Who is the sick astronaut on the ISS? What we know.

NASA will bring four astronauts home from the International Space Station days earlier than planned after one crew member experienced a medical issue that requires further testing. 

In a news conference Thursday evening, agency leaders said the astronaut is "absolutely stable" but needs a full diagnostic workup that can't be done in the lab orbiting 250 miles above the planet. The return from space marks the first controlled medical evacuation from the station in its 25 years of continuous operations, highlighting the rarity of the situation and the limits of treating complex health problems away from Earth.

New NASA Administrator Jared Isaacman said ending the mission a little early is in the best interest of Crew-11. The astronauts — Commander Zena Cardman, pilot Mike Fincke, Japan’s Kimya Yui, and Russia’s Oleg Platonov — will ride SpaceX's Dragon Endeavour capsule back to Earth within days. 

Once Dragon departs, American astronaut Chris Williams, who arrived on a Russian Soyuz spacecraft in November, will be the only U.S. crew member left. Russian colleagues will help Williams manage systems and experiments until Crew‑12's flight launches, potentially as soon as mid‑February. 

Here's what we know about the unprecedented medical evacuation:

Who is the sick astronaut on the ISS?

NASA is deliberately trying to keep the name of the affected astronaut confidential, citing medical privacy practices. During the Jan. 8 briefing, officials didn't disclose identifying information — not even slipping up with any gendered pronouns in their comments. 

What we do know is that just prior to the announcement of the medical evacuation, NASA had canceled a scheduled spacewalk — a venture outside the space station — specifically because of the medical concern that arose Wednesday afternoon. NASA astronauts Fincke and Cardman were expected to perform that operation, related to an installation of solar panels.

JAXA, NASA's Japanese space counterpart, released a statement that clarified Yui is not the crew member dealing with the health issue, according to Japanese news reports. 

What is the medical condition on the ISS?

Though NASA’s chief health and medical officer J.D. Polk said he wouldn't talk specifics about a diagnosis, he and other officials repeatedly referred to the circumstance as "serious." 

NASA has only confirmed that the astronaut is now "stable," but officials haven't said if there was ever a time when the astronaut was not stable. They declined to provide enough details to infer that. 

When asked whether physical preparations for the spacewalk were involved in the problem, Dr. Polk clarified that it was unrelated to pre-breathing procedures. Before a spacewalk, astronauts breathe pure oxygen for a set period to flush nitrogen out of their bloodstream. 

Inside the space station, air is like Earth's, with a mix of nitrogen and oxygen. In the spacesuit, the pressure is much lower and mostly oxygen. A sudden switch from one to the other could cause the dissolved nitrogen in a person's body to form bubbles and cause the bends, like in scuba diving. Pre-breathing is intended to reduce the risk of decompression sickness. 

"This actually had nothing to do with the operational environment in preparing for a spacewalk at all," Dr. Polk said. "This was totally unrelated to any operations on board."

Dr. Polk also emphasized that the medical issue wasn't an injury in the course of work, though he stopped short of saying whether it was some other kind of injury. 

"It's mostly having a medical issue in the difficult areas of microgravity," he said. 

What medical emergencies have happened before? 

Astronauts have handled many medical issues over a quarter-century at the space station, but most fall in the category of urgent-care problems, like toothaches and ear pain. 

Public reports also describe astronauts managing minor injuries and infections, back pain, motion sickness, headaches, and vision changes with telemedicine and the onboard medical kit. They've even dealt with a blood clot in the neck, treating it with injections of a blood-thinning drug and later oral medication, once it arrived on a cargo resupply ship.

"We've had many models … that have said we should have had a medical evacuation approximately every three years in that 25-year history," Dr. Polk said. 

Is the astronaut evacuation an emergency?

Technically speaking, no. NASA describes it as a "controlled expedited return," not an emergency deorbit for the crew. That might seem like semantics, but the difference indicates the situation's perceived level of severity.

An emergency deorbit would mean departing the station as soon as possible, regardless of the landing timing, sea conditions, weather, and other risks, which NASA is capable of doing, Isaacman said. An expedited return, on the other hand, sticks to approved landing zones and mission rules. The latter type of departure indicates the astronaut's life isn't considered in immediate danger.

Despite the crew member remaining stable, NASA wants to bring the person home because the station lacks the full diagnostic hardware normally found in a hospital, officials said.

When will the SpaceX Dragon return to Earth?

NASA hasn't announced an exact landing date yet, but the ship will undock in the "coming days." Officials said they would provide another update by Saturday evening on the departure timeline. NASA will choose the date after teams match the spacecraft’s readiness with weather and recovery conditions.

The splashdown itself should look pretty standard, according to the agency. NASA intends to use the normal recovery convoy to pick up the crew. Every Dragon ship is outfitted with emergency gear, and flight surgeons are always on the recovery ship for every landing, with contingency hospitals waiting on standby when astronauts return. 

"This is no different in that respect," Dr. Polk said.