WASHINGTON, United States (AFP) — Four crewmembers departed the International Space Station (ISS) on Wednesday after a medical issue prompted their mission to be cut a month short — a first for the orbiting laboratory.

A video feed from NASA showed American astronauts Mike Fincke and Zena Cardman, Russian cosmonaut Oleg Platonov and Japanese astronaut Kimiya Yui undocking from the ISS at 2220 GMT, after five months in space.

The US space agency has declined to disclose which crewmember has the health problem or give details about the issue, but it has stressed the return is not an emergency situation.

The affected crewmember “was and continues to be in stable condition,” NASA official Rob Navias said Wednesday.

The SpaceX Dragon capsule carrying the four crewmembers is scheduled to splash down off the California coast overnight at around 0840 GMT Thursday.

“First and foremost, we are all OK. Everyone on board is stable, safe, and well cared for,” Fincke, the pilot of SpaceX Crew-11, said in a recent social media post.

“This was a deliberate decision to allow the right medical evaluations to happen on the ground, where the full range of diagnostic capability exists. It’s the right call, even if it’s a bit bittersweet.”

The Crew-11 quartet arrived at the ISS in early August and had been scheduled to stay onboard the space station until they were rotated out in mid-February with the arrival of the next crew.

James Polk, NASA’s chief health and medical officer, said “lingering risk” and a “lingering question as to what that diagnosis is” led to the decision to bring back the crew earlier than originally scheduled.