Fixing Boeing's leaky Starliner — and returning NASA's stranded astronauts to Earth — is much harder than it sounds

For the past few weeks, NASA astronauts Butch Wilmore and Suni Williams have been stuck on the International Space Station (ISS) after the first crewed voyage of Boeing's new Starliner spacecraft hit a snag. Concerns raised by Boeing and NASA over thruster problems and several helium leaks (helium is used in Starliner's engine system) have prevented the craft from making the return journey as scheduled. NASA has now said the astronauts may have to stay put until the next scheduled crew switchover in August — potentially on another ship.

Boeing and NASA are trying to put a positive spin on the extension by saying they are testing systems required for longer Starliner missions. But the project has already suffered several delays, having originally been set to lift off for the first time with crew in 2017. This, combined with the latest problems raise questions over the whole Starliner programme.

Starliner was noted as having a small helium leak before it even launched. Helium is an inert gas (much like neon or xenon), meaning that it is very unreactive with other materials.

This makes it ideal when coming into contact with rocket fuel and high temperatures, although producing it is an expensive process. It is pressurised and used to push fuel into the engines at the correct rate. Helium leaks can mean that not enough fuel will reach a thruster.

The leak spotted while Starliner was on the launch pad was determined to be negligible and the spacecraft was sent to orbit regardless. However, this turned into a larger problem when additional helium leaks were identified following launch, meaning that several of the spacecraft's small manoeuvring thrusters couldn't be used.

Four of the five thrusters have been repaired while Starliner has been docked to the ISS, but it raises concern for other thrusters cutting out during the return journey to Earth. On Starliner's return, re-entering Earth's atmosphere requires a very specific "angle of attack" to ensure there is not too much friction heating up the vessel.

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