WASHINGTON, March 17 (Reuters) - Butch Wilmore and Suni

Williams, two NASA astronauts who have been stuck on the

International Space Station for nine months, are scheduled to

begin their return to Earth early Tuesday morning on a

long-awaited flight home to cap an unusual mission.

After a replacement crew arrived on the space station

Saturday night, veteran astronauts Wilmore and Williams and two

other astronauts are poised to undock from the ISS at 1:05 a.m.

ET (0505 GMT) Tuesday to begin a 17-hour trip back to Earth.

The astronaut crew are scheduled to splashdown off a Florida

coast - the exact location pending weather conditions - at 5:57

p.m. ET later that day.

Wilmore and Williams were the first crew to fly Boeing's ( BA )

Starliner spacecraft in a key test flight in June. But

after issues with the craft's propulsion system, NASA deemed it

too risky to bring the astronaut duo back home and opted to fold

them into the agency's Crew-9 mission instead while Starliner

returned to Earth empty in September.

NASA astronaut Nick Hague and Russian cosmonaut Aleksandr

Gorbunov, the other two members of Crew-9, will join Wilmore and

Williams. Hague and Gorbunov flew to the ISS in September on a

Crew Dragon craft with two empty seats.

NASA previously planned to return Crew-9 on Wednesday night,

but unfavorable weather later in the week would have complicated

the Crew Dragon capsule's return, leading the agency to move the

return trip up to Tuesday.