Nasa is considering an early return of its crew aboard the International Space Station (ISS) due to an unspecific medical issue. The rare move comes after the agency canceled a spacewalk initially scheduled for Thursday, according to Reuters. “Safely conducting our missions is our highest priority, and we are actively evaluating all options, including the possibility of an earlier end to Crew-11’s mission,” a NASA spokesperson said in a statement on Wednesday night. The astronaut suffering from the medical concern, who was not identified, remains in stable condition, the outlet reported. In a previous statement, NASA revealed it was “monitoring a medical concern with a crew member that arose Wednesday afternoon. The four-person Crew-11 crew consists of U.S. astronauts Zena Cardman and Mike Fincke, Russian cosmonaut Oleg Platonov and Japanese astronaut Kimiya Yu. Fincke, the mission’s commander, and flight engineer Cardman were anticipated to complete a 6.5 hour spacewalk on Thursday to install hardware outside the ISS. Astronauts typically live aboard the ISS for stints of six to eight months, and are equipped with basic medical equipment and medications in the event of an emergency. The crew set off for their current eight-month mission from florida in August and were set to return to Earth in May. Spacewalks are highly strenuous and involve months of training.