A NASA astronaut was taken to the hospital for an undisclosed medical issue after returning from a nearly eight-month space station stay extended by Boeing's capsule trouble and Hurricane Milton, the space agency said on Friday.

A SpaceX capsule carrying three Americans and one Russian parachuted before dawn into the Gulf of Mexico just off the Florida coast after undocking from the International Space Station mid-week.

Soon after splashdown, NASA said one of its astronauts had a “medical issue" and the crew was flown to a hospital in Pensacola, Florida, as a precaution.

The astronaut, who was not identified, was in stable condition and remained at the hospital as a "precautionary measure”, the space agency said in a statement.

The others returned to Houston.

It can take days or even weeks for astronauts to readjust to gravity after living in weightlessness for several months.

The astronauts should have been back two months ago. But their homecoming was stalled by problems with Boeing's new Starliner astronaut capsule, which came back empty in September because of safety concerns. Then Hurricane Milton interfered, followed by another two weeks of high wind and rough seas.

SpaceX launched the four — NASA's Matthew Dominick, Michael Barratt and Jeanette Epps, and Russia's Alexander Grebenkin — in March. Barratt, the only space veteran going into the mission, acknowledged the support teams back home that had “to replan, retool and kind of redo everything right along with us ... and helped us to roll with all those punches”.

Their replacements are the two Starliner test pilots Butch Wilmore and Suni Williams, whose own mission went from eight days to eight months, and two astronauts launched by SpaceX four weeks ago. Those four will remain up there until February.

The space station is now back to its normal crew size of seven — four Americans and three Russians — after months of overflow.