WASHINGTON: Boeing's beleaguered Starliner made its long-awaited return to Earth on Saturday without the astronauts who rode it up to the International Space Station (ISS), after Nasa ruled the trip back too risky.

After years of delays, Starliner launched in June for what was meant to be a roughly week-long test mission — a final shakedown before it could be certified to rotate crew to and from the orbital laboratory.

But unexpected thruster malfunctions and helium leaks en route to the ISS derailed those plans, and Nasa ultimately decided it was safer to bring crew-mates Butch Wilmore and Suni Williams back on a rival SpaceX Crew Dragon — though they'll have to wait until February 2025.

The gumdrop-shaped Boeing capsule touched down softly at the White Sands Space Harbor in New Mexico at 0401 GMT Saturday, its descent slowed by parachutes and cushioned by airbags, having departed the ISS around six hours earlier.

As it streaked red-hot across the night sky, ground teams reported hearing sonic booms. The spacecraft endured temperatures of 3,000ºFahrenheit (1,650ºC) during atmospheric reentry.

Nasa lavished praise on Boeing during a post-flight press conference where representatives from the company were conspicuously absent.

"It was a bullseye landing," said Steve Stich, program manager for Nasa's commercial crew program. "The entry in particular has been darn near flawless."

Still, he acknowledged that certain new issues had come to light, including the failure of a new thruster and the temporary loss of the guidance system.

He added it was too early to talk about whether Starliner's next flight, scheduled for August next year, would be crewed, instead stressing Nasa needed time to analyse the data they had gathered and assess what changes were required to both the design of the ship and the way it is flown.

Ahead of the return leg, Boeing carried out extensive ground testing to address the technical hitches encountered during Starliner's ascent, then promised — both publicly and behind closed doors — that it could safely bring the astronauts home. In the end, Nasa disagreed.

Asked whether he stood by that decision, Nasa's Stich said: "It's always hard to have that retrospective look. We made the decision to have an un-crewed flight based on what we knew at the time and based on our knowledge of the thrusters and based on the modeling that we had."