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Astronomers are concerned proposed budget cuts for the Chandra X-Ray Observatory could lead to its cancellation. (credit: NASA)

A space telescope’s cloudy future

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NASA has a long-standing process for evaluating whether to continue science missions. About every three years, each of NASA’s science divisions conducts a “senior review” of missions that have reached the end of their prime mission but are still operating. The reviews are intended to examine the performance of the missions and the science they are conducting to determine if NASA should keep funding their operations and what changes may be needed, such as efficiencies that can reduce their costs.

“Both missions are operating at extremely high efficiency, and although they are increasingly showing signs of age, both are likely to continue to generate world-class science throughout the next half decade,” the 2022 senior review said of Chandra and Hubble.

That process generally works well, and most missions still in good condition get approved for continued operation by their senior reviews, sometimes with some changes. There are exceptions, though. Last year, the New Horizons team protested a decision by NASA’s planetary science senior review to transfer the mission to heliophysics after 2024, arguing that it would rule out any chance for the spacecraft to perform another flyby of a Kuiper Belt object—if one is found within range of the spacecraft—and potentially lead to the disbanding of its current science team. NASA backtracked and agreed to keep the mission going potentially through the end of the decade, primarily doing heliophysics research but keeping the option open for another Kuiper Belt object flyby.

The senior review model is being tested again with two of its biggest space telescopes. In the most recent senior review of astrophysics missions in 2022, NASA effectively exempted the Chandra X-Ray Observatory and Hubble Space Telescope. Rather than reviewing whether the spacecraft, launched in the 1990s, should continue operations, the senior review convened separate panels to look for efficiencies and other improvements in each telescope.

The final report placed Chandra and Hubble in the first of four tiers of missions considered in the senior review, citing their “immense, broad impact” on the field and potential for joint science with the James Webb Space Telescope. “Both missions are operating at extremely high efficiency, and although they are increasingly showing signs of age, both are likely to continue to generate world-class science throughout the next half decade, operating in concert with JWST as it begins its flagship role,” the June 2022 report stated.

However, by last fall, NASA said it was reconsidering the level of support for both missions, citing broader budget pressures facing the agency in general and astrophysics in particular. “We’re working with the expectation that FY24 budgets stay at the ’23 levels,” said Mark Clampin, director of NASA’s astrophysics division, at an October meeting of the National Academies’ Committee on Astronomy and Astrophysics. “That means that we have decided to reduce the budget for missions in extended operations, and that is Chandra and Hubble.”

That expectation came true last month, when Congress finally completed fiscal year 2024 spending bills, more than five months after the year started. NASA astrophysics received $1.53 billion, compared to $1.51 billion: a small cut in buying power when accounting for inflation.

At that October meeting, and other presentations in subsequent months, NASA did not disclose the magnitude of those potential reductions in the two missions and what they would mean for operations. Clampin said at the time NASA would hold “mini senior reviews” for the missions after the release of the fiscal year 2025 budget proposal to offer recommendations for dealing with those spending reductions.

NASA published its fiscal year 2025 budget request March 11, seeking nearly $25.4 billion for the agency overall—the same as it received in 2023—and nearly $1.58 billion for astrophysics. The proposal also provided the first glimpse of what cuts it envisioned for Chandra and Hubble.

For Hubble, the proposed cut is modest: from $98.3 million allocated in the 2024 spending bill to $88.9 million. NASA suggested in the budget proposal that the cut could be accommodated through efficiencies from the joint operations of Hubble and JWST and through a reduction in grants and fellowships.

“The loss of Chandra would be an extinction-level-event for X-ray Astronomy in the US, a Nobel-prize winning field that began in the US,” the site states.

The cuts to Chandra, though, are steeper. That mission received $68.3 million in fiscal year 2023 and NASA requested a similar amount in 2024. The proposal, though, seeks $41.1 million for Chandra in 2025, a 40% cut from 2023, falling to $26.6 million annually from 2026 through 2028 before dropping to just $5.2 million in 2029, the last of the “outyear” budget projections included in the proposal.

“The Chandra spacecraft has been degrading over the mission lifetime to the extent that several systems require active management to keep temperatures within acceptable ranges for spacecraft operations,” NASA stated in its budget documents. “This makes scheduling and the post processing of data more complex, increasing mission management costs beyond what NASA can currently afford.”

“The reduction to Chandra will start orderly mission drawdown to minimal operations,” it added, without explaining what “minimal operations” meant.

The magnitude of the cut has alarmed X-ray astronomers, who fear that Chandra can’t continue operating with that reduced budget. That was effectively confirmed by the mission itself in a letter issued a week after the budget request by Patrick Slane, director of the Chandra X-ray Center.

He noted the mission has an existing plan for the “closeout” of the mission should it suffer an unexpected mission-ending malfunction, which include documenting spacecraft hardware and mission science and placing data into archives. “The funding levels provided in the new budget plan are consistent with levels for these closeout activities, but lower than can accommodate operation of the Chandra science mission; the minimal operations referred to in the budget document would actually be decommissioning activities.”

He also took issue with the claims that spacecraft operations have become more difficult because of increasing temperatures, noting that this has been an issue since 2005. “Our teams have developed thermal models and processes to manage this situation and have done so with amazing success – experiencing little or no decrease in observing efficiency, which far exceeds the initial requirements for the mission,” he wrote. Those models are incorporated into analysis software “so there is nothing about Chandra’s evolving temperature behavior that makes ‘post processing of data more complex.’”

The budget proposal also quickly led to the creation of, a site that argued that the budget proposal would lead to the cancellation of Chandra. “The loss of Chandra would be an extinction-level-event for X-ray Astronomy in the US, a Nobel-prize winning field that began in the US,” it states, arguing that Chandra can remain a productive space telescope at existing funding levels for up to a decade.

That put NASA officials on the defensive at a March 20 meeting of its Astrophysics Advisory Committee, or APAC. “We don’t want to cancel Chandra,” said Nicola Fox, NASA associate administrator for science, at the meeting. “What we’re looking for is more efficient ways of operating these missions so we can continue them moving forward.”

Both Fox and Clampin said they want to keep both Chandra and Hubble going through efficiencies found in the “mini senior review,” formally known as the Operations Paradigm Change Review. “Are there ways we can maintain scientific productivity with significantly different operating budgets?” Clampin asked at the meeting.

“We don’t want to cancel Chandra,” said Fox. “What we’re looking for is more efficient ways of operating these missions so we can continue them moving forward.”

Many members of APAC, as well as others offering public comments, seemed skeptical that Chandra operations would be viable given the proposed reductions as well as the process. Clampin described that process as a “small review team that would allow us to very quickly assess with the two teams, Hubble and Chandra, if there are ways we can maintain scientific productivity with significantly different operating budgets.”

Astronomers, though, questioned that process and the level of transparency that they, in particular the APAC, would have. “‘Mini senior review’ is probably unfortunate,” said Eric Smith, associate director for research and analysis in NASA’s astrophysics division, when asked by committee members what role, if any, they would have in the review of Chandra and Hubble.

He said that is because the review needs to work quickly: “We need to have the output from this process in time to affect the ’26 budget request, meaning the whole thing has to conclude by the end of May.” The review itself, he said, would take about a month, with the missions required to explain how they would operate within the proposed budget profiles, but with the option of suggesting additional, alternative mission plans and budgets.

“This is our attempt to have the parameter space explored for reduced funding for these missions,” he said of the review. That will, he said, inevitably lead to reduced science. “I don’t think we’re under any illusion you can have the budgets that are there and things just keep going the way that they have.”

That could mean, for example, turning off some instruments on both telescopes. Clampin offered one hypothetical option at the meeting: “Do we, in the age of Webb, want to continue operating a near-IR camera on Hubble?”

Those explanations from NASA officials did not assuage concerns from astronomers about the future of Chandra and the process NASA is using. “This is a surprise to the astrophysics division. It’s a surprise because these missions are so very successful,” said Kelly Holley-Bockelman, chair of APAC, at a meeting the following week of the NASA Advisory Council’s science committee. She also noted the concerns from the committee that the upcoming review is outside the usual senior review process. “We’re all taking this kind of hard.”

It was clear from the public comment session at the APAC meeting that astronomers were taking this hard. Those public comment sessions, required for federal advisory committee meetings, are often periods of silence, with no written or oral comments. At the APAC meeting, though, astronomers chimed in with their concerns about the proposed cuts to Chandra and Hubble.

Among those commenting was Dave Pooley of Trinity University, a member of the Chandra Users’ Committee. He expressed a concern, like that from the site, that cutting Chandra would affect the broader X-ray astronomy community in the US, which relies on Chandra observations and grants to support research related to those observations.

“Sudden budget cuts of this magnitude send a signal that X-ray astrophysics in the US is in an extremely precarious situation,” he said. “It is unclear how and even if X-ray astrophysics in the US will survive these cuts.”

“I don’t think we’re under any illusion you can have the budgets that are there and things just keep going the way that they have,” Smith said.

While Congress has yet to weigh in on the budget proposal, astronomers also worried that Chandra might be at a disadvantage to Hubble, given Hubble’s greater public prominence. Congress, they said, might respond to a public outcry about any potential reductions in Hubble operations, like when NASA canceled two decades ago a final shuttle servicing mission to Hubble, but thought it less likely even a possible cancellation of Chandra would evoke a similar response. Congress also must deal with budget pressures on other parts of NASA’s science portfolio, like the ongoing review of Mars Sample Return and the proposed cancellation of a heliophysics mission, the Geospace Dynamics Constellation.

At the APAC meeting, both Fox and Clampin said they were committed not just to keeping Chandra and Hubble going at reduced spending levels but also to the senior review process that astronomers worry is being short-circuited by the upcoming review.

“We are trying to find cheaper ways to operate Chandra and Hubble so that they are consistent with the budget allocation that we have,” Clampin said. “The decision was done this way because of the very significant impact to the budget that had to be dealt with on a short time scale. We didn’t have time to wait two years,” when the next regular senior review is scheduled.

“We don’t want to change the way we do senior reviews,” Fox said. “When we did the astrophysics senior review, we did not have these budget constraints. We were looking at a very different budget environment.”

Astronomers and others hope that budget environment will improve in 2026 and beyond, when spending caps on non-defense discretionary spending enacted in last year’s debt-ceiling agreement expire. “In ’25 we have our caps that we’re working within, and it’s tough,” said Casey Dreier, chief of space policy at The Planetary Society, during a webinar last week by the Aerospace Industries Association. He expressed hope that NASA’s overall budget, and its science budget, might have room to grow in 2026 and beyond, a scenario he said would likely depend on one party having control of both houses of Congress as well as the White House.

Jean Toal Eisen, a former Senate appropriations staffer who is now vice president of corporate strategy at the Association of Universities for Research in Astronomy, was less sanguine. She cited long-term budget challenges from demands from infrastructure and other programs. “That’s another budget challenge that’s not for ’25 but on the horizon.”

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