets in our Solar System.
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Since its launch, the Hubble Space Telescope has profoundly shaped modern astronomy, with over 22,000 research papers published using its data by 2025. These findings have appeared in peer-reviewed journals and countless conference proceedings. When looking at how long-lasting these contributions are, it's clear that only two percent of Hubble-based papers end up with no citations, compared to about one-third of all astronomy papers overall. On average, studies relying on Hubble data receive roughly twice as many citations as those that don’t. In fact, around 10% of the 200 most-cited papers each year are rooted in Hubble observations.
The Hubble Space Telescope has undeniably advanced astronomical research, but it comes with a steep price tag. A comparison of telescopes showed that papers using HST data receive 15 times as many citations as those from a ground-based telescope like the William Herschel Telescope, which is four meters in diameter. Yet building and maintaining the HST costs about 100 times more than such a telescope.
Deciding between ground- and space-based telescopes involves complex trade-offs. Even before Hubble launched, ground-based techniques like aperture masking interferometry had produced sharper images than Hubble would later achieve, though only for objects about 108 times brighter. Since then, adaptive optics have improved ground-based telescopes’ ability to image faint infrared targets. The choice between using adaptive optics or observing with HST depends on the specific goals of the research. In visible light, adaptive optics work over a small field of view, while Hubble can capture high-resolution images across a wider area. Also, Hubble sees more faint objects because ground-based telescopes are limited by scattered light from Earth’s atmosphere.
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NASA knew it was vital for the Space Telescope to capture the public’s attention, especially since taxpayers had funded its construction and operation. The early years were tough, when the faulty mirror hurt Hubble’s image with people. But after the first servicing mission, the telescope began to recover. The corrected optics delivered stunning photos that helped restore its reputation and showed the value of the investment.
The Space Telescope Science Institute's Office for Public Outreach, set up in 2000, makes sure U.S. taxpayers see what they get from the space telescope program. That office runs HubbleSite.org. The Hubble Heritage Project, based at STScI, shares stunning images with the public. The team includes both amateur and professional astronomers, along with people from other fields. They focus on how beautiful the images are. The project gets limited time to observe objects that don’t capture enough wavelengths for full-color pictures due to scientific needs.
Since 1999, the Hubble European Space Agency Information Centre, or HEIC, has led outreach efforts in Europe. This office is based at the Space Telescope European Coordinating Facility in Munich. HEIC works to share Hubble’s discoveries with the public and educators, especially through news and image releases. Many of these highlight European contributions, showing off ESA's 15% share of the telescope and the work of European scientists. They also produce educational resources, including a video series called Hubblecast, which brings scientific updates to everyone.
The Hubble Space Telescope has been recognized with two Space Achievement Awards from the Space Foundation, one in 2001 and another in 2010, for its outreach activities.
A replica of the Hubble Space Telescope stands on the courthouse lawn in Marshfield, Missouri, the hometown of Edwin P. Hubble, the astronomer after whom the telescope is named. This display brings attention to the legacy of the man whose work helped define modern cosmology and the space telescope that continues to reveal the universe's mysteries.
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The Hubble Space Telescope uses gyroscopes to detect and measure its own rotations, which helps it stay stable in orbit and point accurately at celestial targets. It has six of these rate-sensing gyroscopes installed, though only three are normally needed for full operation. With two gyroscopes, observations are still possible but with some restrictions, and with just one, the telescope can function but faces challenges in maintaining precise pointing. In 2018, if fewer than three gyroscopes were working, the plan was to switch into a single-gyroscope mode. These gyroscopes are part of the Pointing Control System, which also includes magnetic sensors, optical sensors, and two types of actuators called reaction wheels and magnetic torquers.
After the Columbia disaster in 2003, questions arose about whether another mission to service Hubble would be possible, and concerns grew over how long the gyros might last. Engineers responded by creating new software that would let the telescope operate with just two or even one gyroscope, hoping to stretch out the mission’s life. By 2005, they decided to switch to using only two gyroscopes regularly, which extended the telescope’s usable lifetime. That change took place in August 2005, leaving Hubble with two working gyroscopes, two on standby, and two out of order. Then, in 2007, one more gyroscope failed.
When the Hubble Space Telescope needed its final repair in May 2009, only three of its six gyroscopes were still functioning. Engineers found that corrosion in the electric wires powering the motors had been triggered by oxygen used to push the thick suspending fluid through the system. The replacement gyroscopes were built with nitrogen instead, a change expected to improve reliability. During that mission, all six were swapped out. Nearly ten years later, only three had given out—and even then, they lasted longer than the design average.
In 2009, astronauts replaced six gyroscopes on the Hubble Space Telescope—three old-style units prone to a kind of failure called flex-lead failure, and three new ones built to last longer. The first of the old gyroscopes failed in March 2014, followed by a second in April 2018. Then, on October 5, 2018, the final one from that group gave out. Engineers switched on a backup gyroscope, but it didn’t immediately work within normal limits. So Hubble went into “safe” mode while scientists tried to fix it. NASA tweeted on October 22, 2018, that the rotation rates from the spare gyroscope had dropped and were now normal. They said more tests would be done to make sure Hubble could return to full science operations using that one.
The Hubble Operations Team faced a tricky problem when a backup new-style gyroscope failed to operate within its normal range. The solution widely reported as “turning it off and on again” didn’t work. The issue was traced to an air bubble in the fluid around the float inside the gyroscope. On October 18, 2018, the team directed the spacecraft through a series of maneuvers, moving it in opposite directions to try to fix the inconsistency. Only after those moves, and a follow-up set of maneuvers on October 19, did the gyroscope finally function properly again.
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Without those visits to fix and update Hubble, all of its instruments would have eventually stopped working, ending new discoveries. One such instrument, the Space Telescope Imaging Spectrograph, or STIS, had a power system failure in August 2004. The electronics for STIS were built with backups, but the first set failed in May 2001. That problem was fixed during Servicing Mission 4, which happened in May 2009.
The Advanced Camera for Surveys, or ACS, ran into trouble when its main camera primary electronics failed in June 2006. Then, on January 27, 2007, the backup power supply also went down. Only the instrument’s Solar Blind Channel, or SBC, could still function using side-1 electronics. During the SM 4 mission, a new power supply was added for the wide angle channel, but tests showed it didn’t fix the high resolution channel. The Wide Field Channel came back online in May 2009 after STS-125, but the High Resolution Channel stayed non-operational.
On January 8, 2019, the Hubble Space Telescope went into partial safe mode after issues were detected in its Wide Field Camera 3 instrument. NASA later determined the problem was not with the power supply but with faulty telemetry data coming from the instrument’s circuits. The telemetry readings showed incorrect voltage levels, and engineering data from those circuits were also inaccurate. On January 15, NASA said a software issue was to blame. After resetting the affected circuits and boards, the instrument returned to normal operation by January 17. That same day, it completed its first science observations since the incident.
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There is no direct replacement for Hubble as an ultraviolet and visible light space telescope, because near-term space telescopes do not duplicate Hubble's wavelength coverage. Instead, they focus on further infrared bands, which are preferred for studying high redshift and low-temperature objects—generally older and farther away in the universe. These wavelengths are also difficult or impossible to observe from the ground, making a space-based telescope necessary despite the cost. While large ground-based telescopes can image some of the same wavelengths, they sometimes challenge HST in resolution using adaptive optics, have greater light-gathering power, and can be upgraded more easily. Yet they still cannot match Hubble's excellent resolution across a wide field of view with the dark background of space.
The Next Generation Space Telescope project became the James Webb Space Telescope, officially succeeding Hubble. Unlike a bigger version of Hubble, JWST is built to work much colder and farther out at the L2 Lagrangian point, where Earth and Moon interference is reduced. It isn’t designed to be fully serviceable like Hubble, but it does include a docking ring for visits from other spacecraft. One of its main goals is to see the most distant objects in the universe, detecting stars that formed about 280 million years earlier than what Hubble finds. The telescope is an international effort between NASA, the European Space Agency, and the Canadian Space Agency, launching on December 25, 2021, aboard an Ariane 5 rocket. Though it's primarily an infrared instrument, its range reaches down to about 600 nanometers, which includes orange light in the visible spectrum.
A complementary telescope to Hubble and JWST was the European Space Agency's Herschel Space Observatory, which launched on May 14, 2009. Like JWST, Herschel wasn’t designed to be serviced after launch, and it had a mirror larger than Hubble’s, though it observed only in the far infrared and submillimeter wavelengths. It relied on helium coolant, which ran out on April 29, 2013.
The Hubble Space Telescope’s legacy continues with plans for even more powerful instruments, like the Large Ultraviolet Optical Infrared Surveyor, or LUVOIR. This conceptual telescope would be 8 to 16.8 meters across, capable of observing in visible, ultraviolet, and infrared light, with sharper detail than Hubble or Spitzer. The final planning report for the 2020 Astronomy and Astrophysics Decadal Survey suggested a launch around 2039. But eventually, the Decadal Survey recommended combining LUVOIR ideas with the Habitable Exoplanet Observer proposal to create a new 6-meter flagship mission, set to launch in the 2040s.
While ground-based telescopes and proposed Extremely Large Telescopes may outperform the Hubble Space Telescope in light-gathering power and diffraction limit because of their larger mirrors, other factors still give Hubble an edge. Some large ground-based reflectors can match or even exceed Hubble’s resolution using adaptive optics, or AO. However, systems like Lucky Cam produce sharp images only ten to twenty arcseconds wide, whereas Hubble’s cameras capture clear pictures across a field fifteen times wider. Additionally, space telescopes observe the full electromagnetic spectrum, most of which is blocked by Earth’s atmosphere. And because airglow from the atmosphere dims faint objects, the background sky in space remains darker, making deep-field observations easier.
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