Boeing's Starliner return to Earth postponed, NASA says

NASA did not provide a new date on Friday, raising questions about when the mission's two astronauts will return as more testing and technical issues have created more delays.

The return to earth was previously scheduled for Wednesday.

The crew of US astronauts, Butch Wilmore and Suni Williams, lifted off June 5 as a final demonstration to obtain routine flight certification from NASA.

The crewed test of the spacecraft, which has been test-flown to space twice since 2019 without humans aboard, has encountered five failures of its 28 manoeuvring thrusters, five leaks of helium that is meant to pressurise those thrusters, and a slow-moving propellant valve that signalled unfixed issues from the past.

The issues and the additional tests NASA and Boeing have had to do call into question when exactly Starliner will be able to fly its crew home, and add to a list of broader problems Boeing faces with its Starliner program. The company has spent $US1.5 billion ($A2.2 billion) in cost overruns on top of its $US4.5 billion ($A6.7 billion) NASA development contract.

NASA wants Starliner to become a second US spacecraft capable of ferrying its astronauts to and from the ISS alongside SpaceX's Crew Dragon, which has been the agency's primary ride since 2020. Boeing's Starliner program has struggled with software glitches, design problems and subcontractor disputes for years.

When Starliner arrived in the space station's vicinity to dock on June 6, the five thruster failures prohibited the spacecraft from making a close approach until Boeing could implement a fix. The company rewrote software and tweaked some procedures to revive four of them and proceed with a docking.

Starliner's undocking and return to Earth represent the spacecraft's most complicated phases of its test mission. NASA officials have said they want to better understand the cause of the thruster failures, valve issue and helium leaks before Starliner embarks on its roughly six-hour return journey.

While only one thruster remains dead in Starliner's current flight, Boeing encountered four thruster problems during the capsule's uncrewed return from space in 2022.

According to flight rules established jointly by Boeing and NASA, Starliner's manoeuvring thrusters must, at a minimum, allow for "six-degrees of freedom of control," and each have one backup thruster, a NASA spokesperson told Reuters.

That could mean at least 12 of the 28 thrusters - most of which are backups - are required for a safe flight, or potentially fewer, as long as the remaining thrusters have one backup and can work together in a way that doesn't restrict Starliner's freedom of movement in space.

Australian Associated Press