WASHINGTON, March 14 (Reuters) - NASA and SpaceX on

Friday will count down to a long-awaited crewed rocket launch

that will allow them to bring home U.S. astronauts Butch Wilmore

and Suni Williams, who have been stuck on the International

Space Station for nine months.

SpaceX and the U.S. space agency had planned on Wednesday to

launch from Florida a replacement crew of four astronauts, a

mission called Crew-10, but a last-minute issue with the

rocket's ground systems forced a delay.

Now slated for liftoff at 7:03 p.m. ET Friday (2330GMT), the

Crew-10's arrival to the ISS on Saturday night will allow the

return of Wilmore and Williams, two veteran NASA astronauts and

U.S. Navy test pilots who in June 2024 were the first humans to

test-fly Boeing's ( BA ) Starliner spacecraft to the ISS in

June.

But problems with Starliner's propulsion system during its

flight to the ISS delayed what was expected to be an eight-day

stay. NASA deemed it too risky for the astronauts to fly home on

the Boeing ( BA ) craft, which led to the current plan to bring them

home in a SpaceX capsule.

It has also become entangled in politics as President Donald

Trump and his adviser Elon Musk, SpaceX's CEO, claim without

evidence President Joe Biden left "Butch and Suni" on the

station for political reasons.

"We came prepared to stay long, even though we planned to

stay short," Wilmore said, adding that from his standpoint,

politics and played no role in NASA's decision to keep them on

the ISS until Crew-10's arrival.

"That's what your nation's human spaceflight program's all

about, planning for unknown, unexpected contingencies, and we

did that," he said.

The astronaut duo has been doing scientific research and

conducting routine maintenance with the space station's other

astronauts, and have remained safe, NASA has said.

The demands by Trump and Musk for an earlier return were an

unusual intervention in NASA's human spaceflight operations. The

mission previously had a target date of March 26, but NASA

swapped a delayed SpaceX capsule with a different one that would

be ready sooner.

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 pm Saturday, followed by a traditional handover

ceremony that will allow for the Crew-9 crew's departure March

19.

Wilmore and Williams cannot leave until the new Crew-10

craft arrives so that the ISS staffed with enough U.S.

astronauts for maintenance, according to NASA.