Chandra X-ray Observatory Threatened By Budget Cuts

Yet despite this rosy report card, Chandra's future is anything but certain. Faced with the impossible task of funding all of its scientific missions with the relative pittance they're allocated from the federal government, NASA has signaled its intent to wind down the space telescope's operations over the next several years. According to their latest budget request, the agency wants to slash the program's $41 million budget nearly in half for 2026. Funding would remain stable at that point for the next two years, but in 2029, the money set aside for Chandra would be dropped to just $5.2 million.

Drastically reducing Chandra's budget by the end of the decade wouldn't be so unexpected if its successor was due to come online in a similar time frame. Indeed, it would almost be expected. But despite being considered a high scientific priority, the x-ray observatory intended to replace Chandra isn't even off the drawing board yet. The 2019 concept study report for what NASA is currently calling the Lynx X-ray Observatory estimates a launch date in the mid-2030s at the absolute earliest, pointing out that several of the key components of the proposed telescope still need several years of development before they'll reach the necessary Technology Readiness Level (TRL) for such a high profile mission.

With its replacement for this uniquely capable space telescope decades away even by the most optimistic of estimates, the  potential early retirement of the Chandra X-ray Observatory has many researchers concerned about the gap it will leave in our ability to study the cosmos.

The Sky Through X-Ray Vision

Just a few years after the launch of the incredibly powerful James Webb Space Telescope (JWST), it might seem odd that scientists are concerned about the fate of an observatory that was launched 25 years ago.

But for all the capabilities of the JWST, making observations in the X-ray spectrum simply isn't one of them. It was designed almost elusively for performing infrared astronomy, a specialization to which it owes many of its unique design elements. Other well known space telescopes, such as the European Space Agency's Euclid or the legendary Hubble Space Telescope operate primarily in the visible spectrum.

It's not that Chandra is the only space telescope capable of x-ray imaging — but it's unquestionably the best we currently have. Of course, this was by design. While most of the other x-ray telescopes in space are either secondary payloads or otherwise relatively small, Chandra was designed and built as one of NASA's flagship missions. Like Hubble it was part of the Great Observatory program, and was intended to push the state-of-the-art in detection technology for its particular slice of the electromagnetic spectrum, setting the standard for decades to come.

Compared to optical or IR telescopes, x-ray telescopes are ideal for directly imaging high-energy objects such as supernovas and galaxy clusters. It's also possible to make indirect observations by studying how a given object reflects or absorbs ambient x-ray energy. X-ray telescopes are also key for studying what might otherwise be invisible, such as black holes and dark matter.

Rising Temps, Rising Costs?

According to the NASA budget request, winding down Chandra's funding to what it calls "minimal operations" by 2029 isn't just some arbitrary decision. The report specifically cites increased program costs directly related to the age of the spacecraft, or more specifically, the extra effort that's now required to get usable data from onboard systems which are increasingly operating out of spec:

In a post titled "A Letter to the Chandra Community", program director Dr. Patrick Slane addresses these NASA claims directly. While he acknowledges that Chandra has suffered system degradation, he points out that it's hardly unexpected for a spacecraft of this age. Despite being designed for a planned mission duration of 5 years, Chandra is approaching 25 years in orbit. He also admits that it's made observations more difficult than they were when the observatory was launched.

That said, Dr. Slane argues that Chandra's rising operating temperature isn't a new problem, and was first identified as far back as 2005. Over the years the teams have been able to characterize the temperature fluctuations, which vary depending on orbital position and spacecraft attitude, and develop new models that allow them to compensate for any thermally-induced shifts in the data. He says these efforts have been greatly successful, and that even today, Chandra's observation efficiency "far exceeds the initial requirements for the mission."

The documentation for this isn't difficult to find. In the 2016 paper "Evolution of temperature-dependent charge transfer inefficiency correction for ACIS on the Chandra X-ray Observatory", it's explained how the responsiveness of Chandra's Advanced CCD Imaging Spectrometer (ACIS) had changed since launch, and how teams on the ground were adjusting to the system's new characteristics.

While the paper notes that the ACIS had suffered some radiation damage, it identifies the reduced effectiveness of Chandra's thermal control systems to have introduced the largest changes. At the time of publication, now nearly a decade ago, it was already clear that the ACIS temperature was on the rise. The paper notes that in 2000, 99% of observations were made at what it considers "cold" temperatures of below -119.2° C, but by 2015, that figure had dropped to just 33%.

The paper explains that, after gathering sufficient calibration data, algorithms were developed to compensate for the increasingly erratic ACIS temperatures. That said, it did note that the team's initial assumptions that the temperature fluctuations would remain relatively minor were already being challenged. Should instrumentation temperature continue to climb higher, more data would need to be collected to verify their compensation software still worked as expected.

Still, the authors closed the paper with the belief that Chandra would still be able to meet its performance goals even in theis newly dynamic thermal environment.

The Fight Has Only Just Begun

Between the published data on telescope's instrumentation, and the passionate response of Dr. Slane, it would seem the Chandra story isn't quite over yet. There's an excellent case to be made that the thermal issues with the observatory, while a valid and documented concern, are a largely solved problem that doesn't require any additional financial outlay. Why the NASA budget request seems to indicate otherwise is beyond the scope of this article, but with the ever-rising cost of the Artemis program, it's not hard to imagine that the agency is looking to justify cost cutting measures wherever it can.

At the end of his letter, Dr. Slane notes that the Chandra X-ray Observatory would be going through a review in April to further clarify its budgetary needs going forward. After giving the program a closer look, and perhaps after feeling a bit of pressure from the astrophysics community, NASA may yet change its plans for this valuable instrument.