NASA says it is still deciding whether to keep two astronauts at the International Space Station until early 2025 and send their troubled Boeing capsule back empty.

Under the plan being mulled, Butch Wilmore and Suni Williams would catch a ride on SpaceX’s next flight, rather than flying Boeing’s Starliner back to Earth.

That option would keep them at the space station until February.

The test pilots anticipated being away for only a week or so when they rocketed away as Starliner’s first crew. But thruster failures and helium leaks marred the capsule’s trip to the space station, raising doubts about its ability to return safely and leaving the astronauts in limbo.

NASA officials said on Wednesday (US time) they were analysing more data before deciding by end of next week or beginning of the next. These thrusters are crucial for holding the capsule in the right position when it comes time to descend from orbit.

“We’ve got time available before we bring Starliner home and we want to use that time wisely,” NASA space operations mission chief Ken Bowersox said.

Safety chief Russ DeLoach added: “We don’t have enough insight and data to make some sort of simple, black-and-white calculation.”

DeLoach said the space agency wanted to make room for all opinions, unlike what happened with NASA’s two shuttle tragedies, Challenger and Columbia, when dissenting views were ignored.

“That may mean, at times, we don’t move very fast because we’re getting everything out, and I think you can kind of see that at play here,” he said.

Switching to SpaceX would require bumping two of the four astronauts assigned to the next ferry flight, which is targeted for late September. Wilmore and Williams would take the empty seats in SpaceX’s Dragon capsule once that half-year mission ends.

Another complication: The space station has just two parking places for US capsules. Boeing’s capsule would have to depart ahead of the arrival of SpaceX’s Dragon to free up a spot.

Boeing maintains Starliner could still safely bring the astronauts home. Earlier this month, the company posted a list of testing done on thrusters in space and on the ground since liftoff.

NASA would like to keep SpaceX’s crew up there until the replacements arrive, barring an emergency. Those four should have returned to Earth this month, but a seventh month was added to their mission because of the uncertainty over Starliner, keeping them up there until the end of September.

Most space station stays last six months, although some have gone a full year.

Wilmore and Williams are retired Navy captains who spent months aboard the space station years ago. They eased into space station work as soon as they arrived, helping with experiments and repairs.

“They will do what we ask them to do. That’s their job as astronauts,” NASA chief astronaut Joe Acaba said.

“This mission is a test flight and – as Butch and Suni expressed ahead of their launch – they knew this mission might not be perfect.”

Eager to have competing services and backup options, NASA hired SpaceX and Boeing to transport astronauts to and from the space station after the shuttles retired in 2011.

SpaceX’s first astronaut flight was in 2020.

Boeing suffered so much trouble on its initial test flight without a crew in 2019 that a do-over was ordered. Then more problems cropped up, costing the company more than $US1 billion ($1.5 billion) to fix before finally flying astronauts.

-AAP