For the first time, astronauts are being evacuated from the International Space Station (ISS) for medical reasons. The exact nature of the “medical situation” has not been identified for privacy reasons, nor has the astronaut it has affected. But four of the seven crew members on board the ISS are coming home early.

The astronauts returning to Earth are the members of the Crew-11 mission, which launched on 1 August and was originally planned to return in late February. In an 8 January press conference, NASA administrator said that the decision to return the astronauts early was made in part because the mission objectives are nearly all complete, so ending the mission early is not as big of a deal. 

“Because the astronaut is absolutely stable, this is not an emergent evacuation,” said NASA’s chief health and medical officer, , during the press conference. “We’re not immediately disembarking and getting the astronaut down.” Emergency medical evacuations could be performed in a matter of hours, Isaacman said, but in this case the crew will be evacuated in the next few days.

There have been medical incidents aboard the ISS before, but until now all of them have been minor enough that they could be dealt with in orbit. “We have a very robust suite of medical hardware on board the International Space Station, but we don’t have the complete amount of hardware that I would have in the emergency department, for example, to complete a workup of the patient,” said Polk. “The medical incident was sufficient enough that we were concerned enough about the astronaut [and] we would like to complete that workup, and the best way to complete that workup is on the ground.”

The four members of Crew-11 – two NASA astronauts, one Russian cosmonaut and one Japanese astronaut – will ride home aboard the same Dragon spacecraft that shuttled them to the ISS, despite only one of them being ill or injured. Several medical facilities on Earth will be prepared ahead of time to take in the affected crew member for tests and treatment. While this situation is unprecedented, the actual evacuation operation is not significantly different from how crews usually return to Earth at the end of their missions, Isaacman said.

“In the 25 years of the history of the International Space Station we’ve had many models… that have said that we should’ve had a medical evacuation approximately every three years… and we’ve not had one to date,” says Polk.

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The evacuation will leave just one NASA astronaut and two Russian cosmonauts aboard the ISS until the arrival of the Crew-12 mission, which was originally slated to launch around 15 February but may be moved up after this incident.

The most accurate clock in space launches within days and will begin building a highly synchronised network out of the best clocks on Earth. But the project, decades in preparation, will only operate for a few years before it burns up as the International Space Station deorbits at the end of the decade.

The Atomic Clock Ensemble in Space (ACES) is a European Space Agency (ESA) mission that will generate a time signal with unprecedented accuracy and then transmit it via laser to nine ground stations as it passes overhead at 27,000 kilometres per hour. This network of clocks will be in extremely close synchronisation and provide highly accurate timekeeping around the world.

The result is that ACES will be able to test, which says that the passing of time is affected by the strength of gravity, with great accuracy. It will also assist with research on everything from dark matter to string theory.

ACES is scheduled to launch on 21 April aboard a SpaceX Falcon 9 rocket from Kennedy Space Center, Florida. Once at the ISS, the Canadian Space Agency’s robotic arm – Canadarm2 – will attach it to the exterior of ESA’s Columbus laboratory, where it will remain in the vacuum of space.

The package actually comprises two clocks: one called SHM has the ability to remain stable for short periods, which will allow it to help calibrate the other, called PHARAO. Together, these clocks will be so accurate they would lose less than one second over 300 million years – 10 times more accurate than the clocks aboard GPS satellites.

PHARAO is fundamentally modelled on an atomic clock in Paris that occupies an entire room. Miniaturising that technology into something that takes up less than a cubic metre, and can also survive the rigours of a rocket launch and life in space, was no mean feat.

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To generate an accurate clock signal, PHARAO spews a fountain of caesium atoms cooled to near absolute zero and observes their interaction with microwave fields. On Earth, this requires a device up to 3 metres tall, but in microgravity these atoms can be sprayed in a slower-moving and smaller fountain, allowing it to be much smaller.

at ESA says that the device is so sensitive that simply putting a teaspoon near it could create an electromagnetic field strong enough to destroy the clock. “Just to put it in context, it’s better than a thousand million millionth of a second that we’re trying to measure here,” says Weinberg. “So it’s one hell of a challenging job.”

The concept for ACES dates back to the 1990s and was originally planned for launch on the Space Shuttle, which retired in 2011. Once it gets to space, the first signal won’t arrive at an Earthbound clock for a year and a half – it will take around six months to commission the device, and then a year’s worth of measurement will be needed to isolate noise and remove it from the clock signal.

After that, ACES will operate until 2030, after which the ISS will be deliberately crashed into Earth’s atmosphere and burned up. By that point, new super-accurate timepieces known as optical clocks are likely to have made atomic clocks all but obsolete on Earth, although they may not be small or robust enough for use in space by that time.

Weinberg says that at some point ESA will look to launch a new generation of ACES to replace what is lost on the ISS, whatever the most appropriate technology is at the time. “We would be a long way off from doing that, and we would have to gather together the support and the financing and so on to make sure that happened.”