Pilot Sunita "Suni" Williams and Commander Barry "Butch" Wilmore were onboard a test flight to the ISS on Boeing's newest spacecraft on 5 June.

They were meant to stay in orbit for eight days - however, issues with Starliner's propulsion system meant they were left stranded in space for months.

NASA confirmed in August that the two will not return to Earth until 2025, with SpaceX now in charge of rescuing the astronauts on a Crew Dragon flight.

The US's Nick Hague and Russia's Alexander Gorbunov are on board the capsule which blasted off on Saturday evening.

Mr Hague and Mr Gorbunov will not return with Ms Williams and Mr Wilmore - who will take two empty seats made clear on Crew Dragon - until February next year.

By then, the two stranded astronauts will have been in space for eight months.

Speaking before take-off, Mr Hague said: "There's always something that is changing [with spaceflight].

"Maybe this time it's been a little more visible to the public."

Arriving in Cape Canaveral last week, he also said: "We've got a dynamic challenge ahead of us.

"We know each other and we're professionals and we step up and do what's asked of us."

NASA deputy program manager Dina Contella said the two astronauts watched the SpaceX launch from the ISS, with Ms Williams cheering "go Dragon!"

Boeing's Starliner undocked from the ISS and flew back to Earth in September without the crew. It suffered multiple thruster failures and propulsion-system helium leaks on its way to the station.

And in a news conference from space the same month, Ms Williams and Mr Wilmore said the space station had become their "happy place".

"That's how it goes in this business," she said, adding that "you have to turn the page and look at the next opportunity".

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Mr Wilmore also added: "It's been quite an evolution over the last three months, we've been involved from the beginning through all the processes of assessing our spacecraft.

"And it was trying at times. There were some tough times all the way through."

Crew Dragon is set to dock with the ISS by 10.30pm on Sunday. A live stream will be available on NASA's website.