Suni Williams, left, and Butch Wilmore before their June 5 launch from Cape Canaveral. (AP Photo: Chris O'Meara)
In short:
NASA astronauts Suni Williams and Butch Wilmore's return to Earth will be further delayed until at least late March, the space agency says.
The duo had travelled to the ISS in June on what was intended to be a brief test mission, but their return has now been extended by at least nine months.
What's next?
Ms Williams and Mr Wilmore will now return to Earth after the four-member Crew-10 mission, now expected to take off in March, leaves the space station.
NASA astronauts Suni Williams and Butch Wilmore's return to Earth will be further delayed until at least late March, the space agency said, taking what should have been an eight-day stay on the International Space Station (ISS) to more than nine months.
The duo had travelled to the ISS in June on what was intended to be a brief test mission, but their return was extended by eight months to February after the Boeing Starliner capsule they arrived on was deemed unfit to safely return them to Earth.
NASA said Ms Williams and Mr Wilmore, along with NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov, would now return to Earth after the four-member Crew-10 mission, now expected to take off in late March, reaches the space station.
The pair have kept busy during their time on the ISS, helping with repairs as full-time crew members. (NASA via AP)
The agency did not specify an exact date for the return of the astronauts. Colonel Hague and Mr Gorbunov boarded the ISS in September, over three months after Ms Williams and Mr Wilmore.
"Known as a handover period, it [the delay] allows Crew-9 to share any lessons learned with the newly arrived crew and support a better transition for ongoing science and maintenance at the complex," the agency added in the statement on Tuesday.
The Crew-10 mission was originally slated to launch in February. NASA said the delay was to give the teams time to complete processing on a new SpaceX Dragon spacecraft for the mission.
Journey plagued by delays and setbacks
Ms Williams and Mr Wilmore's Starliner test mission was supposed to spell an end to a series of delays and setbacks that had plagued Boeing's space venture since the beginning.
The need for private companies such as Boeing and SpaceX to develop manned spacecraft was created after the US government retired its own space shuttle programs more than a decade ago.
The reality of life as an astronaut
Photo shows Boeing starliner
NASA hired both Boeing and SpaceX to provide an orbital taxi service for its astronauts, however Boeing ran into so many problems on its first unmanned Starliner test flight in 2019 that it had to repeat it.
The 2022 do-over uncovered even more flaws and the repair bill topped $US1 billion ($1.5 billion).
SpaceX, on the other hand, has launched almost a dozen crew ferry flights for NASA since 2020.
NASA officials have stressed that the space agency remains committed to having two competing US companies transporting astronauts.
The goal is for SpaceX and Boeing to take turns launching crews — one a year per company — until the space station is abandoned in 2030 right before its fiery re-entry.
That does not give Boeing much time to catch up, but the company intends to push forward with Starliner, according to NASA.
Thruster problems led NASA to play it safe
As veteran astronauts and retired US Navy captains, Ms Williams and Mr Wilmore would have anticipated hurdles on their test flight — if not to this extent.
But even before they launched on June 5 from Florida's Cape Canaveral, Starliner's propulsion system was leaking helium.
The leak was small and thought to be isolated, but four more leaks appeared after lift-off — and then five thrusters failed as the capsule attempted to dock with the ISS.
Boeing's Starliner docked to the ISS Harmony module's forward port in July. (Supplied via AP: NASA)
Although four of the thrusters were recovered, it gave NASA pause as to whether more malfunctions might hamper the capsule's descent from orbit.
Boeing conducted numerous thruster tests in space and on the ground following the shaky docking, and its scientists were convinced Starliner could safely bring the astronauts back.
But NASA could not get comfortable with the thruster situation, and chose to go with SpaceX for the astronauts' eventual return.
Flight controllers conducted more test firings of the capsule's thrusters after it undocked from the ISS in September for its return to Earth — without its crew.
One of the thrusters failed to ignite, despite NASA Mission Control calling the undocking a "perfect" departure.
Engineers suspect the more the thrusters were fired, the hotter they became, causing protective seals to swell and obstruct the flow of propellant.
They weren't be able to examine any of the parts, however, as the section holding the thrusters was ditched just before re-entry.
Since Starliner's return, three astronauts and one cosmonaut have also returned to Earth from the ISS on a SpaceX capsule, after spending nearly eight months on board.
Ms Williams and Mr Wilmore were not able to join them in that capsule because they are limited to a crew of four.
Reuters/AP