A SpaceX capsule departed the International Space Station (ISS) carrying four astronauts on Wednesday, in an emergency return flight due to the ailing health of a crew member.

This is the first time NASA has cut short a space crew's mission due to a health emergency. Officials did not identify which of the four astronauts on board was facing the said issues, citing privacy concerns.

The affected crew member "was and continues to be in stable condition," NASA official Rob Navias said.

The Crew Dragon capsule, dubbed Endeavor, undocked from the ISS and began its descent orbit at about 5:20 p.m. EST (2220 GMT) and will splash down in the Pacific Ocean off the California coast after a nearly 11-hour flight.

A live NASA webcast showed the departure with the crew members strapped into their seats, seated side-by-side. 

Who's is onboard the Endevaor?

The SpaceX Crew-11 consists of two US astronauts, Zena Cardman, 38, and Mike Fincke, 58; Japanese astronaut Kimiya Yui, 55; and Russian cosmonaut Oleg Platonov, 39.

They had flown to the space station from Florida in August and were originally meant to stay till mid-February.

Finckle, a retired Air Force colonel and the flight's pilot, said everyone was safe: "First and foremost, we are all OK. Everyone on board is stable, safe, 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." 

NASA cites 'lingering risk'

NASA's chief medical officer, James Polk, said the decision was made due to a "lingering risk" and "question as to what that diagnosis is."

Cardman, the other US astronaut on board, was scheduled for a six-hour space walk last week to install hardware outside the space station.

This space walk was canceled on January 7 over a "medical concern" with an astronaut, one which did not involve "an injury that occured in pursuit of operations," Polk had said. 

Edited by: Sean Sinico