Boeing's beleaguered Starliner left the International Space Station (ISS) on Friday, bound for Earth without the astronauts who rode up on the spaceship after NASA deemed the risk too great.

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

But unexpected thruster malfunctions and helium leaks on its way up derailed those plans, and NASA ultimately decided it was safer to bring astronauts Butch Wilmore and Suni Williams back on a rival SpaceX Crew Dragon -- though they'll have to wait until February 2025.

Starliner autonomously undocked from the space station at 6:04 pm Eastern Time (2204 GMT), and is set to land at White Sands Space Harbor in New Mexico at approximately 0403 GMT.

"It is time to bring Calypso home," Williams radioed to mission control, using the spaceship's nickname.

A smooth, uneventful ride is critical not only for salvaging some pride but also for Boeing's prospects of achieving certification in the future.

The century-old aerospace giant had carried out extensive ground testing aimed at replicating the technical issues the spaceship had experienced on its ascent, and devised plans to prevent more problems.

With its reputation already battered by safety concerns affecting its passenger jets, Boeing made assurances in public and in private that it could be trusted to bring the astronauts home, an assessment not shared by NASA

The Starliner program has been plagued by setbacks.

In 2019, during a first uncrewed test flight, a software defect meant the capsule failed to rendezvous with the ISS. A second software bug could have caused a catastrophic collision between its modules, but was caught and fixed just in time.

Then in 2021, with the rocket on the launchpad for a new flight, blocked valves forced another postponement. (AFP)