Washington DC - Two astronauts stuck in space may not be able to return to Earth until 2025 due to issues with Boeing's new Starliner capsule, NASA said on Wednesday.

The two crew members of the Starliner, US astronauts Barry Wilmore and Sunita Williams, have been stranded on the ISS since June, although they were only supposed to stay one week.

After launching from the Cape Canaveral space center in Florida, the spacecraft was only able to dock with the ISS on a second attempt due to engine problems. In addition, the spacecraft suffered a helium leak.

Two options to return the astronauts to Earth are currently being discussed, NASA said during a teleconference.

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The first option involves the pair returning on the Starliner. However, the spacecraft would need to be fixed first.

The other option is to bump two of the four astronauts from the SpaceX Crew 9 mission, due to launch in September after being delayed due to the ongoing troubles. Wilmore and Williams could then return to Earth with the crew in February.

A final decision on when and how the pair will return will be made once NASA had completed a top-level flight readiness review, the agency said.

The Starliner is a partially reusable spacecraft that consists of a capsule around 3 metres high for the crew and a service module. Unlike the Crew Dragon vehicle of Elon Musk's company SpaceX, it does not land on water but on dry land.