NASA and SpaceX canceled Wednesday’s planned launch of the Crew-10 mission to retrieve astronauts from the International Space Station (ISS), citing an issue with the launch pad.

The delay means NASA astronauts Butch Wilmore and Suni Williams will have to wait at least another day or two before they can head home to Earth after nine months in orbit.

Officials called off the launch less than an hour before the scheduled lift-off on Wednesday, with the four astronauts already strapped into their capsule as they awaited a final decision.

NASA said the delay was a result of “a hydraulic system issue with a ground support clamp arm for the Falcon 9 rocket.”

SpaceX officials said the launch is now postponed to Friday at the earliest.

The otherwise routine crew rotation on the space station has garnered attention since the Crew-10 mission must dock at the space station before Wilmore and Williams can be relieved from their extended stay in space.

Wilmore and Williams were part of the first crewed flight of the Boeing Starliner in early June last year, a mission that was supposed to last only eight to 10 days. They managed to successfully dock with the space station, but concerns about the Starliner prevented them from making the return trip as planned. Instead, they joined the Crew-9 mission and extended their stay by several months.

NASA’s crew rotations on the space station usually last for a few months, and one crew member only departs when the other arrives.