Hubble's Back, but Only Using One Gyro

The Hubble Space Telescope has experienced ongoing problems with one of its three remaining gyroscopes, so NASA has decided to shift the telescope into single gyro mode. While the venerable space telescope has now returned to daily science operations, single gyro mode means Hubble will only use one gyro to maintain a lock on its target. This will slow its slew time and decrease some of its scientific output. But this plan increases the overall lifetime of the 34-year-old telescope, keeping one gyro in reserve. NASA is also troubleshooting the malfunctioning gyro, hoping to return it online.

Last week, NASA said that the telescope and its instruments are stable and functioning normally.

Gyroscopes help the telescope orient itself in space, keeping it stable to precisely point at astronomical targets in the distant Universe. Hubble went into safe mode back in November 2023, and then again in April and May 2024 due to the ongoing issue, where the one gyro had been increasingly returning faulty readings.

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

Going in to safe mode suspends science operations, and in the meantime, engineers tried to troubleshoot to figure out why the gyro experiencing the fault-producing issues and doing work-arounds to get the telescope up and running again. The most recent last safe-mode event in May led the Hubble team to transition from a three-gyro operating mode to observing with only one gyro. This enables more consistent science observations while keeping the other operational gyro available for future use.

Launched in 1990, Hubble has more than doubled its expected design lifetime, providing stunning images and scientific discoveries that have changed our understanding of the Universe and re-written astronomy textbooks.  

During its 34-year history, Hubble has had eight out of 22 gyros fail due to a corroded flex lead, which are thin (less than the width of a human hair) metal wires, that carry power in, and data out, of the gyro.  The flex leads pass through a thick fluid inside the gyro and over time, the flex leads begin to corrode and can physically bend or break.

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

Thankfully, for the first 18 years of Hubble’s life in space, the telescope had the advantage of being able to be serviced and upgraded by space shuttle astronauts. 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.

When the final planned Hubble servicing mission was (temporarily) canceled following the space shuttle Columbia disaster, engineers developed and inaugurated a two-gyro mode to prolong Hubble’s life. 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, but it has been running on three since 2018. The three gyros all quit working due to flex lead failures. The retirement of the space shuttle means Hubble has now been operating for 15 years without servicing.

The Hubble Rate Gyro Assembly contains a gyroscope and all of its associated electronics. The gyroscopes are part of Hubble’s pointing system. They provide a frame of reference for Hubble to determine where it is pointing and how that pointing changes as the telescope moves across the sky. They report any small movements of the spacecraft to Hubble’s pointing and Control System. The computer then commands the spinning reaction wheels to keep the spacecraft stable or moving at the desired rate. Credit: NASA

However, during the time it was thought no future servicing mission would happen, the team also devised a one-gyro mode, which will further extend Hubble’s life.

“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 engineers say 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.

NASA says that although one-gyro mode is an excellent way to keep Hubble science operations going, it does have limitations, which include a small decrease in efficiency (roughly 12 percent) due to the added time required to slew and lock the telescope onto a science target. One gyro mode also means it takes additional time for the telescope’s fine guidance sensors to search for the guide stars. Additionally, in one-gyro mode Hubble has some restrictions on the science it can do. For example, Hubble cannot track moving objects that are closer to Earth than the orbit of Mars. Without the full complement of gyros, the motion of these objects are too fast for the telescope to track. Additionally, the reduced area of sky that Hubble can point to at any given time also reduces its flexibility to see transient events or targets of opportunity like an exploding star or an impact on Jupiter. NASA says that when combined, “these factors may yield a decrease in productivity of roughly 20 to 25 percent from the typical observing program conducted in the past using all three gyros.”

Read more about the “new normal” for Hubble’s one-gyro mode at this NASA webpage.

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