WASHINGTON — NASA and Roscosmos continue to disagree on the cause and severity of an air leak in the Russian segment of the International Space Station, one that NASA worries could lead to a “catastrophic failure” of part of a Russian module.

That disagreement was brought to light during a brief meeting of NASA’s ISS Advisory Committee Nov. 13, which recounted a meeting of that committee with its Roscosmos counterpart in Moscow in September to discuss issues with the station.

The major concern has been a small but persistent leak in a vestibule of the Zvezda service module called PrK that separates a docking port from the rest of the module. That leak has existed for several years, and station crews have dealt with the leak by sealing off PrK from the rest of the station when they do not need access to Progress cargo spacecraft docked to the port.

“Although the teams continue to investigate the causal factors for the crack initiation and growth, the U.S. and Russian technical teams don’t have a common understanding of what the likely root cause is or the severity of the consequences of these leaks,” said Bob Cabana, a former NASA astronaut and associate administrator who now chairs the committee.

He said Russian engineers believe the cracks are likely caused by “high cyclic fatigue” from micro-vibrations. NASA, by contrast, believes several factors are at play, including pressure and mechanical stress, residual stress, material properties of the module and environmental exposure.

In September, a report by NASA’s Office of Inspector General (OIG) said “both agencies have narrowed their focus to internal and external welds” as a cause of the leaks, which have not been seen elsewhere in the station.

At a Sept. 27 briefing, NASA officials downplayed the concerns about the leaks, which were first detected in 2019 but earlier this year had grown to their highest rates seen to date, a loss of 1.7 kilograms of air per day, the OIG report stated. At that briefing, agency officials said recent repair work reduced the leak rate by a third.

The leaks, though, remain a cause for concern for the agencies as well as space station crews. At a Nov. 8 briefing, Michael Barratt, a NASA astronaut who returned to Earth in October on the Crew-8 mission after nearly eight months on the station, said his Russian counterparts have been “very open” about the issue but that NASA takes precautions when the hatch to PrK is open.

“We’ve taken a very conservative approach to close a hatch between the U.S. side and the Russian side during those time periods,” he said. “It’s not a comfortable thing but it is the best agreement between all the smart people on both sides, and it’s something that we as a crew live with.”

Cabana said at the committee meeting that there is also no agreement between NASA and Roscosmos on the severity of the issue. “While the Russian team continues to search for and seal the leaks, it does not believe catastrophic disintegration of the PrK is realistic,” he said. “NASA has expressed concerns about the structural integrity of the PrK and the possibility of a catastrophic failure.”

“The Russians believe that continued operations are safe but they can’t prove to our satisfaction that they are, and the U.S. believes that it’s not safe but we can’t prove to the Russians’ satisfaction that that’s the case,” he concluded.

Cabana said the NASA and Roscosmos committee made a joint recommendation to their agencies’ leaders that they would continue to seek a “common understanding of the structural integrity” of PrK and bring in outside experts from academia and industry to support those efforts.

He offered no timetable for those efforts, but said NASA has brought in an independent team to assess the leaks. “This is an engineering problem, and good engineers should be able to reach a solution and agree on it.”

“The station is not young. It’s been up there for quite a while. You expect some wear and tear, and we’re seeing that,” Barratt said at the Crew-8 briefing.

The PrK leak was the primary topic of the meeting, which lasted only about 10 minutes even though the Federal Register notice for the meeting projected it would last an hour. The meeting was the first public meeting by the committee since March 2020, when the committee was chaired by former astronaut Tom Stafford, who passed away this March.

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