Two NASA astronauts who flew to the International Space Station in June aboard Boeing's faulty Starliner capsule will need to return to earth on a SpaceX vehicle early next year, NASA chief Bill Nelson says, deeming issues with Starliner's propulsion system too risky to carry its first crew home.
The agency's decision, tapping Boeing's top space rival to return the astronauts, is one of NASA's most consequential in years.
Boeing had hoped its Starliner test mission would redeem the troubled program after years of development problems and more than $US1.6 billion ($A2.4 billion) in budget overruns since 2016.
Nelson, speaking with reporters at a news conference in Houston, said he discussed the agency's decision with Boeing's new CEO Kelly Ortberg.
"He expressed to me an intention that they will continue to work the problems once Starliner is back safely," Nelson said of Ortberg.
Boeing is also struggling with quality issues on production of commercial planes, its most important products.
Veteran NASA astronauts Butch Wilmore and Suni Williams, both former military test pilots, became the first crew to ride Starliner on June 5 when they were launched to the ISS for what was expected to be an eight-day test mission.
But Starliner's propulsion system suffered a series of 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.
In a rare reshuffling of NASA's astronaut operations, the two astronauts are now expected to return in February 2025 on a SpaceX Crew Dragon spacecraft due to launch next month as part of a routine astronaut rotation mission.
Two of the Crew Dragon's four astronaut seats will be kept empty for Wilmore and Williams.
Starliner will undock from the ISS without a crew and attempt to return to earth as it would have with astronauts aboard.
Boeing struggled for years to develop Starliner, a gumdrop-shaped capsule designed to compete with Crew Dragon as a second US option for sending astronaut crews to and from earth's orbit.
Starliner failed a 2019 test to launch to the ISS uncrewed but mostly succeeded in a 2022 do-over attempt where it also encountered thruster problems.
Its June mission with its first crew was required before NASA can certify the capsule for routine flights but now Starliner's crew certification path has been upended.
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.
NASA's decision, and Starliner's now-uncertain path to certification, will add to the crises faced by Ortberg, who started this month with the goal to rebuild the plane maker's reputation after a door panel dramatically blew off a 737 MAX passenger jet in midair in January.
