Two NASA astronauts stuck on the International Space Station (ISS) for more than two months could be forced to stay until February 2025 because of concerns over the safety of the spacecraft that brought them into orbit.
Commander Barry "Butch" Wilmore and pilot Sunita "Suni" Williams arrived at the ISS in June as the first crew to test Boeing's new Starliner.
The US pair were expected to be back on Earth by now, but have been grounded by issues with the capsule, which suffered thruster failures before it docked - causing their mission to be extended indefinitely.
They may have to stay until February if NASA decides to bring them back on a SpaceX flight, rather than risk using the Starliner capsule.
That would mean they would watch from the space station as the Starliner is cut loose from the ISS to return to Earth without them.
Ken Bowersox, NASA's space operations mission chief, said the US space agency is looking more closely at SpaceX as a back-up, and "could take either path".
Mr Bowersox said during a recent meeting, they "heard from a lot of folks that had concern, and the decision was not clear".
A final decision is expected by mid-August.
It could have a knock-on effect on NASA's next SpaceX taxi flight in September, when two of the astronauts scheduled to fly may be left at home, so there is space for Mr Wilmore and Ms Williams on the return trip in February.
Three NASA astronauts and one Russian are assigned to the flight, but it's not known who could miss out.
Boeing has reiterated its capsule could still safely bring the astronauts home, but the company will need to modify Starliner's software in case it has to return without a crew.
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NASA's commercial crew manager Steve Stich said they hadn't seriously thought about launching a separate SpaceX flight just to bring back Mr Wilmore and Ms Williams.
He said thruster problems have also happened during ground tests and engineers don't know why seals that swell when overheated then shrink back to their proper size.
All but one of the Starliner's five failed thrusters have since been reactivated in orbit.
Starliner needs them to back away from the space station following undocking and to keep the capsule in the proper position for the deorbit.
Technicians are also trying to plug helium leaks in Starliner's propulsion system, which is crucial for manoeuvring.
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SpaceX flew its first NASA crew to the ISS in 2020, nine years after it was hired following the end of the Space Shuttles programme and the next crew flight will be SpaceX's 10th for the agency.
It has been delayed for a month until late September to allow for extra time to decide how to handle Starliner's return.
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