The two astronauts stuck at the International Space Station since June welcomed their new ride home with Sunday's arrival of a SpaceX capsule.

SpaceX launched the rescue mission on Saturday with a downsized crew of two astronauts and two empty seats reserved for Butch Wilmore and Sunita Williams, who will return next year. The Dragon capsule docked in darkness high over Botswana as the two craft soared 420 kilometres above Earth.

NASA switched Wilmore and Williams to SpaceX following concerns over the safety of their Boeing Starliner capsule. It was the first Starliner test flight with a crew, and NASA decided the thruster failures and helium leaks that cropped up after liftoff were too serious and poorly understood to risk the test pilots' return. So Starliner returned to Earth empty earlier this month.

The Dragon carrying NASA's Nick Hague and the Russian Space Agency's Alexander Gorbunov will remain at the space station until February, turning what should have been a weeklong trip for Wilmore and Williams into a mission lasting more than eight months.

Two NASA astronauts were pulled from the mission to make room for Wilmore and Williams on the return leg.

NASA likes to replace its station crews every six months or so. SpaceX has provided the taxi service since the company's first astronaut flight in 2020. NASA also hired Boeing for ferry flights after the space shuttles were retired, but flawed software and other Starliner issues led to years of delays and more than USD 1 billion in repairs.

Starliner inspections are underway at NASA's Kennedy Space Center, with post-flight reviews of data set to begin this week.

"We're a long way from saying, 'Hey, we're writing off Boeing'," NASA's associate administrator Jim Free said at a pre-launch briefing.

The arrival of two fresh astronauts means the four who have been up there since March can now return to Earth in their own SpaceX capsule in just over a week. Their stay was extended a month because of the Starliner turmoil.

Although Saturday's liftoff went well, SpaceX said the rocket's spent upper stage ended up outside its targeted impact zone in the Pacific because of a bad engine firing. The company has halted all Falcon launches until it figures out what went wrong.