NASA today picked Elon Musk’s SpaceX to bring back Sunita Williams and Barry Wilmore from space next year. Eighty days ago, the two astronauts arrived at the International Space Station aboard Boeing’s Starliner for an 8-day mission. They were forced to extend their stay because of major technical issues with the Boeing capsule.

The astronauts are expected to return in February next year, after spending a total of eight months in orbit, on a SpaceX Crew Dragon spacecraft due to launch next month as part of a routine astronaut rotation mission.

NASA chief Bill Nelson said Starliner’s propulsion system is too risky to carry its first crew home. Starliner will undock from the ISS without a crew and attempt to return to Earth as it would have with astronauts aboard.

The space agency’s decision to pick Boeing’s top space rival marked a fresh setback to the Starliner test mission. Boeing had hoped the mission would redeem the troubled program after years of development problems and over $1.6 billion in budget overruns since 2016.

Nelson said he discussed the agency’s decision with Boeing’s new CEO Kelly Ortberg.

The SpaceX Crew-9 mission will take off in late September, but carrying only two passengers instead of the originally planned four.

It will remain moored to the ISS until its scheduled return in February, bringing back its own crew members plus their two stranded colleagues.

The veteran NASA astronauts, both former military test pilots, became the first crew to ride Starliner on June 5 when they were launched to the ISS.

Starliner’s propulsion system suffered multiple glitches beginning in the first 24 hours of its flight to the ISS, triggering months of cascading delays. Five of its 28 thrusters failed and it sprang several leaks of helium, which is used to pressurise the thrusters.

Since Starliner docked to the ISS in June, Boeing has scrambled to investigate what caused its thruster mishaps and helium leaks. The company arranged tests and simulations on Earth to gather data that it has used to try and convince NASA officials that Starliner is safe to fly the crew back home.

But results from that testing raised more difficult engineering questions and ultimately failed to quell NASA officials’ concerns about Starliner’s ability to make its crewed return trip – the most daunting and complex part of the test mission.