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 7:03pm 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.N), opens new tab 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 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.

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If Crew-10 launches as planned on Friday, it will dock to the ISS at 11:30pm 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.