SpaceX postponed a mission on Wednesday to bring the next crew set to work on the International Space Station (ISS) and begin the return of a pair of astronauts back to Earth.

The launch’s postponement was announced on Wednesday evening ahead of what would have been the scheduled launch time.

There was an issue with the hydraulic system on the launch side. It is a ground issue with the launch tower and not a problem with the spacecraft, according to SpaceX.

SpaceX said a backup window of Thursday at 7:48 p.m. ET and Friday at 7:03 p.m. ET would be available.

Barry “Butch” Wilmore and Sunita “Suni” Williams have been in space since June 2024 after they performed the first crewed test flight of Boeing’s Starliner. When they launched, they were only supposed to be on the ISS for about a week.

However, NASA and Boeing officials decided to send the uncrewed Starliner back to Earth in September after several issues and keep Wilmore and Williams onboard until early 2025 when Crew-10 was ready to launch on the Dragon spacecraft. Wilmore and Williams are set to return in the Crew-9 capsule.

The pair integrated with the ongoing Crew-9 mission aboard the ISS and could not return to Earth until Crew-9 completed its six-month mission and were replaced by Crew-10.

Wilmore and Williams assisted the crew with research and other responsibilities. However, NASA officials said the pair were using up more supplies meant for the ISS crew.

Steve Stich, program manager for NASA’s Commercial Crew Program, said that NASA teams spent all summer looking over the data on Starliner and felt there was too much risk with regard to the vehicle’s thrusters.

During a press conference in September, Wilmore said he and Williams did not feel let down by anything during the mission.

“Let down? Absolutely not,” Wilmore said. “It’s never entered my mind. It’s a fair question. I can tell you, I thought a lot about this press conference … and what I wanted to say and convey.”

“NASA does a great job of making a lot of things look easy,” he said, adding, “That’s just the way it goes. sometimes because we are pushing the edges of the envelope in everything that we do.”

If the mission is successful, it’s unclear when exactly Wilmore and Williams will return to Earth on Crew-9.

The crew consists of two NASA astronauts, an astronaut from the Japan Aerospace Exploration Agency and an astronaut from Russia’s Roscosmos.

SpaceX will share a live webcast of the mission beginning one hour and 20 minutes prior to liftoff on its website and on its X account. NASA will also air coverage on its X account.

“During their time on the orbiting laboratory, the crew will conduct new research to prepare for human exploration beyond low-Earth orbit and to benefit humanity on Earth,” SpaceX said on its website.

SpaceX’s contracted missions are part of the larger Commercial Crew Program at NASA, which are certified to perform routine missions to and from the ISS.