A SpaceX capsule departed the International Space Station on Wednesday carrying a four-member crew on an emergency return flight to Earth necessitated by an undisclosed serious medical condition afflicting one of the astronauts aboard.

The Crew Dragon capsule carrying two US NASA astronauts, a Japanese crewmate and a Russian cosmonaut undocked from the space station and began its descent from orbit at about 5:20 p.m. EST (2220 GMT). It was headed for a splashdown in the Pacific Ocean off the California coast early on Thursday.

If all goes as planned, the capsule dubbed Endeavor will parachute into the sea following a return flight of about 10-1/2 hours, capped by a fiery re-entry through Earth’s atmosphere, concluding a 167-day mission.

Live video from a NASA webcast of the departure showed the capsule separating from the ISS and drifting away from the orbiting laboratory as the two vehicles soared some 260 miles (418 km) over the Earth south of Australia.

The astronauts were seen strapped into the crew cabin, seated side by side and wearing their helmeted white and black space suits as the undocking proceeded.

Mystery medical issue

The plan to bring all four members of Crew-11 home a few weeks ahead of schedule was announced January 8, with NASA Administrator Jared Isaacman saying one of the astronauts faced a “serious medical condition” that required immediate medical attention on the ground.

This marks the first time NASA has cut short the mission of an ISS crew because of a health emergency.

NASA officials have not identified which of the four crew members was experiencing a medical issue or described its nature, citing privacy concerns.

The crew consists of US astronauts Zena Cardman, 38, and Mike Fincke, 58, Japanese astronaut Kimiya Yui, 55, and Russian cosmonaut Oleg Platonov, 39. They arrived at the space station following a launch to orbit from Florida in August.

Fincke, a retired Air Force colonel who was the station’s designated commander, and Cardman, a rookie astronaut and geobiologist assigned as flight engineer, had been scheduled to conduct a six-hour-plus spacewalk last week to install hardware outside the station. The spacewalk was canceled on January 7 over what NASA then characterized as a “medical concern” with an astronaut.

NASA Chief Health and Medical Officer James Polk later said the medical emergency did not involve “an injury that occurred in the pursuit of operations.”

In an Instagram post a few days ago, Fincke, wrapping up the fifth space mission of his NASA career, wrote that the four members of Crew-11 “are all OK,” adding, “Everyone on board is stable, and well cared for.”

“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,” Fincke wrote.

Crew-12, the 12th regular crew rotation mission flown by SpaceX to the ISS, is expected to launch in mid-February with four more astronauts. In the meantime, the space station remains occupied by NASA astronaut Christopher Williams and two cosmonauts who flew to the ISS aboard a Russian Soyuz spacecraft in November.