Two NASA astronauts who flew to the International Space Station in June in Boeing’s malfunctioning Starliner capsule will be scheduled to return to Earth early next year in a SpaceX vehicle, NASA said Saturday. NASA has deemed problems with the Starliner’s propulsion system too risky to bring the first crew home.

The agency’s decision, tapping Boeing’s biggest space rival to send the astronauts back, is one of NASA’s most consequential in years. Boeing had hoped the test mission would salvage the Starliner program after years of development problems and more than $1.6 billion in budget overruns since 2016.

Boeing is also struggling with quality problems in the production of commercial airplanes, the company’s main products.

Veteran NASA astronauts Butch Wilmore and Suni Williams, both former military test pilots, were the first crew members to fly aboard the Starliner on June 5, when they launched to the ISS for what was expected to be an eight-day test mission.

But Starliner’s propulsion system suffered a series of failures that began in the first 24 hours of its flight to the ISS, causing months of consecutive delays. Five of its 28 thrusters failed, and multiple leaks of helium, which is used to pressurize the thrusters, occurred.

In a rare reshuffle of NASA’s astronaut operations, the two astronauts are now expected to return in February 2025 on a SpaceX Crew Dragon spacecraft scheduled to launch next month as part of a routine astronaut rotation mission. Two of the Crew Dragon’s four astronaut seats will remain empty for Wilmore and Williams.

Starliner will undock from the ISS without a crew and attempt to return to Earth, just as it would with astronauts on board.

Boeing struggled for years to develop Starliner, a candy-shaped capsule that was supposed to compete with Crew Dragon as the second U.S. option for flying astronaut crews to and from Earth orbit.

Starliner failed in an uncrewed test launch to the ISS in 2019, but largely succeeded in a 2022 retry attempt, which also included some problems with its booster. A crewed mission was required before NASA would certify the capsule for routine flights, but now Starliner’s certification path has been turned on its head.

Since Starliner docked with the ISS in June, Boeing has raced to investigate what caused the booster problems and helium leaks. The company has conducted tests and simulations on Earth to gather data it has used to convince NASA officials that Starliner is safe to fly the crew home.

But the results of those tests raised even more difficult technical questions and ultimately failed to assuage NASA officials’ concerns about Starliner’s ability to make the crewed return trip. That was the most daunting and complex part of the test mission.

NASA’s decision, and the Starliner’s now uncertain path to certification, will deepen the crisis facing Boeing’s new CEO, Kelly Ortberg. Ortberg began work this month on rebuilding the company’s reputation after a door panel blew off a 737 MAX passenger jet in January.