30 years after its first repair mission, Hubble continues to observe the cosmos

by Justin Davenport

30 years ago, on the very early morning of Dec. 2, 1993, the Space Shuttle Endeavour and the STS-61 crew launched on a high-stakes mission to repair the Hubble Space Telescope. 30 years after the successful flight, which enabled Hubble to finally fulfill its promise to revolutionize our view of the Universe, the veteran observatory is still making new discoveries and may get a new lease on life deeper into the 21st century.

Out of focus: Hubble after launch

After the Hubble Space Telescope was launched on April 24, 1990, by the Space Shuttle Discovery and the crew of STS-31, there was initial excitement in finally launching the long-planned and long-awaited orbiting observatory. As far back as the 1960s, plans had been made for a large observatory in space, which could see above the blurring effects and wavelength attenuation of Earth’s atmosphere.

However, within weeks of the observatory’s launch, engineers and scientists realized that something was wrong with Hubble’s optics. Two months after STS-31, NASA had to announce that the billion-dollar observatory was not functioning as it should. This was due to a mirror that was precisely ground to the wrong shape — by just 1/50th the width of a human hair — a decade earlier at the Perkin-Elmer facility in Danbury, Connecticut.

The mirror suffered from what is known as “spherical aberration.” This means that not every ray of light would converge at the same focus point when reflected by Hubble’s mirror. The telescope could still observe the universe, but its clarity was not what it needed to be for the ground-breaking observations that were planned and promised. The telescope could still observe certain parts of the ultraviolet (UV) and infrared spectrum that were not visible on Earth. However, Hubble was built to observe visible light, not infrared and UV.

Thus, Hubble was not able to achieve its full potential as an astronomical instrument and was criticized on late night television and painted as a symbol of NASA’s failures. U.S. Senator from Maryland Barbara Mikulski, whose state hosts the Goddard Space Flight Center, where Hubble is commanded from to this day, labeled the telescope a “techno-turkey.”

The first image released from the Hubble Space Telescope in 1990. After the image was released the spherical aberration was discovered during attempts to refine Hubble’s focus. (Credit: NASA)

A new pair of glasses: fixing Hubble

While the telescope and NASA were taking a big hit to their reputations and revised observational programs were prepared for the orbiting observatory, the space agency set out to find a fix for the optical system. Since the mirror was ground so precisely to a wrong shape — as a result of an improperly assembled measuring device called a “null corrector” used to figure the primary mirror — corrective lenses could be prepared to allow the telescope’s instruments to receive focused light.

The repair plan involved two instruments: the Wide Field and Planetary Camera-2 (WF/PC2), which would replace the original Wide Field and Planetary Camera (WF/PC) and feature built-in corrective optics, and the Corrective Optics Space Telescope Axial Replacement (COSTAR) box which would correct the light for three axial instruments. The High Speed Photometer (HSP), which was one of four instruments mounted in boxes just above the telescope’s mirror, would be sacrificed and COSTAR would be installed in its place.

The HSP was the instrument least affected by the spherical aberration, and it could be used to rapidly measure the light from astronomical objects. The other axial instruments, the Faint Object Camera (FOC), the Faint Object Spectrometer, and the Goddard High Resolution Spectrograph (GHRS), would receive corrected light from COSTAR. COSTAR would achieve this with pairs of small corrective mirrors on the end of motorized arms, which would send corrected light to the instrument they were aligned with.

Supernova 1987a’s remnant as imaged by the Hubble Space Telescope in 1990 with the ESA-provided Faint Object Camera. Even with the optical flaw, HST was able to perform science. (Credit: NASA/ESA)

The Hubble Telescope also had pointing issues that needed to be fixed for it to reach its full observing potential. One particular concern was “jitter,” which occurred every time Hubble crossed Earth’s day/night terminator line due to temperature changes. New solar panels with silver insulation covering the guides, which would keep the panels taut after deployment, were commissioned for a 1993 servicing mission.

While the instruments and other replacement items were being built and checked out, Hubble observed storms on Saturn, the remnant of Supernova 1987a, and other targets. A new image processing method was invented that could remove some aberration effects for certain observations, and some Shuttle flights in 1992 and 1993 had EVAs added to them to test tools and techniques needed on the Hubble repair mission.

The STS-61 stack being rolled out for launch. During flight preparations, Endeavour had to be rolled from Pad 39A to 39B due to contamination in 39A’s rotating service structure. (Credit: NASA)

STS-61: the mission to save Hubble

The seventh and final Shuttle mission of 1993 was STS-61, the first Hubble Space Telescope servicing flight. The early 1990s had been a difficult time for NASA, with the Hubble mirror issue being only one of the agency’s many problems. As such, the pressure for STS-61 to deliver and fix Hubble was high.

The space station program — what would ultimately become the International Space Station — had come very close to being canceled by Congress, not once, but twice, as a recession and post-Cold War budget cuts trimmed space spending. The Space Shuttle fleet had suffered from hydrogen leaks delaying flights in the summer of 1990, the same summer when the Hubble mirror issue became known.

Rendering of the Galileo probe orbiting Jupiter with its high gain antenna not deployed correctly. This was one of the setbacks that NASA endured in the early 1990s. (Credit: NASA)

In addition, the Galileo probe to Jupiter, deployed by the Space Shuttle Atlantis, had been unable to extend its high-gain antenna, and its mission goals were threatened. The Mars Observer probe had been lost just before it was to orbit the Red Planet. The agency was under pressure, and many were questioning the value of space exploration.

Space Shuttle Endeavour started its spaceflight career with an improvised and successful rescue of the Intelsat 6 communications satellite during the STS-49 mission. Now, Endeavour and the astronauts of STS-61 would be called upon to rescue the credibility of one of the signature projects in NASA and spaceflight history, to restore the scientific promise of the Hubble Space Telescope.

Crew portrait for STS-61 taken two months before launch. (Credit: NASA)

The STS-61 crew had been named well before the flight, and they were all veterans of the program with prior flight experience. Mission commander Richard Covey had flown three prior Shuttle missions, including the STS-26 return to flight in 1988. Pilot Ken Bowersox was on his second flight, having flown on the STS-50 Spacelab mission aboard Columbia.

Mission Specialist 1 (MS1), Kathryn Thornton, had a pair of flights to her name, including STS-49, where she gained EVA experience. Mission Specialist 2 (MS3), Claude Nicollier, from Switzerland, was an ESA astronaut who had flown on the Shuttle once before, helping to deploy the EURECA and TSS satellites on STS-46.

Astronauts Akers and Thornton – left to right – training to replace the WF/PC with the corrected WF/PC2 at the Johnson Space Center in Houston. All four EVA astronauts were cross-trained on each others’ tasks. (Credit: NASA)

The third mission specialist (MS3), Jeffrey Hoffman, was on his fourth flight. As an astronomer, he was particularly knowledgeable about the Hubble Space Telescope. He also had EVA experience from his first flight in 1985 and was also a mission specialist on the long-delayed ASTRO-1 astronomy Spacelab mission in 1990. Additionally, Hoffman and Nicollier were crew mates on the aforementioned STS-46 mission.

Story Musgrave and Thomas Akers rounded out the seven-person crew. Both men had EVA experience, with Musgrave being on his fifth spaceflight and Akers on his third. Musgrave had been one of two astronauts on STS-6 to conduct the first-ever Shuttle spacewalk, while Akers had flown on STS-49 and had been one of the astronauts involved in the retrieval of Intelsat 6 on the first three-person EVA in history.

STS-61 launched into the pre-dawn darkness on Dec. 2, 1993, at 4:27 AM EST. (Credit: NASA)

On Dec. 2, 1993, at 4:27 AM EST (09:27 UTC), after a 24-hour delay due to out-of-limit weather conditions for a contingency return to launch site abort, Endeavour launched from Launch Complex 39B at the Kennedy Space Center in Florida. The shuttle reached orbit and started a two-day chase to rendezvous with the Hubble Space Telescope.

Endeavour’s robotic arm, operated by Claude Nicollier, grappled the observatory on Dec. 4, which was flight day three for the mission. Astronauts and flight controllers got their first look at the Hubble Telescope since its launch in April 1990, and they found one thing immediately apparent. One of the solar panels had a recognizable kink in it, which would be problematic for retracting the panel and returning it to Earth.

The Hubble Space Telescope just before its capture on flight day three of STS-61. Note the kink in one of the solar panels. (Credit: NASA)

While MS2 Nicollier would be at the controls of the shuttle’s Canadarm for the entire flight, the other mission specialists would alternate their EVA duties. A record five spacewalks were planned for this mission, with the first one coming up on the next flight day, and pairs of astronauts would alternate the EVA activity to allow for sufficient rest.

On Dec. 5, or flight day four, astronauts Musgrave and Hoffman moved into Endeavour’s cargo bay to start the critical repair work. After setting up their tools, Hoffman mounted himself to the shuttle’s Canadarm robotic arm while Story Musgrave was the free-floater. The first order of business was to give Hubble six healthy gyroscopes, which they did by replacing a pair of rate sensing units, each one with two gyroscopes.

Astronaut Jeff Hoffman working on gyroscope installation during EVA 1 of STS-61. Note the “dings” — small space debris hits — on the exterior. (Credit: NASA)

Musgrave and Hoffman also replaced a pair of electrical control units that controlled two of the three rate sensing units, changed eight fuse plugs, and installed protective covers on various parts of the observatory. Their only real issue was with the latches on the gyro door, and they eventually solved this. After seven hours and 50 minutes outside, the astronauts returned to the airlock.

Flight day five, on Dec. 6, was designated for another high-priority task: the replacement of the solar panels. The resolution of the observatory’s pointing issues was as critical to fulfilling the observatory’s potential as fixing the optical system. The gyroscope replacement was part of this task, and it was hoped that the new solar panels would fix the “jitter” issue.

The damaged solar panel after it was jettisoned by astronaut Kathryn Thornton on the Canadarm. (Credit: NASA)

Astronauts Thornton and Akers conducted the second spacewalk of STS-61. Kathryn Thornton mounted on the Canadarm while Tom Akers was the free-floating astronaut. One of the solar arrays did not retract properly due to the kink near the bottom solar panel, so the astronauts detached the damaged array before Thornton released the array overboard, allowing the array to float freely away into space.

One of the new arrays was successfully installed in place of the damaged one, after which the telescope was rotated 180 degrees on its fitting in the orbiter’s payload bay. The other array was removed and mounted to a pallet in the payload bay, and the other new array was successfully added. Thornton and Akers returned to the airlock after six and a half hours.

Astronaut Story Musgrave during EVA 3, before the astronauts installed the corrected WF/PC2. Musgrave is in the white suit, Jeff Hoffman used solid red stripes. (Credit: NASA)

Once the solar arrays and gyroscopes were changed out and tests on the gyros were passed successfully, the repairs to the optical system could begin on Dec. 7, which was Flight day six. Astronauts Musgrave and Hoffman teamed up to replace the telescope’s original WF/PC camera with the optically corrected WF/PC2, which passed its aliveness test shortly thereafter.

The process took a lot less time than was envisaged before the flight – 40 minutes instead of four hours – and was successfully completed. Hoffman, being on the Canadarm, changed a pair of magnetometers on the top of the telescope near the aperture door to finish out the EVA. They got back to the airlock after six hours and 47 minutes. After the spacewalk, and before the next one, Pilot Bowersox boosted Endeavour and Hubble from the initial 594 by 587-kilometer orbit to a 596 by 594-kilometer circular orbit.

Astronauts Tom Akers and Kathryn Thornton working on the COSTAR install on EVA 4. Akers is in the suit with the diagonal stripes on the left while Thornton used broken red stripes. (Credit: NASA)

Flight day seven, on Dec. 8, was the day the axial instruments on Hubble would get their corrective optics. Astronauts Thornton and Akers opened the doors to the telescope’s aft shroud, removed the powered-down HSP box, and then practiced with it to help them with the COSTAR installation. The astronauts installed COSTAR successfully, closed the equipment bay doors, and added a co-processor and additional memory to upgrade the telescope’s onboard computer. The EVA lasted six hours and 50 minutes.

One final spacewalk remained before the Hubble Space Telescope could be redeployed. On Dec. 9, which was Flight day eight, Astronauts Musgrave and Hoffman entered the shuttle’s payload bay and replaced the solar array drive unit on the telescope.

Space Shuttle Endeavour and the payload bay taken from near the top of the Hubble Space Telescope. (Credit: NASA)

Ground controllers commanded Hubble to extend its new solar arrays to deploy position, but the arms failed to extend, so the astronauts had to crank the deployment mechanism by hand manually. The new solar arrays were successfully deployed afterward, and the astronauts fitted an electrical connection to the GHRS.

The last repair task for the mission was to place a pair of covers fashioned on orbit to the new magnetometers at the top of the telescope to prevent debris shedding, which had happened on flight day six when Jeff Hoffman replaced the original pair of magnetometers, which could have been degraded by ultraviolet exposure.

The newly repaired Hubble Space Telescope shown before its redeployment on STS-61. Note the new solar arrays with the insulation on the deployment guides. (Credit: NASA)

The fifth and final EVA ended after seven hours and 21 minutes, bringing the total spacewalk time for STS-61 to 35 hours and 28 minutes, a record for a shuttle mission. The telescope’s high-gain antenna was deployed, and the crew prepared to release the newly repaired telescope.

Hubble was redeployed on flight day nine after some troubleshooting of a data interface unit on the observatory. The aperture door was opened, and the telescope was deployed afterward at 5:26 AM EDT (10:26 UTC) on Dec. 10, 1993. Endeavour landed safely on Runway 33 at the Kennedy Space Center at 12:26 AM EST (05:26 UTC) on Dec. 13.

Before and after: M100 imaged on Nov. 27, 1993 to the left, and on Dec. 31, 1993 to the right. (Credit: NASA)

Hubble’s new lease on life

Hubble’s first public imagery since the repair mission showed the public just how far the observatory had come in a very short time. The spherical aberration and the telescope’s “jitter” during terminator crossings were completely fixed. The telescope’s images of the galaxy M100, taken before and after the repair, showed that Hubble could now fulfill its full potential as the observatory that would be the biggest advancement in astronomy since Galileo invented the telescope.

Over the following years and decades, the Hubble telescope has made many discoveries that have completely transformed our understanding of the Universe. Among other things, Hubble proved the existence of black holes and discovered that the universe was expanding and speeding up, discovering “dark energy” in the process.

Hubble also imaged Comet Shoemaker-Levy 9’s impacts on Jupiter, possible plumes of water from Jupiter’s moon Europa, storms on Saturn, and the ice giants Uranus and Neptune. Furthermore, Hubble discovered Kuiper Belt objects, imaged the aftermath of an asteroid collision, and the impact of the DART spacecraft on the asteroid moon Dimorphos.

Image of debris falling from the asteroid moon Dimorphos after it was impacted by DART. Taken by the Hubble Telescope in December 2022. (Credit: NASA, ESA, and David Jewitt/UCLA)

In addition to the telescope’s observations of our solar system, Hubble also made the first discovery of an atmosphere around an exoplanet, an impressive feat given that exoplanets had not yet been directly observed when Hubble was being constructed. Hubble has also made deep field images that showed galaxies only 500 million years after the Big Bang — at the very limit of what is observable in visible light.

What’s more, Hubble has discovered that galaxies are surrounded by invisible “dark matter,” and follow-on telescopes, such as the recently launched Euclid and the Nancy Grace Roman Telescope, will work to understand dark matter and dark energy further. This is only a very partial list of the discoveries the Hubble Space Telescope has made since its first repair mission thirty years ago.

The Hubble Ultra Deep Field, taken in 2003-2004. This image shows objects at the very limit of visible light detection. (Credit: NASA/ESA/S. Beckwith/STScI/HUDF Team)

STS-61 was only the first of five Space Shuttle missions to repair and upgrade the Hubble Space Telescope. Today, the Hubble Telescope has solar arrays that are much smaller than the original arrays yet generate 20 percent more power, along with new instruments that are much more advanced than the instruments originally flown aboard the veteran observatory.

As part of the observatory’s continuing upgrades, old equipment — including the COSTAR — was brought back to Earth. COSTAR was replaced and returned to Earth in 2009 as newer axial instruments featured their own corrective optics. Hubble’s current complement of instruments includes the Wide Field Camera 3, the Cosmic Origins Spectrograph, the Advanced Camera for Surveys, the Space Telescope Imaging Spectrograph, and the Near Infrared Camera and Multi-Object Spectrograph.

New methods of operating Hubble have been devised since its launch, such as a “two-gyro” mode that allows for observations without using the normally required three gyros. During STS-125, the last Shuttle servicing mission in 2009, Hubble received a soft docking ring fitted to its aft structure just underneath the aft shroud for a future deorbit mission.

Artist’s rendering of a Crew Dragon performing a reboost maneuver for the Hubble Space Telescope. (Credit: Mack Crawford for NSF/L2)

The future of Hubble

The Hubble Space Telescope has operated well since its final servicing mission in 2009. Although the much larger James Webb Space Telescope has dominated astronomy headlines since its launch and deployment in 2022, Hubble is still performing critical and ground-breaking observations, sometimes in coordination with Webb.

Recently, a proposal for a new servicing and reboost mission to the veteran space telescope was revealed. Jared Isaacman and the Polaris Program proposed a mission that would see a SpaceX Crew Dragon dock to the aft end of Hubble and its LIDS soft docking ring that was installed in 2009. Astronauts could then reboost the telescope and also replace flight-critical units, keeping Hubble operational well into the 21st century.

The Hubble Space Telescope after the final Shuttle servicing mission in May 2009. Notice the LIDS docking ring on the aft shroud. (Credit: NASA)

During a recent interview with NSF’s Jack Beyer, Isaacman laid out the case for a new servicing and reboost mission to Hubble. “Hubble is not healthy, even though it’s doing amazing science alongside, especially alongside James Webb. The growth of Ph.D. papers published that draw on Hubble as the scientific instrument has been exponential. So, it’s not winding down; it’s the exact opposite. And that’s because you can pair it with other instruments, again, like James Webb, but it’s not healthy.”

Isaacman further stated while discussing the risk-reward of a mission to Hubble, “I mean, if you’ve got one healthy computer, I recall, I think you’re at two and a half healthy gyros. If you lose that other half of the one that’s quasi; you lose 25% of the observables.“

This week, there was a timely reminder that it has been 14 years since the last Hubble servicing mission. Gyroscope issues have forced the veteran telescope into safe mode three times in the past few days, and NASA is working to recover operational capability. Hubble can now operate and observe with one gyro if necessary, which was not imaginable when the telescope was first launched.

The Polaris 2 mission will feature EVA activity in any case, and it is likely the mission would be the one that would go up to the Hubble Telescope if NASA decides to pursue this mission. A reboost and the replacement of key items would allow the telescope to operate for 10-15 more years.

(Lead image: The Hubble Space Telescope as it was configured from 1993 to 2002. Taken from Space Shuttle Columbia in March 2002. Credit: NASA/ESA)

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