The crew was scheduled to launch on February 11, Elon Musk's SpaceX company said this week it was grounding all flights on its Falcon 9 rocket while it investigates an unspecified issue.

This late uncertainty is just the most recent twist for the SpaceX Crew-12 mission, which includes Americans Jessica Meir and Jack Hathaway, French astronaut Sophie Adenot and Russian cosmonaut Andrey Fedyaev.

They will be replacing Crew-11, which returned to Earth in January, a month earlier than planned, during the first medical evacuation in the space station's history.

NASA has declined to disclose any details about the health issue that cut the mission short.

However, the scientific laboratory, which orbits 400 kilometres above Earth, has since been staffed by a skeleton crew of three.

Because of the medical evacuation, NASA moved the date of the Crew-12 launch forward a few days.

The launch had also overlapped with NASA's first mission to fly astronauts around the Moon in more than half a century.

The launch window for the Artemis 2 mission had been set for February 6-11 -- until leaks detected this week during final tests pushed the date back to March 6.

- 'One day, that will be me' -

Once the astronauts finally get on board, they will be one of the last crews to live on board the football field-sized space station.

Continuously inhabited for the last quarter century, the ageing ISS is scheduled to be pushed into Earth's orbit before crashing into an isolated spot in the Pacific Ocean in 2030.

The ISS, once a symbol of warming post-Cold War relations, has been a rare area of continued cooperation between the West and Russia since Moscow invaded Ukraine in 2022.

However, the space station has not entirely avoided the tensions back on Earth.

In November, Russian cosmonaut Oleg Artemyev -- who had long been planned to be a member of Crew-12 -- was suddenly taken off the mission.

Reports from independent media in Russia suggested he had been photographing and sending classified information with his phone. Russian space agency Roscosmos merely said he had been transferred to a different job.

His replacement Fedyaev, has already spent some time on the ISS as part of Crew-6 in 2023.

During their eight months on the space station, the four astronauts will conduct many experiments, including research into the effects of microgravity on their bodies.

Meir, who previously worked as a marine biologist studying animals in extreme environments, will serve as the crew's commander.

Adenot will become the second French woman to fly to space, following in the footsteps of Claudie Haignere, who spent time on the Mir space station.

When Adenot saw Haignere's mission blast off, she was 14 years old.

"It was a revelation," the helicopter pilot said recently.

"At that moment, I told myself: one day, that will be me."

Among other research, the European Space Agency astronaut will test a system that uses artificial intelligence and augmented reality to allow astronauts to carry out their own medical ultrasounds.

The mission is scheduled to launch from Cape Canaveral, Florida at 1100 GMT on February 11. If called off, launches can also be attempted on the following two days.

dl/rl/ane

ISS A/S