Despite a known helium leak, the launch went ahead on June 5, launching veteran Nasa astronauts Barry 'Butch' Wilmore and Sunita 'Suni' Williams into orbit.

The mission was meant to last eight days, but almost three weeks later, the pair are still on the space station – and no one knows when they'll come home.

Multiple helium leaks, five dead manoeuvering thrusters and a propellant valve fail means crews in space and on the ground in Houston are racing to fix the craft. 

So far, Nasa has rescheduled the six-hour return journey three times. There is currently no official scheduled return, although sources inside Nasa suggested the target is July 6.

But the team cannot wait indefinitely.

When will the Nasa astronauts return to Earth?

Starliner can stay docked at the ISS for up to 45 days, according to comments by Nasa's commercial crew manager Steve Stich, because the Harmony module that it is attached to has limited fuel.

This means a cut-off of July 21 – the craft arrived at the ISS on June 6, a day after lift-off.

If absolutely necessary, such as if more problems arise that mission officials cannot fix in time, it could stay docked for up to 72 days, relying on various backup systems, according to a person familiar with flight planning. This would be August 17.

What is wrong with the Boeing Starliner capsule?

To set off back to Earth, Starliner's propulsion system will need to be fixed, which is needed to back the capsule away from the ISS and position it to dive through Earth's atmosphere. Many of Starliner's thrusters have overheated when fired, and the leaks of helium – used to pressurise the thrusters – appear to be connected to how frequently they are used, according to Mr Stich.

He said recent test-firings of the thrusters while Starliner remains docked gave mission teams confidence in a safe return, though tests and reviews are ongoing.