As NASA prepares to send four astronauts around the moon for the 10-day Artemis II mission, a veteran space flier's unexplained illness in orbit is spotlighting one of the biggest risks of deep-space travel: the need for medical systems in case of emergencies.

NASA astronaut Michael Fincke said a sudden episode aboard the International Space Station (ISS) in January left him unable to speak and forced NASA's first-ever medical evacuation from the orbiting laboratory. Doctors have ruled out a heart attack, Fincke told the Associated Press, but they still don't know what caused the medical issue.

NASA was able to get Fincke (along with the three other members of the crew) back to Earth relatively quickly from the ISS. But that may not be the case for the longer lunar missions the agency envisions under the Artemis program. As NASA works toward building a lasting human presence on and around the moon — including plans for a $20 billion lunar base — unexplained medical events like Fincke's may become harder to dismiss as one-off scares.

"It was completely out of the blue. It was just amazingly quick," Fincke said, according to the Associated Press. "My crewmates definitely saw that I was in distress. It was all hands on deck within just a matter of seconds."

Fincke, the pilot on NASA's SpaceX Crew-11 mission to the ISS, flew alongside NASA astronaut Zena Cardman, Japan Aerospace Exploration Agency astronaut Kimiya Yui and Roscosmos cosmonaut Oleg Platonov. Crew-11 conducted research on astronaut health and other space-station science while helping to keep the orbital laboratory running.

The episode hit on Jan. 7, 2026, while Fincke was eating dinner after preparing for a spacewalk. He said he felt no pain, and the event lasted around 20 minutes. Seeing Finke's distress, his crewmates quickly called flight surgeons back on Earth.

Fincke was more than five months into the mission when the medical issue happened. NASA used the station's ultrasound machine during the emergency, he said, and the agency is now reviewing astronaut medical records to see if anything similar has happened in space.

The incident shortened Crew-11's mission as NASA canceled the next day's spacewalk and brought Fincke and his three crewmates home early on Jan. 15.

For now, Fincke says he feels fine. Artemis II is nearing its scheduled April 2026 launch date, and Fincke was never scheduled for that mission ‪—‬ but the medical mystery highlights a key weakness that NASA must overcome before astronauts can safely live and work beyond Earth.

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