Cracks are appearing on the International Space Station (ISS), and retired NASA astronaut Bill Shepherd says they're a "fairly serious issue."

After Russian cosmonauts spotted the cracks on the station's Zarya module, Vladimir Solovyov, flight director of the Russian segment of the ISS, publicly revealed the discovery in August.

The cracks don't pose a danger to astronauts at this time, NASA says, and the agency told Insider last month that nobody had identified "new potential leak sites" on the station.

But in a House committee hearing on Tuesday, Shepherd told Congressional representatives that "there are probably other cracks we haven't found yet."

"As far as I know, the Russian engineers and the NASA engineers – they've analyzed it – they don't exactly understand why these cracks are appearing now," Shepherd said.

Shepherd has flown to orbit four times on the Space Shuttles. He worked on the International Space Station Program when its first modules were launching, and he commanded the first crew to the station in 2000. He said at the hearing that he'd learned more about the cracks in two meetings of NASA's ISS Advisory Committee, which he recently joined.

The cracks are "quite small – they look like scratches on the surface of the aluminum plate," Shepherd said, adding, "there are probably something like half a dozen of them."

NASA did not immediately respond to a request for comment.

'This is bad'

Shepherd told the House committee that currently, the cracks are not long enough to pose a "serious problem."

But last month, Solovyov told state-owned news agency RIA: "This is bad and suggests that the fissures will begin to spread over time," according to a Reuters report translating his statement.

Solovyov did not share how extensive the cracks were at the time.

Shepherd didn't say whether NASA and Russia plan to further investigate the cracks beyond the analysis they already finished.

In the past, both space agencies have taken their time when investigating and repairing issues that don't threaten the safety of astronauts or interfere with ISS operations.

The space station is getting old

The ISS has been orbiting Earth for 20 years, and it's showing signs of age. Russia's side of the space station hosts some of its oldest components, and the cracks are the latest in a series of issues in those modules.

Last year, a toilet on the segment went bust, temperatures mysteriously increased, and an oxygen-supply system broke down.

In September 2019, another space-station module, Zvezda, which provides living quarters for the cosmonauts, started leaking air. That wasn't an immediate danger to astronauts, and they eventually found the hole and patched it with Kapton tape.

Russian media previously reported that Solovyov told the Russian Academy of Sciences: "There are already a number of elements that have been seriously damaged and are out of service. Many of them are not replaceable. After 2025, we predict an avalanche-like failure of numerous elements onboard the ISS."

Even Russia's newest module – a spacecraft called Nauka, which it launched to the ISS in July – has experienced serious problems. Shortly after it docked to the station, Nauka began unexpectedly firing its thrusters. This caused the entire ISS to spin around 540 degrees and flip upside down before flight controllers regained control an hour later.

NASA has the funds to keep operating the ISS through 2024, and it's aiming to get an extension from Congress to continue the station's activities through 2028.

But Shepherd said that NASA should first solve the mystery of the Zarya module's new cracks.

soyuz capsule docking with zarya on the ISSSoyuz spacecraft docking with the ISS Zarya module, 2009 (NASA)

"Getting to the bottom of this is a fairly serious issue," Shepherd said. "I don't think the station's in any immediate danger. But before we clear the station for another so many years of operational use, we should better understand this."

The ISS will eventually be retired and push itself into the atmosphere to burn up. After that, NASA doesn't want to build a new station; the agency is recruiting private companies to do that instead.

It's currently evaluating about a dozen space-station proposals from various companies, with the aim of distributing $400 million among two to four of them.

Eventually, NASA hopes to be one of many customers on private commercial space stations.

The agency has already awarded Axiom Space $140 million to fly modules up to the ISS that will eventually detach from it to become their own space station. Axiom aims to launch its first module to the ISS in 2024.

China, meanwhile, launched the first piece of its own space station earlier this year, and astronauts completed their first three-month mission there last week.

This article was originally published by Business Insider.

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