NASA has postponed a spacewalk outside the International Space Station (ISS) and announced the early return of its crew due to a medical issue that arose on Wednesday (Jan. 7) with one of the astronauts on board, the agency said.

The spacewalk was planned for 8 a.m. ET on Thursday (Jan. 8) to finish preparing a power channel where a new solar array is set to be installed on the ISS. American astronauts Mike Fincke and Zena Cardman were scheduled to exit the space station for 6.5 hours in what would have been Cardman's first spacewalk. (Fincke has already performed nine spacewalks.)