Nasa astronauts stranded on the International Space Station (ISS) after the failure of Boeing’s Starliner may be stuck there until 2025, the US space agency has admitted.

Commander Barry ‘Butch’ Wilmore and pilot Sunita ‘Suni’ Williams arrived at the ISS in June in what was supposed to be an eight-day mission and first human test flight for Starliner.

But after problems with the propulsion system and thrusters, the pair were unable to get home and Nasa is unsure whether Starliner – which remains attached to the space station – can be fixed.

This week, Nasa said it was considering using the upcoming SpaceX Crew-9 mission to bring Mr Wilmore and Ms Williams home.

Under the rescue plan, just two astronauts instead of four would travel to the space station in a reconfigured SpaceX Crew Dragon.

Mr Wilmore and Ms Williams would then travel back in the spare seats, but the return journey is not scheduled until February 2025, leaving the pair facing another six months in space.

The ISS usually holds a long-term crew of seven, but with Mr Wilmore and Ms Williams on board there are currently nine astronauts crammed into the limited facilities.

Ken Bowersox, Nasa’s director of space operations, said: “At some point we need to bring Butch and Sunny home.

“While they’re up there we have extra crew, we have extra hands and they can do a lot more work but they’re also using up more consumables more supplies so we have to maintain that balance. And at some point we need to bring bring those folks home and get back to a normal crew size on the ISS.

“We are in a situation where we’ve got multiple options. We don’t just have to bring a crew back on Starliner, we could bring them back on another vehicle.”

Boeing Starliner project plagued by problems

The Starliner project has been beset with problems from the outset.

After the space shuttle was decommissioned in 2011, Nasa was forced to use the Russian Soyuz to travel to the ISS, and so invited bids for a new all-American crew vehicle.

In 2014, Nasa chose SpaceX and Boeing to design two crew modules capable of ferrying astronauts to the ISS,

But while SpaceX completed the brief in just six years, launching its first astronauts in November 2020, Boeing lagged years behind amid ongoing technical issues.

Numerous test launches were delayed or scrapped at the last minute, and even the inaugural flight with Mr Wilmore and Ms Williams on June 5 was temporarily postponed while engineers dealt with a “sticky valve”.

When Starliner eventually launched, more helium leaks were detected and five of the 28 thrusters failed as it approached the ISS.

The stuck pair were forced to undertake manual manoeuvres in space in a docking sequence that took an hour longer than planned.

Nasa is currently troubleshooting the issues at the White Sands Test Facility in New Mexico, and has found that a Teflon seal appears to be swelling and blocking the fuel flow.

But if engineers cannot work out how to fix the leaks and thruster issues they have another problem looming. In order for SpaceX Crew Dragon to dock at the space station, Starliner will need to be detached and return to Earth autonomously, so that the rescue ship can use its vacated port.

The capsule, however, does not have the contingency software needed to undock without crew on board, and if there is insufficient thruster power, there are fears it may not be able to move far enough from the space station to avoid a collision.

‘It’s been a very complicated timeframe’

Nasa said it was confident that undocking could be achieved without it hitting the ISS.

Dana Weigel, Nasa’s International Space Station program manager, said: “We’ve got sufficient fault tolerance for undocking so we’re not concerned about the immediate proximity around the ISS.

“Even if the vehicle did nothing and just drifted out, it’s on a safe trajectory for the ISS, so that’s an added layer of conservatism. There is a safe complete drift-out trajectory if none of the thrusters were working.”

Boeing issued a statement saying it continued to “believe in Starliner’s capability and its flight rationale”.

“If Nasa decides to change the mission, we will take the actions necessary to configure Starliner for an uncrewed return,” a Boeing spokesperson said.

Steve Stich, program manager for Nasa’s commercial crew program added: “I want to reiterate we haven’t made a decision yet relative to return Butch and Sunny on Starliner or on[Crew Dragon].

“It’s been a very complicated timeframe and there’s been a lot of work with Boeing and the SpaceX team.”