NASA astronauts prepare for homecoming after nine-month mission

Butch Wilmore and Sunita Williams, two veteran NASA astronauts who have been stuck on the International Space Station (ISS) for nine months, are scheduled to begin their return to Earth on Tuesday morning on a long-awaited flight home to cap an unusual mission.

After a replacement crew arrived on the space station Saturday night, Wilmore, Williams and two other astronauts are poised to undock from the ISS at 1.05 am ET (09.05 am UAE time) Tuesday to begin a 17-hour trip back to Earth.

The space station departure is the start of a highly anticipated end to the drawn-out saga of "Butch and Suni", who had been part of a key test mission with Boeing's Starliner spacecraft that went wrong last year. The mission was initially expected to last eight days.

A Crew Dragon capsule from Elon Musk's SpaceX will be their ride home, part of a contingency plan devised by NASA last year.

The failed test mission was another blow to Boeing's space unit, which has struggled for years to bring Starliner to market to compete with SpaceX's Crew Dragon, a dominant vehicle in the global human spaceflight domain.

More recently, US President Donald Trump and his close adviser Elon Musk - SpaceX's CEO - have sought to blame without evidence former President Joe Biden for the astronauts' plight, adding political drama to an already unusual situation for NASA's human spaceflight programme.

After their autonomous undocking from the ISS, the astronaut crew is scheduled to splash down in the Gulf of Mexico at 5:57 pm ET Tuesday (1.57 am Wednesday UAE time), with the exact location depending on local weather conditions. They will be flown to NASA's Johnson Space Center for a few days of routine post-mission medical checks.

Wilmore and Williams were the first crew to fly Boeing's Starliner in a test flight for the capsule in June.

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. Starliner returned to Earth empty in September.

NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov, the other two members of Crew-9, flew to the ISS in September on a Crew Dragon craft with two empty seats. They will join Wilmore and Williams on Tuesday's return trip.

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

Wilmore and Williams' mission turned into a normal NASA rotation to the ISS and they have been doing scientific research and conducting routine maintenance with the station's other five astronauts.

The ISS is a football field-sized research lab that has been housed continuously by international crews of astronauts for nearly 25 years, a key platform of science diplomacy mainly managed by the US and Russia.