Butch Wilmore and Suni Williams flew to the orbital lab in June last year, on what was supposed to be a days-long roundtrip to test out Boeing's Starliner on its first crewed flight.

But the spaceship developed propulsion problems and was deemed unfit to fly them back, instead returning empty without more major problems.

Ex-Navy pilots Wilmore and Williams, 62 and 59 respectively, were instead re-assigned to the NASA-SpaceX Crew-9 mission, which saw a Dragon spacecraft fly to the ISS last September with a team of two, rather than the usual four, to make room for the "stranded" pair.

Then, early Sunday, a relief team called Crew-10 docked with the station, their arrival met with broad smiles and hugs as they floated through the hatch.

Crew-10's arrival clears the way for Wilmore and Williams to depart, along with American Nick Hague and Russian cosmonaut Aleksandr Gorbunov.

Hatch closure is set for 10:45 pm (0245 GMT), followed by final checks and undocking at 1:05 am.

If all goes smoothly, the Dragon craft will deploy its parachutes off the coast of Florida for an ocean splashdown, where a recovery vessel will retrieve the crew.

- 'Unbelievable resilience' -

Wilmore and Williams' stay surpasses the standard six-month ISS rotation but ranks only sixth among US records for single-mission duration.

Frank Rubio holds the top spot at 371 days in 2023, while the world record remains with Russian cosmonaut Valeri Polyakov, who spent 437 consecutive days aboard the Mir station.

That makes it "par for the course" in terms of health risks, according to Rihana Bokhari of the Center for Space Medicine at Baylor College.

Challenges such as muscle and bone loss, fluid shifts, and readjusting to gravity are well understood and well managed.

"Folks like Suni Williams are actually known for their interest in exercise, and so I believe she exercises beyond what is even her normal prescription," Bokhari told AFP.

Still, the unexpected nature of their extended stay -- away from their families and initially without enough packed supplies -- has drawn public interest and sympathy.

"If you found out you went to work today and were going to be stuck in your office for the next nine months, you might have a panic attack," Joseph Keebler, a psychologist at Embry-Riddle Aeronautical University, told AFP.

"These individuals have shown unbelievable resilience."

- Trump weighs in -

Their unexpected stint also became a political lightning rod, with President Donald Trump and his close advisor, Elon Musk -- who leads SpaceX -- repeatedly suggesting former president Joe Biden abandoned the astronauts and refused an earlier rescue plan.

"They shamefully forgot about the Astronauts, because they considered it to be a very embarrassing event for them," Trump posted on Truth Social on Monday.

Such accusations have prompted an outcry in the space community, especially as Musk offered no specifics and NASA's plan for the astronauts' return has remained unchanged since their Crew-9 reassignment.

Trump has also drawn attention for his bizarre remarks, referring to Williams, a decorated former naval captain, as "the woman with the wild hair" and speculating about the personal dynamic between the two.

"They've been left up there -- I hope they like each other, maybe they love each other, I don't know," he said during a recent White House press conference.