Reading the Tea Leaves: The Future of the Hubble and Chandra Space Telescopes

When it comes to astronomy in space, sometimes building the rockets is easier than funding for the science.

On July 25th, the U.S. Senate announced a proposed budget for NASA for the next fiscal year (FY2025). Under the new bill, NASA would receive an increase to $25.434 billion, above the agency's request of $25.383 billion and in line with the incremental yearly growth over the past decade. (NASA still doesn't take up much space, though, at around 0.5% of the overall federal budget.)

However, while on the surface the Senate's budget looked to hold good news for legacy astronomy programs such as the Chandra X-ray Observatory and the Hubble Space Telescope, a closer look leaves these iconic space telescopes' futures unclear.

What's in Store for Hubble and Chandra

The Senate's bills follow the release of the House's own spending bills, which provided only $25.178 billion in funding for NASA — below the agency's request. However, while the Senate's larger budget is thus good news overall for NASA as a whole, the fate of legacy missions, and especially Chandra, is less clear.

Hubble and Chandra were launched as two of NASA's Great Observatories, and they are the only two still operating. Having served the astronomy community and the general public for 34 and 25 years, respectively, these iconic missions are also getting on in years: Hubble recently transitioned to a more limited one-gyro mode of operations in June, and Chandra has had to grapple with workarounds to deal with its main detector's downgraded performance over time. Despite those challenges, both missions continue to fuel productive science.

The spending bill from the House, however, didn't agree. It specified a 10% drop in funding for Hubble, from $98.3 million in FY24 to $88.9 million in FY25, and even deeper cuts of 40% for Chandra, from $68.3 million down to just $41.1 million. The latter change would leave Chandra without the funds to continue operations.

NASA formed a review panel in response to that bill, looking at potential ways forward for both missions under that leaner fiscal environment. The panel found that Hubble's smaller cut left some options for the iconic observatory: The mission could terminate all infrared operations, leaving that to JWST, or it could cut funding for astronomers using Hubble's capabilities for their science. It could also operate with a leaner staff, though that could prove risky if problems were to arise.

But the deeper cuts for Chandra left little choice: The team would have to begin moving into closeout operations for the observatory starting in 2025. And that would leave the U.S. without a flagship space mission to study X-rays — a type of astronomy that can only be done from space. (NASA has other, smaller X-ray missions, such as NUSTAR and NICER, and it collaborated with Japan on the XRISM mission, but none of these see the universe with eyes as sharp as Chandra's.)

Now, the new Senate proposal makes future options for both Chandra and Hubble less dire. The bill includes line items for both observatories, specifying "not less than" $98.3 million for Hubble (the same as FY24, with no 10% drop), and "up to" $72.1 million for Chandra, which might give it full funding for operations.

That's great news for Hubble, but the phrasing leaves Chandra's options open.

"The 'up to' language is supportive but not binding," says Patrick Slane, director of the Chandra X-ray Center. "Our discussions with NASA headquarters have led to significant progress on preserving important core elements of the Chandra mission, but not its full capabilities."

The fight's not over yet: The spending bills from the House and Senate will need to be reconciled before being signed into law, and it's uncertain when that will happen. While the current fiscal year ends September 30th, the fact that it's a presidential election year could mean some final decisions on funding might not occur until November.

"As a result, NASA and other agencies have to look at the individual bills and 'read the tea leaves' in formulating their budget decisions," Slane says.

See more beautiful images from Chandra in a celebration of the observatory's 25th anniversary in the July 2024 issue of Sky & Telescope.

The Moon, Mars, and Beyond

Most of NASA's budget is earmarked for exploration, and here the House and Senate bills are in alignment with each other, more or less. Proposed funding supports the agency's Artemis initiative, which includes various missions to and around the Moon, as well as the International Space Station.

Both the House and the Senate also support funding the Near-Earth Object Surveyor to ready the asteroid-finding mission for launch in 2028.

Additionally, the spending bills are in agreement that the struggling Mars Sample Return mission must move forward. The House bill offers $650 million for the mission in FY2025, with launch no later than 2031, while the Senate version instead encourages NASA to "select a realistic mission architecture" and would then fund what NASA needs to make that happen. The selection and cost estimate would be due within 30 days of the bill's enactment.

Time will tell what the final budget resolution will bring, as the fate of legacy observatories hangs in the balance.