Four astronauts returned safely to Earth early Thursday after an undisclosed serious medical condition affecting one of them forced an end to their International Space Station (ISS) mission a few weeks early.
Their SpaceX capsule splashed down in the Pacific off California, capping a 10-hour-plus descent from the space station and fiery re-entry through Earth's atmosphere.
It was the first time that the National Aeronautics and Space Administration (Nasa) has cut short the rotation of an ISS crew due to a health emergency.
The Crew Dragon spacecraft dubbed Endeavour parachuted into calm seas off San Diego about 12.45am PST (0845 GMT).
The finale of the abbreviated mission was carried live by a Nasa-SpaceX webcast.
Moments later, several dolphins were visible swimming near the capsule, their dorsal fins breaking the surface of the ocean, as the spacecraft bobbed gently upright in the water.
"On behalf of SpaceX and Nasa, welcome home," mission control told the crew moments after landing.
"It's so good to be home, with deep gratitude to the teams that got us there and back," Nasa astronaut Zena Cardman, 38, replied.
Joining her on the return voyage were fellow US astronaut Mike Fincke, 58, Japanese astronaut Kimiya Yui, 55, and Russian cosmonaut Oleg Platonov, 39.
In less than an hour, SpaceX recovery teams had secured their heat-scorched capsule and hoisted it onto the deck of a retrieval vessel, then helped the astronauts out of the spacecraft for their first breath of fresh air in nearly 24 weeks.
Each of the crew members, still garbed in helmeted white-and-black space suits, smiled and gave a thumbs-up as they emerged and were helped to their feet.
It was not evident from their appearance which one was ailing.
Unable to bear their own weight on Earth after spending months in microgravity, the four were each assisted onto special gurneys and escorted to an onboard medical station for routine checkups at sea.
Afterward they were to be flown to a local hospital for further medical exams, SpaceX said.
The decision to bring all four home early was announced January 8, with Nasa Administrator Jared Isaacman saying one of the astronauts faced a "serious medical condition" that required immediate attention of doctors on the ground.
Isaacman was also present at mission control for the splashdown Thursday.
Nasa officials have not identified the crew member of concern or described the nature of the medical issue, citing privacy requirements.
Fincke, a retired Air Force colonel who has now logged five missions to space, and Cardman, a rookie astronaut and geobiologist, 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 characterised 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”.
As the 11th regular ISS crew flown to orbit by SpaceX, Cardman, Fincke, Yui and Platonov arrived at the space station following a launch from Florida in August.
They departed on Wednesday afternoon on a 10 1/2-hour flight home, ending a 167-day mission.
Crew-12 is expected to launch to the space station in mid-February with four more astronauts.
In the meantime, the orbiting laboratory remains occupied by Nasa astronaut Christopher Williams and two cosmonauts who flew to the ISS aboard a Russian Soyuz spacecraft in November.
The Russian Roscosmos space agency operates alongside Nasa on the outpost, and the two agencies take turns transporting a citizen of the other country to and from the orbiter – one of the few areas of bilateral co-operation that still endure between the United States and Russia.
Continuously inhabited since 2000, the ISS seeks to showcase multinational co-operation, bringing together Europe, Japan, the United States and Russia.
Located some 400km (248.5 miles) above Earth, the ISS functions as a testbed for research that supports deeper space exploration – including eventual missions to return humans to the Moon and onward to Mars.
