The Falcon 9 rocket propelling the NASA-SpaceX Crew-10 mission was set to blast off from Florida's Kennedy Space Center on Wednesday at 7:48 pm (2348 GMT).

But with around 45 minutes left on the countdown clock and the four member team strapped in, the launch was scrubbed because of a technical issue.

"There was an issue with the hydraulic system on the ground side," NASA launch commentator Derrol Nail said, adding that "everything was fine with the rocket and the spacecraft itself."

A Federal Aviation Administration space advisory showed the next launch opportunity is on Thursday, with a backup on Friday.

NASA duo Butch Wilmore and Suni Williams have been stranded aboard the ISS since June after their Boeing Starliner spacecraft developed propulsion issues and was deemed unfit for their return. It came back empty.

Wilmore and Williams were initially slated for an eight-day mission but were reassigned to Crew-9 after its astronauts arrived in September aboard a SpaceX Dragon. The spacecraft carried only two crew members instead of the usual four to make room for Wilmore and Williams.

That Crew-9 spacecraft now docked at the ISS can only return to Earth after the one carrying Crew-10 arrives.

"We came up prepared to stay long, even though we plan to stay short," Wilmore said in a recent news conference. "That's what your nation's human space flight program is all about, planning for unknown, unexpected contingencies."

Crew-9 was set to depart on Sunday, weather permitting, for an ocean splashdown off the Florida coast. It was not immediately clear if that timeline was still feasible.

Along with Wilmore and Williams, NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov will also be aboard the returning Dragon capsule.

Space remains an area of cooperation between the United States and Russia despite the Ukraine conflict, with cosmonauts traveling to the ISS aboard SpaceX Crew Dragons and astronauts doing the same via Soyuz capsules launched from Kazakhstan.

- Political flashpoint -

Wilmore and Williams's prolonged stay has recently become a political flashpoint, as President Donald Trump and his close advisor Elon Musk have accused ex-president Joe Biden's administration of abandoning the pair.

SpaceX boss Musk has suggested, without providing specifics, that he had offered Biden a "rescue" mission outside of the routine crew rotations.

However, with Trump now in office for nearly two months, the astronauts are still set to return as originally planned.

The issue recently sparked a heated online exchange between Musk and Danish astronaut Andreas Mogensen, whom Musk called a slur for mentally disabled people. Several retired astronauts quickly came to Mogensen's defense.

One astronaut who backed Musk however was Wilmore, who offered contradictory statements in last week's press conference.

"I can only say that Mr. Musk, what he says is absolutely factual," he said, seemingly endorsing the SpaceX founder's version of events, before adding "politics is not playing into this at all."

The Crew-10 team consists of NASA astronauts Anne McClain and Nichole Ayers, Japan's Takuya Onishi, and Russia's Kirill Peskov.

McClain, the mission's commander, will be making her second trip to space.

"I'm looking forward to breaking bread with those guys, talking to them, giving them big hugs," she said of Wilmore and Williams.

During their mission, the new crew will conduct a range of scientific experiments, including flammability tests for future spacecraft designs and research into the effects of space on the human body.

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