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.