According to NASA director Bill Nelson, it is too risky to send astronauts Butch Wilmore and Suni Williams back to Earth on the defective Starliner.

Two NASA astronauts who flew to the International Space Station in June aboard Boeing’s malfunctioning Starliner capsule will return to Earth aboard a SpaceX vehicle early next year, NASA Administrator Bill Nelson said.

On Saturday, he told reporters that problems with Starliner’s propulsion system made it too dangerous to bring the first crew home.

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.

“NASA has decided that Butch and Suni will return with Crew-9 next February, and that Starliner will return unmanned,” Nelson said at a news conference in Houston.

He added that he discussed the agency’s decision with Boeing’s new CEO, Kelly Ortberg.

“He indicated to me that he planned to continue to resolve the issues once Starliner is safely back,” Nelson said of Ortberg.

Since Starliner docked with the ISS in June, Boeing has been actively investigating the cause of the thruster problems and helium leaks.

The company also conducted tests and simulations on Earth to gather data, trying to convince NASA officials that it was safe to fly the Starliner crew home.

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

Amy Thompson, a Florida-based aerospace and science journalist, said NASA is putting crew safety first by deciding not to send the astronauts back on the Boeing Starliner.

“The big concern here is: What do the leaks do to the spacecraft during re-entry? What happens if it gets superheated in the atmosphere? The same goes for the thrusters… These are things that NASA can now test without putting the crew at risk,” Thompson told Al Jazeera.

NASA’s decision, and Starliner’s now-uncertain path to certification, will add to the crises Ortberg faces. The CEO began his tenure this month with the goal of rebuilding the planemaker’s reputation after a door panel dramatically blew off a 737 MAX passenger jet in January.