The space agency said returning Sunita “Suni” Williams and Barry “Butch” Wilmore onboard the Boeing spacecraft was too risky as engineers still hadn’t fully deduced what went wrong with its thrusters.
The news means Starliner will now return to Earth without crew early next month, while two of the four astronauts originally due to blast off on the upcoming SpaceX Crew-9 mission will lose their seats.
After a series of delays, Starliner finally blasted off to the ISS in June on its historic first crewed mission. The spacecraft was due to come back after just a week, but issues with both the thrusters and helium leaks mean their stay will now extend to eight months.
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Previously, both Boeing and NASA have strongly denied any suggestion the pair would not return on Starliner, despite the potential to send it back home autonomously without crew.
Over the last two months, teams on Earth have been trying to duplicate the loss of performance in the thrusters, with researchers concluding that a Teflon seal had heated and expanded, which constrained the flow of oxidiser.
However, on Saturday, Steve Stich, NASA commercial crew program manager, revealed there was still “too much uncertainty” in predicting how the thruster would perform after undocking.
“There was just too much risk for the crew, and so we decided to pursue the uncrewed test flight,” he said.
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Ken Bowersox, NASA’s associate administrator for space operations, added that one important factor in making the call was that teams didn’t know much about how they could use the thrusters on the way back home “before we encounter a problem”.
In a boost to Boeing, though, Bowersox said the decision was a close call and only made last week.
“With more time, we might have gotten a lot smarter, but we’re just at the point where we need to bring Starliner home,” he said.
Bill Nelson, NASA’s administrator, also attempted to downplay criticism of Boeing by arguing spaceflight is risky “even at its routine” and a test flight “by nature is neither safe nor routine”.
“So the decision to keep Butch and Suni aboard the ISS and bring the Boeing Starliner home uncrewed is a result of a commitment to safety,” he said. “Safety is our North Star of what we are trying to do in a very hostile environment, in which if you make a mistake, it’s very unforgiving.”
The current Starliner mission is the final test flight before NASA certifies the vehicle for regular operational missions.
Starliner was initially due to blast off to the ISS in early May, but the first attempt was scrubbed at the last minute because of a faulty valve on the rocket’s Centaur upper stage.
A subsequent try was also repeatedly delayed, this time due to a helium issue. Finally, a third problem was found to be linked to a flange in a thruster in the spacecraft’s service module.
May’s scrubbed launches are the latest in years of issues for Starliner, which Boeing hopes will be able to regularly send US astronauts into space much like SpaceX’s Dragon capsule.
Starliner’s first attempt at a flight without humans onboard failed in 2019 due to software glitches, but it eventually docked with the ISS in May 2022.
Adam Thorn
Adam is a journalist who has worked for more than 40 prestigious media brands in the UK and Australia. Since 2005, his varied career has included stints as a reporter, copy editor, feature writer and editor for publications as diverse as Fleet Street newspaper The Sunday Times, fashion bible Jones, media and marketing website Mumbrella as well as lifestyle magazines such as GQ, Woman’s Weekly, Men’s Health and Loaded. He joined Momentum Media in early 2020 and currently writes for Australian Aviation and World of Aviation.
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