The test flight pilots will remain in space until February.

NASA decided Saturday that returning two astronauts to Earth in Boeing’s troubled new capsule was too dangerous, and they’ll have to wait until next year to fly home with SpaceX. What should have been a weeklong test flight for the duo will now take more than eight months.

The experienced pilots have been stuck on the International Space Station since early June. A series of propulsion failures and helium leaks in the new capsule ruined their trip to the space station, and they found themselves waiting while engineers ran tests and debated what to do about it.

After nearly three months, the decision finally came from NASA’s top brass on Saturday. Butch Wilmore and Suni Williams will return to the SpaceX spacecraft in February. Their empty Starliner capsule will separate in a week or two and attempt to return to autopilot.

As test pilots for the Starliner, the astronauts were supposed to oversee this crucial final stage of the journey, with a landing in the American desert.

It’s a blow to Boeing, adding to the company’s safety concerns about the aircraft. Boeing had been counting on the Starliner’s first crewed flight to revive the troubled program after years of delays and spiraling costs. The company has insisted that Starliner is safe based on all recent propulsion tests in space and on the ground.

The two retired Navy captains with extensive spaceflight experience, Mr. Wilmore, 61, and Ms. Williams, 58, expected surprises when they agreed to the mission to develop “a new spacecraft, but not quite that much.”

Before their June 5 launch from Cape Canaveral, Florida, they said their families had resigned themselves to the uncertainty and stress that had plagued their careers for decades. At their on-orbit press conference last month, they said they had confidence in the ongoing thruster tests. They added that they had no complaints and were happy to be working on the space station.

Mr. Wilmore’s wife, Diana, was equally stoic in an interview earlier this month with WVLT-TV in Knoxville, Tenn., their hometown. She was already preparing to postpone until next February: “We just have to get by.”

Few options

The SpaceX capsule currently stationed at the space station is intended for the four residents who have been there since March. They will return in late September, and their stay will be extended by a month because of the Starliner saga. NASA said it would be too dangerous to cram two more people in there, except in an emergency.

The Russian Soyuz capsule that docked is more compact, carrying only three people – two of them Russians who have completed their year-long stay.

Mr. Wilmore and Ms. Williams will then wait for SpaceX’s next taxi ride. It is scheduled to launch in late September with two astronauts instead of the usual four for a routine six-month stay. NASA removed two to make room for Mr. Wilmore and Ms. Williams on the return trip in late February.

NASA said it had not seriously considered a request for a rapid, independent rescue from SpaceX. Last year, the Russian space agency had to urgently bring a replacement Soyuz capsule for three men whose original spacecraft was damaged by space debris. The change extended their mission beyond a year, a U.S. record for space endurance still held by Frank Rubio.

Starliner's problems began long before its final flight.

Bad software ruined the first uncrewed test flight in 2019, forcing a restart in 2022. Then came problems with the parachute and others, including a helium leak in the capsule’s propulsion system that canceled a launch attempt in May. The leak was eventually deemed isolated and small enough not to pose a problem. But other leaks emerged after liftoff, and five thrusters failed.

All but one of these small batches have been restarted. But engineers remain puzzled as to why some of the seals bulged, clogging the propellant lines, and then returned to their normal size.

These 28 thrusters are essential. In addition to being necessary for rendezvous with the space station, they keep the capsule pointed in the right direction at the end of the journey while larger engines steer the craft out of orbit. A crooked arrival could spell disaster.

While the Columbia disaster was still fresh in the minds of many (the shuttle broke apart upon re-entry in 2003, killing all seven people on board), NASA launched an open debate about Starliner’s ability to return. Dissenting opinions were stifled during Columbia’s ill-fated flight, just as they had been during Challenger’s 1986 flight.

Despite Saturday's decision, NASA is not abandoning Boeing.

NASA embarked on its Commercial Crew Program a decade ago, wanting two competing American companies to ferry astronauts into the post-shuttle era. Boeing won the largest contract: more than $4 billion, compared to $2.6 billion for SpaceX.

With the stations already delivered, SpaceX successfully completed its first nine astronaut flights in 2020, while Boeing was mired in design flaws that cost the company more than $1 billion. NASA officials remain hopeful that Starliner’s problems can be fixed in time for another crewed flight in a year or two.