NEW YORK (AP) — NASA canceled its first spacewalk of the year and may bring its crew back early from the International Space Station due to an onboard medical issue.

The spacewalk was supposed to happen on Thursday, but was postponed because of an unspecified medical concern with an astronaut, who was not identified, the space agency said.

NASA said the crew member is now stable, but it’s “actively evaluating all options” including an early end to the mission.

“Safely conducting our missions is our highest priority,” said NASA spokesperson Cheryl Warner in a statement.

The U.S.-Japanese-Russian crew of four have been at the orbiting lab since August after launching from Florida.