The extended difficulty of catching a ride home for two astronauts stranded in space for months is "trying", NASA astronauts Suni Williams and Butch Wilmore said on Friday. 

The pair will remain on the International Space Station (ISS) until at least February after the Boeing Starliner capsule they used to travel to the station returned to earth last week, after NASA ruled the capsule's technical problems posed too much risk for them to return with it. 

Their eight-day mission that began in June is now expected to last more than eight months.

As spacecraft pilots, "you don't want to see it go off without you, but that's where we wound up."

While they never expected to be up there nearly a year, as Starliner's first test pilots, they knew there could be problems that might delay their return. "That's how things go in this business," Ms Williams said.

The pair have requested absentee ballots so they can vote in the upcoming US presidential elections on November 5. 

Now full-fledged station crew members, the pair are chipping in on routine maintenance and experiments across the ISS.

Ms Williams will take over command of the space station in a few more weeks, Mr Wilmore told reporters during a news conference — only their second since blasting off from Florida on June 5.

The duo, along with seven others on board, welcomed a Soyuz spacecraft carrying two Russians and an American earlier this week, temporarily raising the station population to 12, a near record. 

Two more astronauts will fly up on SpaceX later this month with two capsule seats left empty for the NASA pair to make the return leg in February.

The transition to station life was "not that hard" since both had previous stints there, said Ms Williams, who logged two long space station stays years ago.

Mr Wilmore noted that if his adjustment wasn't instantaneous, it was "pretty close."

Both said they were missing their families and friends, but messages of support from across the blue globe they were orbiting helped.

Boeing's Starliner flight was the first crewed mission for the company, and endured a series of failures including thruster issues and helium leaks before it arrived at the ISS in early June.

Mr Wilmore said he and Ms Williams were "absolutely not" let down by NASA's decision to keep them orbiting for longer, saying the Boeing capsule had issues they were "just not comfortable" with.

The capsule landed safely back on Earth in the New Mexico desert earlier this month, but Boeing's path forward in NASA's commercial crew program remains uncertain.

The space agency hired SpaceX and Boeing as an orbital taxi service a decade ago after the shuttles retired. SpaceX has been flying astronauts since 2020.

AP