Nasa has said it will return a four-person crew aboard the International Space Station (ISS), cutting short their mission a month early because of a "serious medical condition" affecting one of the astronauts.
The agency did not disclose the name of the crew member or the medical condition, citing health privacy, but said the person was in a stable condition.
"This is not an emergency evacuation," said a Nasa official, adding: "We always err on the side of the astronaut's health."
On Wednesday, Nasa abruptly cancelled a spacewalk due to take place on Thursday, when two astronauts were set to step outside the ISS, citing a "medical concern".
The decision to bring back the crew early was announced by Nasa Administrator Jared Isaacman and other agency officials at a news conference on Thursday.
They gave few details, but said the medical issue was not related to space operations, and was not an injury.
An update was expected within 48 hours on the timeline for the astronauts' return, they added.
This is the first early evacuation in the history of the ISS, which has been continuously inhabited since 2000.
The four-person team is called Crew-11, and consists of Nasa astronauts Zena Cardman and Mike Fincke, Kimiya Yui from Japan's JAXA space agency, and a Russian cosmonaut, Oleg Platonov.
One American astronaut will remain aboard the ISS, officials say, and will be accompanied by two Russian cosmonauts.
Dr James Polk, Nasa's chief health and medical officer, told reporters this is the first time in Nasa's over-65 year history that a mission would return early due to a medical issue.
Crew-11 launched to the ISS in August last year on a SpaceX Crew Dragon and were expected to remain in orbit for about six months, returning around next month, after being replaced by another four-person crew a few days earlier.
The ISS has basic medical equipment, supplies and communication systems that let doctors on Earth talk privately to astronauts in space, assess their condition and advise treatment, much like a secure video or phone consultation with a GP.
Returning the four-person crew early could delay some experiments and maintenance tasks until the new crew arrive next month, according to Dr Simeon Barber, a space scientist at the Open University.
"The space station is a big, complex feat of engineering, it's designed to be operated by a certain minimum level of crew," he said.
He added that the remaining crew would probably be forced to "dial back on some of the more experimental work and focus more just on the housekeeping and keeping the station healthy, waiting for the full complement of crew to be restored."
Source : BBC