Two astronauts who have spent months aboard the International Space Station will have to stay there months longer after Nasa decided on Saturday they could not return on Boeing’s troubled Starliner space vehicle. They will return instead on a SpaceX capsule next year.

That decision finally brings clarity to the saga of the two Nasa astronauts, Sunita Williams and Butch Wilmore, who docked at the space station as part of a test flight of the Boeing vehicle. It also extends months of difficult problems experienced by Boeing, a dominant aerospace company that has faced embarrassing setbacks in its much larger civilian aviation and defense divisions this year.

“A test flight by nature is neither safe nor routine,” Bill Nelson, the Nasa administrator, said during a news conference, “and so 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.”

Williams and Wilmore were to have stayed at the ISS for at least eight days. Problems almost inevitably pop up during test flights like this one, the first with people aboard the Starliner spacecraft, and it was not at first a surprise that their stay stretched out a couple of weeks longer.

But even after lengthy analysis and ground testing, engineers still could not say with certainty why several of Starliner’s thrusters had malfunctioned before the capsule and the astronauts docked with the space station in June.

That lingering uncertainty spurred unease and led Nasa leaders to decide they should not risk the lives of Williams and Wilmore on Starliner. Instead they elected to rely on a different spacecraft — the Crew Dragon, built by Elon Musk’s SpaceX — for the return trip.

Starliner will undock and return to Earth in early September without anyone aboard. The next launch of a SpaceX Crew Dragon is scheduled for no earlier than September 24.

It will carry only two astronauts instead of the usual crew of four for a six-month mission at the space station. That leaves two seats for Williams and Wilmore, who will remain in orbit and become full-fledged members of the space station crew.

That will extend their stay at the space station to eight months.

The four astronauts — Williams, Wilmore and the two astronauts launching on Crew Dragon in September — are to return to Earth around February.

New York Times News Service