CAPE CANAVERAL, Fla. (AP) — A launch pad problem prompted SpaceX to delay a flight to the International Space Station on Wednesday to replace NASA’s two stuck astronauts.

The new crew needs to get to the International Space Station 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. 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.