SpaceX delayed its planned flight to the International Space Station (ISS) on Wednesday due to a launch pad issue, postponing the mission until at least Friday.

Axar.az reports that the delay affects a crew meant to replace NASA astronauts Butch Wilmore and Suni Williams, who have been stranded in orbit for nine months due to failures in Boeing’s Starliner capsule.

The problem arose less than four hours before liftoff, involving a hydraulic system needed to release one of the rocket’s support arms. Despite the crew being strapped in and awaiting takeoff, SpaceX ultimately canceled the launch with under an hour remaining.

Wilmore and Williams were originally set to return after a weeklong Starliner test flight, but major malfunctions forced NASA to keep them aboard the ISS and arrange for their return via SpaceX. The incoming U.S., Japanese, and Russian crew will replace them once the delayed launch proceeds.