After a launch pad problem forced a flight delay Wednesday, SpaceX will try again Friday to send a mission to the International Space Station 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. On Wednesday evening, concerns over a critical hydraulic system arose less than four hours before the Falcon rocket's planned 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. Already strapped into their capsule, the four astronauts - NASA astronauts Anne McClain and Nichole Ayers, along with Japan Aerospace Exploration Agency astronaut Takuya Onishi and Russian cosmonaut Kirill Peskov - awaited a final decision, which came down with less than an hour remaining in the countdown. SpaceX canceled for the day. Friday's planned launch is scheduled for 7:03 p.m. Eastern Time. Once at the space station, the U.S., 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.