Four astronauts are buckling up for an unexpected return from the International Space Station (ISS) after one of them suffered a "serious medical condition".
It's the first time in its 25-year history that a mission is being cut short due to such an incident.
NASA will not say which of the four astronauts is affected due to confidentiality, nor will it state what the problem is.
The person's condition is said to be stable and the space agency's medical chief, Dr. James Polk, said last week they were "erring on the side of caution".
The crew in the Dragon Endeavour spacecraft undocked just after 10.20pm UK time and drifted away from the space station.
Splashdown off California is being targeted for 8.41am on Thursday, according to an ISS post on X.
The crew is made up of Americans Mike Fincke and Zena Cardman, Russian Oleg Platonov and Japanese astronaut Kimiya Yui.
They arrived at the ISS via a Space X mission in August and were intending on staying for at least six months.
Sky News science correspondent Thomas Moore says up to now, ISS astronauts, backed by a flight surgeon and other medics on the ground, have been able to deal with all medical incidents.
These have included a blood clot in the jugular vein of an unnamed astronaut a few years ago.
The ISS has more than 190 drugs on board as well as medical equipment, including an ultra-sound machine, defibrillator and an intravenous therapy kit.
The emergency means the first planned spacewalk of 2026 has been cancelled. Cardman and Fincke were planning to venture outside to make preparations for some new solar panels.
"I'm proud of the swift effort across the agency thus far to ensure the safety of our astronauts," said NASA boss Jared Isaacman.
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The evacuation leaves three others behind at the ISS; NASA's Chris Williams and Russia's Sergei Mikaev and Sergei Kud-Sverchkov.
They arrived in November and are not due home until the summer.
NASA is planning on taking the 400-tonne space station - which circles the Earth at around 17,500mph - out of orbit in late 2030 or early 2031.
The structure will be brought down in the ocean, and the vision is for privately-funded space stations to fill the gap.
(c) Sky News 2026: International Space Station astronauts begin evacuation due to 'serious medical condition'
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