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The Hubble Space Telescope at the end of the final shuttle servicing mission to it in May 2009. (credit: NASA)

Hubble limps along

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For months, one of the three remaining working gyroscopes on the Hubble Space Telescope, designated Gyro 3, has been malfunctioning. A problem with the gyro would trigger a safe mode, taking the telescope offline for days while engineers worked to get the gyro working again, allowing observations to resume.

“We do not see Hubble as being on its last legs,” Crouse concluded.

“Gyro 3, to be frank, has always performed a little bit out-of-family on orbit,” said Patrick Crouse, project manager for Hubble at NASA’s Goddard Space Flight Center, during a media telecon last week. “It’s been ongoing work since 2018 on the operations team to learn to live with this gyro and make the best of it.”

However, it’s no longer possible to live with Gyro 3. On May 24, that gyro malfunctioned again, putting Hubble into another safe mode. A week and a half later, NASA announced that it concluded that, unlike previous malfunctions, the gyro could not be brought back into service.

“After completing a series of tests and carefully considering our options, we have made the decision that we will transition Hubble to operate using only one of its three remaining gyros,” Mark Clampin, director of NASA’s astrophysics division, said on the call.

NASA decided to implement a plan created years ago for just such a scenario, as the telescope’s aging gyros—installed on the final shuttle servicing mission 15 years ago—began to fail. Once Hubble fell below the three gyros needed for normal operations, engineers would switch the spacecraft to operate with just a single gyro, putting the other into reserve.

Doing so is intended to extend the life of Hubble: rather than run both gyros simultaneously, one can be turned off until the other fails, conserving its lifetime. The two remaining gyros are both “enhanced” models with lifetimes five times longer than the standard gyro’s 50,000-year lifetime. One has 142,000 hours and the other nearly 90,000 hours, and neither has experienced issues like those that plagued Gyro 3.

That gives NASA confidence that Hubble, launched in 1990, can continue to operate for perhaps another decade. Crouse said there was at least a70% chance that at least one of the remaining two gyros should still be working in the mid-2030s. Other Hubble systems, including its instruments, are also working well. “We do not see Hubble as being on its last legs,” he concluded.

Switching to the single-gyro mode, though, has drawbacks, primarily because Hubble cannot shift from one target to another as quickly or precisely as before. Crouse said that while Hubble can perform observations an average of 85 orbits per week in normal operations, that will be reduced to 74 orbits a week in single-gyro mode, a 12% loss of observing time.

“We don’t want stop using Hubble and we don’t want to stop using Chandra,” Clampin said. “We want to find a cheaper way to do it.”

The single-gyro mode also means a larger portion of the sky will no longer be observable at a given time because of a bigger exclusion zone to avoid accidently observing the Sun. That will primarily affect “time-domain” astronomy where observations of a particular object at a specific time are needed; NASA noted that the whole sky will still be visible over the course of the year.

The single-gyro mode also prevents observations of objects close to the Earth that require better tracking. Crouse said Hubble will no longer be able to observe objects closer than Mars, like the Moon and some near Earth asteroids, but noted such observations account for only about one percent of recent Hubble investigations.

Astronomers have been dreading the inevitable switch to single-gyro observations because of those restrictions. A 2016 study by a team at the Space Telescope Science Institute, which handles Hubble science operations, estimated a 25 percent reduction in “scientific productivity” of Hubble should it operate with one or two gyros, although the paper didn't elaborate on how such productivity was defined.

Clampin, asked about the study in the call, said he was not familiar that analysis or its conclusion, but acknowledged there would be scientific impacts. “Even with those limitations, Hubble is still doing a broad range of science observations, so I don’t personally see this as a major restriction on its ability to do important science,” he argued.

The gyro problem comes at an awkward time for NASA and Hubble. In March, NASA announced it would work to reduce the costs of operating both Hubble and the Chandra X-Ray Observatory, the other of the original “Great Observatories” space telescopes still in service. NASA cited for Chandra in particular technical problems with the aging observatory, although astronomers took issue with those claims (see “A space telescope’s cloudy future,” The Space Review, April 1, 2024.)

NASA established a committee formally called the Operations Paradigm Change Review to look at ways to reduce the operating costs of both missions, possibly by turning off instruments or other reductions in observing capabilities. Clampin, for example, said in March that Hubble’s near-infrared observing capability may no longer be valuable now that the much larger James Webb Space Telescope is operating in the same wavelengths.

The change in Hubble observations, and concomitant reduction in scientific productivity, comes as that review is nearing completion. Clampin said in the media telecon that the switch to single-gyro observations would not affect the outcome of that review.

The limitations in Hubble observations caused by the change in observing modes “are not what I would call serious impediments to continuing science operations,” he said, and thus would not affect any major conclusions of the review.

Speaking June 10 at a NASA town hall session held during a meeting of the American Astronomical Society (AAS), Clampin said NASA expected to receive a final report from the review soon. “In due course, we will make an announcement about how we plan to proceed forward,” he said, which will include a town hall with astronomers to discuss any changes to the two space telescopes.

That review included the option of shutting down either or both telescopes, although he said that was something of a last-ditch option. “We don’t want stop using Hubble and we don’t want to stop using Chandra,” he said later in the town hall. “We want to find a cheaper way to do it, but we wanted to make sure they were looking at all of the options that were possible.”

Jared Isaacman wearing an EVA spacesuit he plans to test on the upcoming Polaris Dawn mission. He has pushed for a Hubble servicing mission that could be carried out on the second Polaris program flight. (credit: SpaceX)

Reboosting or repairing Hubble

The series of problems in recent months with Gyro 3 prompted calls for a private servicing mission to Hubble. Those stemmed from a study NASA announced in September 2022 it was undertaking with SpaceX to examine how a Crew Dragon spacecraft could be used to raise Hubble’s orbit or even service it (see “NASA-SpaceX study opens final chapter for Hubble Space Telescope,” The Space Review, October 3, 2022).

That study was completed more than a year ago, but neither NASA nor SpaceX provided any details about what the study found or whether NASA would pursue any kind of commercial servicing mission. The agency also solicited other concepts for a commercial reboost mission, receiving eight responses to a request for information (RFI) early last year.

“It is not like anyone was going to wing it, especially after a joint study was assembled to determine generally how a successful mission could be achieved,” Isaacman said of a Crew Dragon servicing mission.

As NASA’s review of that study and the responses to the RFI dragged on, and Hubble went in and out of safe mode because gyro problems, one key individual became impatient. Jared Isaacman, the billionaire who led the Inspiration4 private astronaut mission in 2021 and later established the Polaris program of private missions, was involved in the NASA/SpaceX study. He envisioned it as the second of three missions Polaris planned, after a Crew Dragon mission now scheduled to launch this summer to test the ability to perform spacewalks from the spacecraft and before a crewed Starship test flight.

In a tweet in November, he said going forward with a Crew Dragon reboost and/or servicing missions “should be an easy risk/reward decision.” The benefits of a “free” (to NASA) mission included extending the life of Hubble and demonstrating commercial space capabilities, he argued. He didn’t, though, elaborate on the risks or how such a mission would work.

In May, National Public Radio reported on emails, obtained under a Freedom of Information Act request, at NASA about the study. The agency had asked experts, including former astronauts who flew on shuttle servicing missions to Hubble, to review the report. Some raised concerns about the spacewalks required to do any repairs, given that SpaceX has yet to demonstrate the ability to do spacewalks from Crew Dragon. That spacecraft also lacks a robotic arm that was essential for the shuttle servicing missions, they noted.

One of the people consulted by NASA was Keith Kalinowski, a retired Hubble operations expert, who concluded in one of the released emails that while a reboost mission might be feasible, a commercial spacewalk to repair Hubble was “unnecessary and risky.”

Isaacman weighed in on the NPR article on X, arguing again NASA was missing an opportunity not to pursue a servicing mission. “It is not like anyone was going to wing it, especially after a joint study was assembled to determine generally how a successful mission could be achieved,” he wrote.

But he also suggested that the opportunity had passed. “Had a mission been flown, and I was happy to fund it, I believe it would have resulted in the development of capabilities beneficial to the future of commercial space and along the way given Hubble a new lease on life.”

At last week’s briefing about Hubble’s switch to single-gyro operations, Clampin said that NASA would not pursue the SpaceX/Polaris servicing proposal or other commercial options, at least for now. “Our position right now is that, after exploring the current commercial capabilities, we are not going to pursue a reboost right now,” he said.

“It was not a go/no-go kind of study,” Clampin said of the NASA/SpaceX study. “There was never a recommendation to go forward.”

For NASA, the risks of a reboost or servicing mission outweighed the rewards. “Our assessment also raised a number of considerations, including potential risks such as premature loss of science and some technology challenges,” he said. “We believe we need to do some additional work to determine whether the long-term science returns will outweigh the short-term science risks.”

Clampin also offered a different characterization of the study’s conclusions, particularly about servicing. Isaacman, in his tweet last month, said that the study “arrived at a formal recommendation” on a commercial servicing mission.

“It was not a go/no-go kind of study,” Clampin said. “It was a feasibility study to help us understand some of the issues and challenges we might have to face.” He noted options for servicing Hubble, like adding new gyros installed on its exterior, were included in the study but “really just notional concepts” not explored in any serious level of technical detail.

“There was never a recommendation to go forward,” he added.

Unfortunately, neither NASA nor SpaceX have released the final report from that study, or even a summary of it, to allow people to judge these conflicting claims, even though language in the agreement suggests that such a summary is allowed. “NASA may publish unclassified and non-Proprietary Data resulting from work performed under this Agreement,” the text of the Space Act Agreement between NASA and SpaceX for this study states. “The Parties will coordinate publication of results allowing a reasonable time to review and comment.”

Clampin, in both the call with reporters and the AAS town hall meeting, left the door open to revisiting the idea of a commercial reboost or servicing mission in the future. “It was clear after doing the study that there are a lot of things we needed to think about,” he said at the town hall. “It’s not that we didn’t want to do it, but we thought it needed a lot more development work before we could proceed in what we could consider a safe way, both for the spacecraft, the telescope, and the people inside the spacecraft.”

“I don’t think there was a big disagreement amongst any of the parties involved in the study that that was needed,” he concluded.

He acknowledged, though, that some kind of mission to change Hubble’s orbit will be needed by the mid-2030s, when Hubble currently projected to deorbit. “The general feeling is that we either need to bring it in with a controlled reentry or reboost it to a higher orbit so it can continue to operate,” he said. “We have to do something then, no matter what.”

For now, though, Hubble will continue to operate—a little less efficiently, like so many of us as we age—and offer a window on the universe for astronomers, as engineers contemplate what might be possible to make it last even longer.

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