LOS ANGELES, California: Cutting short their mission at the International Space Station, four astronauts returned safely to Earth early on January 15.

One of the four astronauts reported an undisclosed serious medical condition, forcing the four to return home early. It was the first time NASA had cut short an ISS crew's rotation due to a health emergency.

Their SpaceX capsule splashed down in the Pacific off California, after a 10-hour-plus descent from the space station and fiery re-entry through Earth's atmosphere.

The Crew Dragon spacecraft called Endeavour landed in the ocean off San Diego at about 12:45 a.m. local time. NASA and SpaceX showed the landing live online. Soon after, dolphins were seen swimming near the capsule as it floated in the water.

The commander, NASA astronaut Zena Cardman, said over the radio, "It's good to be home." The other crew members were U.S. astronaut Mike Fincke, Japanese astronaut Kimiya Yui, and Russian cosmonaut Oleg Platonov.

Within an hour, SpaceX recovery teams lifted the capsule onto a ship and assisted the astronauts. It was their first fresh air in almost 24 weeks. All four smiled and gave a thumbs-up as they came out, and it was not clear who had the medical problem.

Because their bodies were weak after months in low gravity, the astronauts could not walk on their own. They were placed on special stretchers and taken to a medical area on the ship for checks. After that, they were to be flown to a nearby hospital for more tests.

NASA said on January 8 that the crew would return early because one astronaut had a serious medical condition requiring immediate treatment. NASA has not said which astronaut it was or what the problem is, for privacy reasons.

Fincke and Cardman were scheduled to perform a spacewalk last week, but it was canceled due to a medical concern. NASA later said the problem was not caused by an accident during work.

The four astronauts arrived at the space station after launching from Florida in August and spent 167 days in space. They left the station on January 14 and flew for about 10 and a half hours before landing in the ocean.

During the return, the capsule passed through Earth's atmosphere and became extremely hot. The astronauts' space suits and cooling systems helped keep them safe.

Parachutes slowed the capsule before it gently hit the water. Infrared video showed the parachutes opening during the final fall.

The next SpaceX crew, called Crew-12, is expected to launch to the space station in mid-February. Until then, the station is being staffed by one NASA astronaut and two Russian cosmonauts who arrived on a Russian spacecraft in November.