Nasa has started an evacuation of astronauts from the International Space Station for the first time in history.
A SpaceX Dragon capsule carrying four astronauts left the ISS on Wednesday afternoon after one of them suffered a medical emergency.
Crew-11 boarded their spacecraft on Wednesday afternoon, sealing the hatch at 2.29pm ET (7.29pm GMT).
The capsule departed the orbital outpost just under three hours later, beginning a near 10-hour trip back to Earth.
Splashdown is anticipated off the California coast at around 3.40am local time (11.40am GMT) on Thursday.
The crew will fire their engines briefly at roughly 2.50am to slow down, allowing gravity to draw it towards Earth.
Nasa duo Zena Cardman and Mike Fincke are aboard alongside Japanese astronaut Kimiya Yui and Russian cosmonaut Oleg Platonov.
One of the Crew-11 spacemen experienced a serious medical episode last week, though Nasa has not disclosed which astronaut was affected.
The SpaceX Dragon capsule (pictured) left the ISS on Wednesday afternoon
|NASA
Dr James Polk, Nasa's chief medical officer, said the astronaut was in a stable condition and faced no immediate danger.
New Nasa boss Jared Isaacman made the decision to bring the crew home early as a precautionary measure.
The medical situation was deemed serious enough to warrant additional treatment on Earth.
Dr Polk clarified that the astronaut's condition was unrelated to a cancelled spacewalk on January 8 - or any other station operations.
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PICTURED: The moment the Dragon capsule decouples from the ISS
|NASA
Crew-11 reached the space station on August 1 last year, having originally been scheduled to remain in orbit until late February.
The four astronauts were due to depart only after their replacements arrived aboard a fresh SpaceX Dragon capsule.
Replacement team Crew-12's launch had been planned for no earlier than February 15.
This represents the first occasion in the station's history that a crew has had their mission terminated ahead of schedule due to health concerns.
Nasa duo Zena Cardman and Mike Fincke, Japanese astronaut Kimiya Yui and Russian cosmonaut Oleg Platonov are on their way home
|GETTY
Officials have also confirmed that any potential acceleration of Crew-12's launch will not affect the upcoming Artemis II mission.
Mr Isaacman described the two launches as "totally separate campaigns," suggesting there should be no complications in launching Artemis on schedule.
Artemis II will be the first crewed spaceflight to orbit the moon since 1972 - and is set to blast off in a matter of weeks.