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.”

Pilot whales are highly social animals and have a natural instinct to look out for one another.

Volunteers hope to bring the 15 stranded whales together in a “nice tight group” to help them re-familiarise with one another and swim out together, Hawkes said.

They will attempt to refloat the whales this afternoon, but time is tight. “We have to do all of that before the tide turns and drops again,” Hawkes said.

The group is calling for volunteers to help with the refloating.

New Zealand’s conservation department has deployed rangers, a boat and a drone to Farewell Spit to monitor any further strandings.

Mass strandings regularly occur at Farewell Spit, located on the northern-most tip of the South Island.

The conservation department describes it as a “naturally occurring ‘whale trap'” located along a migratory route for long-finned whales.

“Whales may be easily deceived and caught out by the gently sloping tidal flats and a rapidly falling tide,” it said in a statement on Thursday.

In February 2017, more than 400 long-finned pilot whales washed up there – the largest stranding in New Zealand for more than 100 years.

[BBC]