Uh oh. Hubble's Having Gyro Problems Again

The Hubble Space Telescope has gone through its share of gyroscopes in its 34-year history in space. Astronauts replaced the gyros during the last servicing mission in 2009, bringing it back up to six (three with three spares), but they only last so long. Last week, HST went into safe mode because one of the gyros experienced fluctuations in power. NASA paused the telescope’s science operations today to investigate the fluctuations and perhaps come up with a fix.

With this one gyro experiencing problems, only two of the gyros remain fully operational. HST works best with three gyros, and so engineers are working to understand the issue and hopefully figure out a way to fix it remotely. However, several years ago, engineers figured out a way to still conduct science operations with only a single gyro.

HST entered safe mode on April 23, 2024 when the one gyro sent faulty readings. This particular gyro also caused Hubble to enter safe mode last November after returning similar faulty readings. The gyroscopes are part of Hubble’s Pointing Control System, which includes three Fine Guidance Sensors, reaction wheels and the gyros. This allows Hubble to track stars with incredible accuracy, helping the telescope find its way as it scans the heavens, as well as keep Hubble locked onto to its targets.

To work correctly, Hubble must be able to stay focused on a target without deviating more than 7/1000th of an arcsecond, or about the width of a human hair seen at a distance of a mile.

Hubble team created a contingency plan in preparation for a time when the spacecraft might find itself with less than three working gyros again. The team developed a two-gyro mode that substitutes other sensors for one missing gyro. Although less efficient, two-gyro mode allows Hubble to continue collecting ground-breaking science data.

The end of a Hubble gyro reveals the hair-thin wires known as flex leads. They carry data and electricity inside the gyro. Credit: NASA

NASA said that Hubble gyros fail over time, usually because of “wear and tear” of thin (less than the width of a human hair), metal wires, called flex leads that carry power in, and data out, of the mechanism. Hubble’s flex leads pass through a thick fluid inside the gyro. Over time, the flex leads begin to corrode and can physically bend or break.

During its 34-year history, Hubble has had eight out of 22 gyros fail due to a corroded flex lead. For example, in 1999, four out of six gyros had failed, with the last one failing about a month before a servicing mission was scheduled to replace them (and do other upgrades to the telescope). This meant Hubble sat in safe mode waiting for the space shuttle and astronauts to arrive.

Engineers developed a two-gyro mode when the final planned Hubble servicing mission was (temporarily) canceled following the space shuttle Columbia disaster. The mission was reinstated after outcry from scientists and the public, and so NASA figured out a way to mitigate the risks of flying the space shuttle. Servicing Mission 4 replaced all six gyros one last time in 2009.

With his feet firmly anchored on the shuttle’s robotic arm, astronaut Mike Good maneuvers to retrieve the tool caddy required to repair the Space Telescope Imaging Spectrograph during the final Hubble servicing mission in May 2009. Periodic upgrades have kept the telescope equipped with state-of-the-art instruments, which have given astronomers increasingly better views of the cosmos. Credits: NASA

However, during the time it was thought no future servicing mission would happen, the observatory was proactively put into two-gyro mode to prolong its life. During this time, the team also devised a one-gyro mode, which could further extend Hubble’s life if needed.

“We knew gyros would be a limiting factor so we started to working on a reduced gyro mode to extend their life,” the director of the Space Telescope Science Institute Ken Sembach told me back in 2015 for my book, “Incredible Stories From Space.” “As it turned out, we did need that reduced gyro mode, and now they aren’t [as big of a] limiting factor for Hubble because we now know how to use the gyro resources in a new way. That added a longer life to the mission we didn’t think we would have.”

While the difference between two-gyro mode and one gyro-mode is negligible, one-gyro mode provides the option to have one of the remaining gyros placed in reserve. As of now, three of the six gyros onboard Hubble have had a flex lead fail and are no longer functional. NASA has not announced if the faulty readings are due to flex lead fail or another issue. If this gyro fails, the team will invoke one-gyro mode.

NASA did say that all of the science instruments are in good shape and they anticipate Hubble will “continue making groundbreaking discoveries, working with other observatories throughout this decade and possibly into the next.”

Hubble launched in 1990, and recently celebrated its 34th anniversary. While everyone expected HST would revolutionize astronomy, I don’t think anyone expected it would continue to be such a productive, world-class observatory even more than a thirty years after it launched. But, please, let’s keep it going for as long as possible!

Nancy Atkinson

Nancy has been with Universe Today since 2004, and has published over 6,000 articles on space exploration, astronomy, science and technology. She is the author of two books: "Eight Years to the Moon: the History of the Apollo Missions," (2019) which shares the stories of 60 engineers and scientists who worked behind the scenes to make landing on the Moon possible; and "Incredible Stories from Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos" (2016) tells the stories of those who work on NASA's robotic missions to explore the Solar System and beyond. Follow Nancy on Twitter at and and Instagram at and

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