NASA and SpaceX set for mission to swap out stuck astronauts Butch Wilmore, Sunita Williams

Washington: NASA and SpaceX hope on Friday to launch a long-awaited crewed rocket that will let them bring home US astronauts Butch Wilmore and Suni Williams, stuck on the International Space Station for nine months.

SpaceX and the US space agency had planned to launch a replacement crew of four astronauts from Florida on Wednesday, a mission called Crew-10, but a last-minute problem with the rocket's ground systems forced a delay.

NASA on Thursday said SpaceX had resolved the issue - flushing a suspected pocket of air out of a hydraulic clamp arm - and that the weather was 95 percent favorable for a Friday launch.

Slated for liftoff at 7:03 p.m. ET Friday (2330 GMT), the Crew-10 will reach the ISS on Saturday night.

Butch Wilmore and Suni Williams, two veteran NASA astronauts and US Navy test pilots, became the first humans to test-fly Boeing's Starliner spacecraft to the ISS.

But problems with Starliner's propulsion system during the flight forced an extension of their planned eight-day stay as NASA deemed it too risky for them to fly home on the craft, which returned to Earth empty.

The mission has also become entangled in politics as President Donald Trump and his adviser Elon Musk, SpaceX's CEO, say without evidence that former president Joe Biden left the astronauts on the station for political reasons.

"We came prepared to stay long, even though we planned to stay short," Wilmore said, adding that he did not believe NASA's decision to keep them on the ISS until Crew-10's arrival had been affected by politics.

"That's what your nation's human spaceflight program's all about," he said, "planning for unknown, unexpected contingencies. And we did that."

NASA says they have had to remain on the ISS to maintain its minimum staffing level.

Having seen their mission turn into a normal NASA rotation to the ISS, Wilmore and Williams have been doing scientific research and conducting routine maintenance with the other astronauts.

'Unusual' flight preparation process

Trump and Musk's demand for an earlier return was an unusual intervention, and NASA brought forward the Crew-10 mission from March 26, swapping a delayed SpaceX capsule for one that would be ready sooner.

The pressure from Musk and Trump has hung over a NASA preparation and safety process that normally follows a well-defined course.

NASA's Commercial Crew Program manager Steve Stich said SpaceX's "rapid pace of operations" had required NASA to change some of the ways it verifies flight safety.

The agency had to address some "late breaking" issues, NASA space operations chief Ken Bowersox told reporters, including investigating a fuel leak on a recent SpaceX Falcon 9 launch and deterioration of a coating on some of the Dragon crew capsule's thrusters.

Bowersox said it was hard for NASA to keep up with SpaceX: "We're not quite as agile as they are, but we're working well together."

When the new crew arrives aboard the station, Wilmore, Williams and two others - NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov - can return to Earth in a capsule that has been attached to the station since September, as part of the prior Crew-9 mission.

If Crew-10 launches as planned on Friday, it will dock to the ISS at 11:30 p.m. ET Saturday (0330 GMT Sunday), followed by a traditional handover ceremony that will allow for the Crew-9 crew's departure on March 19.