After more than three decades of revolutionary cosmic discovery, the Hubble Space Telescope is getting a new spin on its operations—and entering what may be the twilight of its epochal mission.
Hubble is still in good health as it orbits at 17,000 miles per hour some 320 miles above Earth, NASA says, but the observatory has been in “safe mode” since May 24. This is only the latest in an increasingly regular series of safe-mode events that have suspended the telescope’s science operations. Most of these recent disruptions have been linked to faulty readings from a glitchy gyroscope, or gyro, a device that uses the angular momentum of a fast-spinning wheel to measure the speed and motion of Hubble’s turns as it tracks targets across the sky. Ground controllers have repeatedly reset the troublesome gyro, only for the erroneous readings to soon reappear.
Without gyroscopic guidance, the telescope would almost be flying blind, unable to accurately lock onto most celestial objects. And without Hubble—which even now boasts the sharpest space-based view of the heavens in visible and ultraviolet light—a crucial window on the universe would close. Hubble remains heavily “oversubscribed”—the term astronomers use when research proposals for a telescope outnumber its limited available observing time. And it offers potent synergies with new and upcoming powerful space telescopes for studying exoplanets, exploding stars and distant galaxies.
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Hubble carries six gyros, all installed in 2009 during the last space shuttle mission to service the telescope. Including the problematic gyro, only three of the six remain active. Normally all of these three gyros work in tandem to efficiently and precisely steer and point the telescope.
But a new “normal” is now at hand. During a press conference on June 4, NASA officials announced that the space agency is transitioning Hubble to “one-gyro mode,” a contingency plan for continuing science operations despite the telescope’s dwindling number of usable gyros. The transition will leave Hubble with a single fully functional gyro held in reserve and will require the reconfiguration both the spacecraft and ground control, as well as revised plans for all scientific observations. But the telescope should be back in action by mid-June, officials say.
“Operationally, we believe this is our best approach to support Hubble science through this decade and into the next since most of the observations it takes will be completely unaffected by this change,” said Mark Clampin, director of NASA’s astrophysics division, at the press conference. “One-gyro mode actually returns Hubble to consistent science operations, which is important, and it allows us to keep another gyro working, although we will maintain it powered up in reserve for potential future use.”
Devised more than 20 years ago in the aftermath of the space shuttle Columbia disaster that paused plans for future Hubble servicing missions, the one-gyro mode involves a multistep process that relies on other onboard instruments to substitute for the telescope’s failed gyros. The first step uses Hubble’s magnetometers and sun sensors together with one gyro to determine the spacecraft’s orientation with respect to Earth and the sun. This allows Hubble to point within 10 degrees of a target, after which the observatory’s star trackers and then its fine guidance sensors each perform adjustments, ultimately sharpening its on-target aim to within 150 milliarcseconds.
NASA says this approaches the accuracy achieved with the normal three-gyro mode, which is akin to pointing a laser beam at a dime more than 200 miles away—and, for some of Hubble’s most delicate observations, holding it there for up to 24 hours at a time. The telescope has, in fact, relied on a version of the one-gyro mode for all its science observations since 2021, using three gyros for slewing to a target, then stabilizing its view via a single gyro and a fine guidance sensor once data collection begins.
“We still believe there’s a very high reliability and likelihood that we can operate Hubble very successfully doing groundbreaking science through the rest of the [2020s] and into the 2030s,” said Patrick Crouse, Hubble’s project manager at NASA’s Goddard Space Flight Center, at yesterday’s press event. “And we still come to [the] conclusion that [there’s a] greater than 70 percent probability of operating at least one gyro through 2035…. So we do not see Hubble as being on its last legs. And we do think it’s a very capable observatory. And we’re poised to do exciting science with the other observatories on orbit and those that will join us on orbit.”
Yet the one-gyro mode will still place new limits on Hubble’s performance. The observatory will now require more time to seek out any given target, making it more sluggish and reducing the efficiency of scheduling its observations. This sluggishness also means Hubble will now slew too slowly to track objects interior to the orbit of Mars, although Crouse noted such observations historically account for less than 1 percent of the telescope’s work. The change will also significantly decrease the area of the sky accessible to Hubble at any given time, hindering its ability to study supernovas and other short-lived astrophysical outbursts that demand rapid response.
“One-gyro mode reduces the area of sky that Hubble can observe by approximately half, but that’s at any one time, and the entire sky comes into view over the course of the year,” says Tom Brown, Hubble’s mission head at the Space Telescope Science Institute (STScI).
“Basically, if you’re an astronomer hoping to use Hubble to study transient phenomena, a shift to one-gyro mode might make you a little bit nervous,” says Edward Cheng, a former Hubble project scientist at NASA, who is now retired. “But most other observations, I don’t think, will be greatly affected. My personal sense is that Hubble will still be 100 percent subscribed after this change.”
Citing estimates from a 2016 study, at the press conference Crouse acknowledged that the transition to one-gyro mode could decrease Hubble’s overall scientific productivity by up to 25 percent.
That reduction, paired with the trending uptick in safe-mode events and the gradual decay of Hubble’s orbit—which is projected to pull the telescope into a fiery atmospheric reentry sometime in the mid-late 2030s—has rekindled calls from some corners for NASA to mount another crewed servicing mission to once again rescue its most iconic observatory.
That intervention would require at minimum a rendezvous with Hubble to reboost it to a higher altitude, but could also entail extravehicular activity by a crew to install new gyros or star trackers. Without the benefit of a space shuttle, this time the process would rely on a different vehicle, such as SpaceX’s Crew Dragon spacecraft, which remains untested as a support vehicle for spacewalking astronauts. In collaboration with SpaceX, billionaire and private astronaut Jared Isaacman has proposed such a mission to NASA and has even offered to essentially fund it himself. But the space agency’s reception has been chilly because of concerns for Hubble’s science, as well as the safety of any crew.
During the press conference, Clampin made it clear that NASA’s reticence remains unchanged. “After exploring the current commercial capabilities, we are not going to pursue a reboost right now,” he said. “While the reboost is an option for the future, we believe we need to do some additional work to determine whether the long-term science return will outweigh the short-term science risk.... For the time being, we are stepping back.”