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This included issues with some of Starliner's thrusters, which would be needed to slow the spacecraft for re-entry into the Earth's atmosphere, and leaks of helium gas in the propulsion system.

"It doesn't sink," says Mr Heald, explaining how it uses four horizontal and vertical thrusters to move in any direction.

However, technical issues including helium leaks and thruster malfunctions meant that the Starliner was unsafe for their return.

It found that crew members had believed there were problems with the ship's thruster when they could not change the vessel's direction.

They might get more interesting again if the young thruster Lennon Miller keeps progressing so rapidly.