Suni Williams and Butch Wilmore have been stuck in space for nine months. (AP: Chris O'Meara)
In short:
SpaceX has delayed a rescue mission for the return of two astronauts stranded on the International Space Station.
The delay was due to a last-minute technical issue with the rocket's launch pad.
What's next?
It was not immediately clear when the next launch opportunity would be, but SpaceX and NASA could try to launch again in the coming days.
SpaceX has delayed a mission for the return of two astronauts stranded on the International Space Station (ISS) over a last-minute technical issue with the rocket's launch pad.
NASA launch commentator Derrol Nail said there was "an issue with the hydraulic system on the ground side," but "everything was fine with the rocket and the spacecraft itself".
Concerns over the critical hydraulic system arose less than four hours before the Falcon rocket's planned evening lift-off from NASA's Kennedy Space Center.
It was not immediately clear when the next launch opportunity would be.
The reason for the scrub suggests that SpaceX and NASA could try to launch again in the coming days.
Stranded US astronauts to return home after nine months in space
Photo shows three people in a space capsule
NASA had been set to launch a SpaceX rocket from Florida carrying a replacement crew for the ISS in a mission that would set up the return of US astronauts Butch Wilmore and Suni Williams.
The pair have been stuck in space for nine months after a trip on Boeing's faulty Starliner.
The US space agency had moved up the mission by two weeks after President Donald Trump and SpaceX CEO Elon Musk called for the astronauts to be brought back earlier than NASA had planned.
A planned eight-day stay on the orbiting station has dragged on for Mr Wilmore and Ms Williams, a pair of veteran astronauts and US Navy test pilots.
Starliner returned to Earth without them last year.
SpaceX's rocket had been scheduled to launch from Kennedy Space Centre in Cape Canaveral at 7:48 pm, local time, with a crew of two US astronauts and one astronaut each from Japan and Russia.
'Roller coaster' for families
Mr Wilmore and Ms Williams have been working on research and maintenance with the space station's other astronauts and have remained safe, according to NASA.
Ms Williams told reporters earlier this month that she was looking forward to seeing her family and pet dogs upon returning home.
"It's been a roller coaster for them, probably a little bit more so than for us," Ms Williams said.
"We're here, we have a mission — we're just doing what we do every day, and every day is interesting because we're up in space and it's a lot of fun."
Butch Wilmore and Suni Williams inspecting safety hardware aboard the International Space Station. (NASA via AP))
The flight, known as Crew-10, normally would be considered a routine astronaut rotation.
It had become entangled in politics as Mr Trump and Mr Musk sought to blame former president Joe Biden for the delayed return of Mr Wilmore and Ms Williams without evidence.
The demands by Mr Trump and Mr Musk for an earlier return were an unusual intervention in NASA's human spaceflight operations.
Astronauts will travel back in capsule
The mission previously had a target date of March 26, but NASA swapped a delayed SpaceX capsule with a different one that would be ready sooner.
When the new crew arrives aboard the station, Mr Wilmore and Ms Williams and two others — NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov — can return to Earth.
They are set to travel back in a capsule that has been attached to the station since September, as part of the prior Crew-9 mission.
Mr Wilmore and Ms Williams cannot leave until the new Crew-10 craft arrives in order to keep the ISS staffed with enough US astronauts for maintenance, according to NASA.
Mr Wilmore and Ms Williams flew to the station in June as the first test crew of the Starliner, which suffered propulsion system issues in space.
The Boeing's Starliner spacecraft launched Butch Wilmore and Suni Williams to the International Space Station. (AP: NASA)
NASA deemed it too risky for the astronauts to fly home on the Boeing craft, which led to the current plan to bring them home in a SpaceX capsule.
Boeing built Starliner under a $US4.5 billion ($7.1 billion) contract with NASA to compete with SpaceX's Crew Dragon capsule.
Last year's mission had marked Starliner's first test flight with astronauts aboard, a requirement before NASA could certify the capsule for routine astronaut missions.
Starliner's development has been plagued with engineering issues and cost overruns since 2019, putting it far behind SpaceX's Crew Dragon, which was developed under a similar NASA contract worth at least $US4 billion.
Reuters/Wires