The ISS has been leaking air for 5 years, and engineers still don’t know why

Officials from NASA and Russia’s space agency don’t see eye to eye on the causes and risks of small but persistent air leaks on the International Space Station.

That was the word from the new chair of NASA's International Space Station Advisory Committee last week. The air leaks are located in the transfer tunnel of the space station's Russian Zvezda service module, one of the oldest elements of the complex.

US and Russian officials "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 retired NASA astronaut who took the helm of the advisory committee earlier this year. Cabana replaced former Apollo astronaut Tom Stafford, who chaired the committee before he died in March.

The transfer tunnel, known by the Russian acronym PrK, connects the Zvezda module with a docking port where Soyuz crew and Progress resupply spacecraft attach to the station.

Air has been leaking from the transfer tunnel since September 2019. On several occasions, Russian cosmonauts have repaired the cracks and temporarily reduced the leak rate. In February, the leak rate jumped up again to 2.4 pounds per day, then increased to 3.7 pounds per day in April.

This prompted managers to elevate the transfer tunnel leak to the highest level of risk in the space station program's risk management system. This 5×5 "risk matrix" classifies the likelihood and consequence of risks. Ars reported in June that the leaks are now classified as a "5" both in terms of high likelihood and high consequence.

NASA reported in September that the latest round of repairs cut the leak rate by a third, but it did not eliminate the problem.

An engineering problem

"The Russian position is that the most probable cause of the PrK cracks is high cyclic fatigue caused by micro-vibrations," Cabana said on November 13. "NASA believes the PrK cracks are likely multi-causal, including pressure and mechanical stress, residual stress, material properties, and environmental exposures."

The ISS is aging. Zvezda and the PrK launched in July 2000 and will mark a quarter-century in orbit next year. NASA wants to keep the space station operating until at least 2030, while Roscosmos, Russia's space agency, has committed only through 2028.

Roscosmos has shared sample metals, welds, and investigation reports with NASA to assist in the study of the cracks and leaks. In a report published in September, NASA's inspector general said NASA's ISS Vehicle Office at Johnson Space Center in Houston said the leaks are "not an immediate risk to the structural integrity of the station."

This is because managers have implemented mitigations to protect the entire station in the event of a structural failure of the PrK.

Crew members aboard the space station are keeping the hatch leading to the PrK closed when they don't need to access the Progress cargo freighter docked at the other end of the transfer tunnel. Russian cosmonauts must open the hatch to unpack supplies from the Progress or load trash into the ship for disposal.

But NASA and Roscosmos disagree on when the leak rate would become untenable. When that happens, the space station crew will have to permanently close the hatch to seal off the PrK and prevent a major failure from affecting the rest of the complex.

"The station is not young," said Michael Barratt, a NASA astronaut who returned from the space station last month. "It's been up there for quite a while, and you expect some wear and tear, and we're seeing that."

"The Russians believe that continued operations are safe, but they can't prove to our satisfaction that they are," said Cabana, who was the senior civil servant at NASA until his retirement in 2023. "And the US believes that it's not safe, but we can't prove that to the Russian satisfaction that that's the case.

"So while the Russian team continues to search for and seal the leaks, it does not believe catastrophic disintegration of the PrK is realistic," Cabana said. "And NASA has expressed concerns about the structural integrity of the PrK and the possibility of a catastrophic failure."

Closing the PrK hatch permanently would eliminate the use of one of the space station's four Russian docking ports.

NASA has chartered a team of independent experts to assess the cracks and leaks and help determine the root cause, Cabana said. "This is an engineering problem, and good engineers should be able to agree on it."

As a precaution, Barratt said space station crews are also closing the hatch separating the US and Russian sections of the space station when cosmonauts are working in the PrK.

"The way it's affected us, mostly, is as they go in and open that to unload a cargo vehicle that's docked to it, they've also taken time to inspect and try to repair when they can," Barratt said. "We've taken a very conservative approach to closing the hatch between the US side and the Russian side for those time periods.

"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 and adapt."