Why are the Boeing Starliner astronauts still in space?

The two astronauts testing out Boeing’s new Starliner spacecraft were supposed to begin making their way back to Earth on Wednesday night but instead they will stay on the International Space Station (ISS).

The vessel's return to Earth had already been delayed because of issues with some of its thrusters and leaks of the helium gas which pushes fuel into the propulsion system. Nasa is carrying out a high-level review of the technical problems before deciding when to bring its astronauts home. Suni Williams and Butch Wilmore are in no danger, but what has gone wrong with the spacecraft and what does this mean for their journey home?

Starliner was launched on 5 June despite there being a small leak of helium gas. Helium is used to push propellent to the thruster systems used for maneuvering in space and slowing down to re-enter the Earth’s atmosphere. The leak was extremely small and engineers believed that it would not affect the mission and so went ahead with the launch.