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.