What you need to know about Nasa's SpaceX Crew 9 ‘rescue mission’

A "rescue mission" due to bring home two astronauts stuck at the International Space Station (ISS) has arrived at its destination, Nasa and SpaceX have said.

Butch Wilmore and Suni Williams have been trapped in Space after the Boeing Starliner capsule they arrived on in June was beset by problems and deemed unfit to return them to Earth.

NASA astronaut Nick Hague and Roscosmos cosmonaut Aleksandr Gorbunov boarded the ISS shortly after the Dragon capsule docked at the station on Sunday night.

Wilmore and Williams, who were the first crew to fly on the troubled Starliner, are now due to return home with Hague and Gorbunov on Crew Dragon in February next year, as what was supposed to be an eight-day mission has turned into an eight-month ordeal.

How did Crew 9 become a rescue mission?

Wilmore and Williams boarded their Boeing Starliner spacecraft to the ISS in June.

After years of delays, technical glitches and supply chain mishaps, it was meant to be a successful final test before Nasa could certify Starliner for routine flights.

Wilmore and Williams arrived on 6 June and were due to stay at the ISS for about eight days, but Starliner's propulsion system malfunctioned and Nasa deemed the troublesome thrusters unsafe for the return journey – leaving the pair without a ride home.

In August, it was announced that the two former military test pilots would instead be brought home in Crew Dragon capsule manufactured by SpaceX, the latest humiliation for Boeing at the hands of Elon Musk's dominant space company.

Nasa administrator Bill Nelson said at the time: “Space flight is risky, even at its safest and even at its most routine, and a test flight by nature is neither safe nor routine, and so the decision to keep Butch and Suni aboard the International Space Station and bring the Boeing Starliner home uncrewed is a result of a commitment to safety."

The Crew-9 mission was originally set to be launched no earlier than 18 August, but was pushed back a month to allow more time for the analysis of issues with Boeing's Starliner spacecraft.

The launch was then pushed back again, until 28 September, due to Tropical Storm Helene.

What will happen next?

The two Nasa astronauts will come back to Earth on board the Dragon craft with a planned return date of February 2025.

By that point, although they will have spent eight months in space, both astronauts have said they are ‘grateful’ for the extra time on board the ISS.

Wilmore said in September: “We are pushing the edges of the envelope in everything that we do. And it is not easy. It's not an easy thing to do, but that's not why we do it. Maybe we do it because it's hard.”

Williams added: “We're both navy. We've both been on deployments. We’re not surprised when deployments get changed.”

In the meantime, the two will continue their work formally as part of the Expedition 71/72 crew through to February 2025.