Elon Musk’s SpaceX will use one of its capsules to bring home astronauts stuck at the International Space Station next year, in a fresh setback for rival Boeing Co that also raises questions about how Nasa will staff the orbiting lab moving forward.

Boeing’s flawed Starliner craft will return without people on board in early September, the US space agency said during a Saturday news conference announcing its decision.

The contingency plan means that Nasa astronauts Barry “Butch” Wilmore and Sunita “Suni” Williams will hitch a ride home on SpaceX’s rival Crew Dragon capsule during a six-month mission, called Crew-9, slated to launch in late September. That would put them back on US soil in February - months later than they had planned to come home.

“The decision to keep Butch and Suni aboard the International Space Station and bring the Boeing Starliner home uncrewed is a result of a commitment to safety,” Nasa administrator Bill Nelson told reporters, citing the loss of two Space Shuttle crews in the agency’s past.

Nelson later added that he was 100% certain Starliner would launch with crews again.

This is another significant blow to the Starliner programme. In 2019, Boeing botched an uncrewed test flight of the capsule that failed to reach the space station as planned. This was followed by years of delays and glitches that cost the company some US$1.6 billion in additional charges.

As a result, Boeing was roughly seven years late in launching its first crew on Starliner, while fellow Nasa partner SpaceX continued to launch Nasa crews routinely on its Crew Dragon spacecraft. Now Boeing must face the embarrassment of having its rival carry home astronauts that Starliner was supposed to bring back.

The two astronauts, who arrived at the ISS on Boeing’s CST-100 Starliner test flight on June 6, were originally to remain for roughly a week. They’re now facing an eight-month stay in orbit.

Meanwhile, SpaceX will need to reconfigure the seats on its Crew-9 Dragon capsule to allow for a launch of only two astronauts instead of the planned four, and allow for Wilmore and Williams to return with them.

Astronauts will separately reconfigure SpaceX’s Crew-8 capsule, currently docked at the ISS, to be able to carry six astronauts home in the case of an emergency.

“SpaceX stands ready to support Nasa however we can,” SpaceX president Gwynne Shotwell said in a social media post.

Nasa made its decision against a backdrop of a close US presidential race between Vice President Kamala Harris and former president Donald Trump. Harris is chair of the National Space Council, so the effects of the decision may reverberate in the weeks before the election.

Nelson said politics wasn’t a factor. “Nasa is not only bipartisan, it is nonpartisan, and that’s the way we try to operate this agency.”

The stakes of the decision were high regardless of world events.

“Frankly, every call Nasa makes has similar potential ramifications when you’re dealing with human spaceflight,” said Lori Garver, the former deputy administrator of Nasa. “Obviously the worst thing would be loss of crew.”

Boeing now faces questions about its future with Nasa. Starliner’s crewed flight to the station was part of a critical test to determine whether the spacecraft could regularly carry people to and from the ISS. Like Musk’s SpaceX, Boeing holds a contract with Nasa to routinely bring crews to the space station until its planned retirement in 2030.

Nelson said he recently spoke with new Boeing chief executive officer Kelly Ortberg, who told him the company was committed to continuing the Starliner programme. “He expressed to me an intention that they will continue to work the problems once Starliner is back safely and that we will have our redundancy and our crewed access to the space station,” Nelson said.

Boeing continues to focus on the safety of the crew and spacecraft, a company spokesperson said in a statement. The company is executing the mission as determined by Nasa and preparing the spacecraft for a safe and successful uncrewed return.

Williams and Wilmore successfully launched on Starliner June 5, becoming the first two fliers to journey inside the vehicle to space. Before the pair took off, Nasa and Boeing had spotted a helium leak in Starliner, but ultimately determined the mission could continue without fixing the source of the leak.

However, as Starliner attempted to dock with the space station, the vehicle experienced additional helium leaks, and a handful of the vehicle’s thrusters - engines the capsule uses to maneuver through space - failed and had to be rebooted. But Starliner still docked June 6.

While Starliner’s been attached to the ISS, Nasa and Boeing engineers have been running a series of tests and analysing data to determine why the thrusters failed. They’ve also run thruster tests on the ground at Nasa’s White Sands facility.

Those tests were not entirely conclusive, so opinions at Nasa varied on how the thrusters failed and whether Starliner was dependable enough to bring Wilmore and Williams home.

On Saturday, after repeated delays for additional analysis, Nasa said one of the key factors in its decision was uncertainty over how the thrusters would behave on the return to Earth.

“As we got more and more data over the summer and understood the uncertainty of that data, it became very clear that the best course of action was to return Starliner uncrewed,” Nasa official Steve Stich told reporters.

Nasa originally picked both Boeing and SpaceX to have separate options for getting people to and from the ISS. That way, if one was grounded because of a problem, Nasa still had an American-made vehicle to staff the ISS.

“When one stumbles, that’s not good for the US space programme,” said Clayton Swope, the deputy director of the Aerospace Security Project at the Center for Strategic and International Studies. “We all are hopeful that there is a robust and competitive and diverse space ecosystem in the US, and that’s what Nasa is looking for in this case.”

The helium leaks and thruster issues will need to be studied and fixed before Starliner’s next flight, though that would have been the case if Williams and Wilmore had returned on the vehicle.

If Starliner arrives safely on Earth without a crew on board, it’s possible NAsa may still certify the vehicle for future crewed missions. However, Nasa didn’t say how that will happen or if another crewed test flight will be required.

“We’ll have to sit down and talk about the certification aspects after the flight,” Stich said. “It’s a little premature to do that at this point.”

Garver, who helped to spearhead Nasa’s commercial crew programme, said it has had success.

“You got a fully operational human spaceflight capability for a fraction of the cost of what it would have been for Nasa to develop it separately,” Garver said of SpaceX. “I think having two providers is required for ultimately where we want to go, which is a robust economic sphere in low-Earth orbit.”