A launch pad issue forced SpaceX to delay its mission to the International Space Station (ISS) on Wednesday, postponing the arrival of a replacement crew for NASA astronauts Butch Wilmore and Suni Williams, who have been stranded in orbit for nine months.The new crew must reach the ISS before Wilmore and Williams can return home. The delay came after concerns arose about a critical hydraulic system less than four hours before launch. Engineers investigated an issue with one of the two arms holding the Falcon rocket to its support structure, which must retract before liftoff.The four strapped-in astronauts awaited a final decision as the countdown continued, but with less than an hour to go, SpaceX called off the launch. The company has yet to confirm a new date but indicated the next attempt could happen as soon as Thursday night.Once at the ISS, the incoming U.S., Japanese, and Russian astronauts will take over for Wilmore and Williams, who have been aboard since June. The two test pilots were originally set to return on Boeing's Starliner capsule, but critical failures during its first crewed flight forced NASA to extend their stay and transfer them to SpaceX for their journey home.Upon arrival at the ISS, the U.S., Japanese, and Russian astronauts will take over duties from Wilmore and Williams, who have been in orbit since June. The two test pilots were forced into an extended stay on the ISS after Boeing's Starliner capsule suffered major malfunctions during its inaugural crewed mission.Originally planned as a one-week test flight, Starliner's mission was cut short due to critical in-flight breakdowns. NASA ultimately decided to return the capsule empty and reassigned Wilmore and Williams to SpaceX for their long-awaited journey home.​In October 2024, SpaceX's Crew-8 mission successfully returned four astronauts to Earth after an extended stay aboard the ISS. The crew—NASA astronauts Matthew Dominick, Michael Barratt, Jeanette Epps, and Russian cosmonaut Alexander Grebenkin—splashed down safely in the Gulf of Mexico near Pensacola, Florida, on October 25, 2024, at 3:29 a.m. ET.Their mission, initially slated for six months, was prolonged due to technical issues with Boeing's Starliner spacecraft, which had been intended for their return. ​The delays were compounded by the need to retrofit SpaceX's Crew Dragon Endeavour with additional seating to accommodate potential emergencies and by adverse weather conditions, including Hurricane Milton.Despite these challenges, the crew's return marked the longest mission for SpaceX's Crew Dragon to date, with the astronauts spending 235 days in space.Upon landing, the astronauts were transported to a medical facility for standard evaluations, underscoring the mission's success and SpaceX's pivotal role in ensuring the safe return of astronauts amidst unforeseen complications. ​This is a developing news story and will be updated as more information is available.Reporting by the Associated Press contributed to this story.