NASA has decided it’s too risky to bring two astronauts back to Earth in Boeing’s troubled new capsule, and they’ll have to wait until next year for a ride home with SpaceX. What should have been a weeklong test flight for the pair will now last more than eight months.

The seasoned pilots have been stuck at the International Space Station since the beginning of June. A cascade of vexing thruster failures and helium leaks in the new capsule marred their trip to the space station, and they ended up in a holding pattern as engineers conducted tests and debated what to do about the flight back.

After almost three months, the decision finally came down from NASA’s highest ranks on Saturday. The astronauts, Butch Wilmore and Suni Williams, will come back in a SpaceX capsule in February. Their empty Starliner capsule will undock in early September and attempt to return on autopilot with a touchdown in the New Mexico desert.

As Starliner’s test pilots, the pair should have overseen this critical last leg of the journey.

“A test flight by nature is neither safe nor routine,” said NASA Administrator Bill Nelson. The decision “is a result of a commitment to safety.”

Mr. Nelson said lessons learned from NASA’s two space shuttle accidents played a role. This time, he noted, open dialogue was encouraged rather than crushed.

“This has not been an easy decision, but it is absolutely the right one,” added NASA’s associate administrator, Jim Free.

It was a blow to Boeing, adding to the safety concerns plaguing the company on its airplane side. Boeing had counted on Starliner’s first crew trip to revive the troubled spacecraft program after years of delays and ballooning costs. The company had insisted Starliner was safe based on all the recent thruster tests both in space and on the ground.

Boeing did not participate in Saturday’s news conference by NASA, but released a statement: “Boeing continues to focus, first and foremost, on the safety of the crew and spacecraft.” The company said it is preparing the spacecraft “for a safe and successful return.”

Rand Corp.’s Jan Osburg, a senior engineer who specializes in aerospace and defense, said NASA made the right choice. “But the U.S. is still left with egg on its face due to the Starliner design issues that should have been caught earlier.”

Mr. Wilmore, 61, and Ms. Williams, 58, are both retired Navy captains with previous long-duration spaceflight experience. Before their June 5 launch from Cape Canaveral, they said their families bought into the uncertainty and stress of their professional careers decades ago.

During their lone orbital news conference last month, the astronauts said they had trust in the thruster testing being conducted. They had no complaints, they added, and enjoyed pitching in with space station work.

Mr. Wilmore’s wife, Deanna, said she and their daughters, along with family and friends, “were praying for a safe return on whatever spacecraft that may be.” While they are disappointed that he will be away longer, “we know that it’s the Lord’s plan,” she said via text.

Flight operations director Norm Knight said he talked to the astronauts Saturday and they fully support the decision to postpone their return.

There were few options.

The SpaceX capsule currently parked at the space station is reserved for the four residents who have been there since March. They will return in late September, their routine six-month stay extended a month by the Starliner dilemma. NASA said it would be unsafe to squeeze two more into the capsule, except in an emergency.

The docked Russian Soyuz capsule is even tighter, capable of flying only three — two of them Russians wrapping up a yearlong stint.

So Mr. Wilmore and Ms. Williams will wait for SpaceX’s next taxi flight. It’s due to launch in late September with two astronauts instead of the usual four. NASA is yanking two to make room for Wilmore and Williams on the return flight in late February.

NASA said no serious consideration was given to asking SpaceX for a quick stand-alone rescue. Last year, the Russian Space Agency had to rush up a replacement Soyuz capsule for three men whose original craft was damaged by space junk. The switch pushed their six-month mission to just over a year.

Despite Saturday’s decision, NASA isn’t giving up on Boeing. Mr. Nelson said he is “100%” certain that Starliner will fly again.

NASA went into its commercial crew program a decade ago wanting two competing American companies ferrying astronauts in the post-shuttle era. Boeing won the bigger contract: more than $4 billion, compared with SpaceX’s $2.6 billion.

With station supply runs already under its belt, SpaceX aced its first of now nine astronaut flights in 2020, while Boeing got bogged down in design flaws that set the company back more than $1 billion. NASA officials still hold out hope that Starliner’s problems can be corrected in time for another crew flight in another year or so.

Associated Press