When two American astronauts blasted off on a test mission to the International Space Station on 5 June, they were expecting to be back home in a matter of days.

But things didn't quite go to plan.

In fact, Barry “Butch” Wilmore and Sunita Williams are still there, floating high above the Earth nearly two months later.

The pair - who are stuck indefinitely - now face the sudden prospect of missing the summer entirely and even spending Christmas and New Year in space.

Mr Wilmore, 61, and Ms Williams, 58, flew a Boeing Starliner spacecraft to the station. It was the first flight of its kind with people on board and was a test designed to see how the new spacecraft performs before it is used more regularly.

Problems, however, emerged as it made its approach. These included leaks in its propulsion system and some of its thrusters shutting down.

So while they made it to the space station safely, they will need an alternative mode of transport to get home if the Starliner is not deemed safe to return to Earth.

At a news briefing on Wednesday, Nasa officials said no firm decisions had been made when it comes to next steps.

"Our prime option is to return Butch and Suni on Starliner," Steve Stich, manager of Nasa's Commercial Crew Program, said. "However, we have done the requisite planning to make sure we have other options open.”

One potential option being considered, they said, is to attach the two astronauts to a mission that is scheduled to launch in September, and return them to earth with that mission in February 2025.

That flight to the space station will be made by a SpaceX Crew Dragon craft. The initial plan was for four crew members to be aboard, but two of the seats could be left empty if needed.

That plan would mean the astronauts would spend more than eight months – rather than eight days – aboard the International Space Station (ISS).

If the Crew Dragon is used, the Starliner craft would be returned to Earth without any crew, under computer control.

Nasa officials said it could take a week or more for a final decision to be made.

Ken Bowersox, Nasa's director of space operations, told reporters the chances of an uncrewed return of the Starliner “have increased a little bit based on where things have gone over the last week or two.”"That's why we're looking more closely at that option to make sure that we can handle it," he said.

Using a SpaceX craft to return the astronauts would be a blow to Boeing, which has for years tried to compete with the company and its more experienced Crew Dragon.

Earlier this week, Nasa used a SpaceX rocket to deliver more food and supplies to the ISS, including extra clothes for the two astronauts.

Last month, in a short press briefing, the pair said they were "absolutely confident” in the return trip and Starliner was “truly impressive.”

This is the third stint aboard the ISS for Ms Williams, a retired Navy helicopter pilot, while Mr Wilmore is a former fighter jet pilot who has been to space twice before.

“We've been thoroughly busy up here, integrated right into the crew,” Ms Williams told reporters during a recent briefing call.

“It feels like coming back home. It feels good to float around. It feels good to be in space and work up here with the International Space Station team," she said. "So yeah, it’s great to be up here.”

Boeing was hoping that the maiden Starliner mission would pave the way for regular use of its capsule for missions back and forth to the station. The Space X Crew Dragon has been approved for Nasa missions since 2020.

Although the astronauts will spend much more time in space than they initially planned, others have spent much longer periods above the Earth's surface. Russian Valeri Polyakov spent 437 days in space in aboard the Mir space station in the mid-1990s.

Last year, Frank Rubio returned from the ISS after 371 days, the longest time an American has spent in space.

And Russia's Oleg Kononenko, also currently aboard the ISS, is the first person to have spent more than 1,000 days in space during the course of their career.

In their briefings and interviews, the two Americans have been upbeat about their situation. "I’m not complaining that we’re here for a couple extra weeks," Ms Williams said last month.

As things stand, the pair may be there for many more weeks to come.