A SpaceX mission to return NASA’s two stranded astronauts from the International Space Station (ISS) was delayed on Wednesday after a launch pad problem.

The new crew needs to get to the International Space Station (ISS) before Butch Wilmore and Suni Williams can head home after nine months in orbit.

Concerns over a critical hydraulic system arose less than four hours before the Falcon rocket’s planned evening liftoff from NASA’s Kennedy Space Center in Florida.

As the countdown clocks ticked down, engineers evaluated the hydraulics used to release one of the two arms clamping the rocket to its support structure. This structure needs to tilt back right before liftoff.

Already strapped into their capsule, the four astronauts awaited a final decision, which came down with less than an hour remaining in the countdown. SpaceX canceled for the day.

Officials later said the launch was off until at least Friday.

Once at the space station, the US, Japanese, and Russian crew will replace Wilmore and Williams, who have been up there since June.

The two test pilots had to move into the space station for an extended stay after Boeing’s new Starliner capsule encountered major breakdowns in transit.

Starliner’s debut crew flight was supposed to last just a week, but NASA ordered the capsule to return empty and transferred Wilmore and Williams to SpaceX for the return leg.