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Boeing’s Starliner did not leave astronauts Butch Wilmore and Suni Williams in immediate peril, but propulsion problems turned their roughly eight-day test flight into a months-long stay aboard the International Space Station. NASA delayed a crewed return because engineers could not establish enough confidence in how the spacecraft’s thrusters and helium system would perform together during the trip home. Starliner eventually returned without its crew; Wilmore and Williams came back on SpaceX’s Crew Dragon in March 2025. As of March 2026, key propulsion and helium issues were still unresolved, according to NASA’s inspector general.
A test flight became a long wait
NASA’s Boeing Crew Flight Test was meant to demonstrate that Starliner could carry astronauts to and from the International Space Station, paving the way for regular crew rotations alongside SpaceX’s Crew Dragon. It was a test flight, not a routine operational mission. Starliner launched on June 5, 2024, with NASA astronauts Butch Wilmore and Suni Williams aboard and docked with the station on June 6. The mission was initially expected to last about eight days. NASA’s mission FAQ summarizes the flight and the subsequent return plans.
During the approach to the station, some of Starliner’s reaction-control-system (RCS) thrusters performed unexpectedly or were taken out of service, and engineers identified helium leaks in the service module’s propulsion system. The problems did not prevent docking: the spacecraft reached the ISS, and some thrusters were recovered or returned to service. But getting to the station was only one part of the safety question. NASA also had to decide whether the vehicle could reliably undock, maneuver, deorbit, reenter and land with the anomalies understood well enough to carry its crew.
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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Calling the astronauts “stranded” captures the dramatic change in their plans, but needs qualification. They spent far longer in orbit than intended, yet they were aboard a functioning space station with supplies, crew support and a contingency path home. NASA did not describe them as being in immediate danger. The issue was how to bring them back with an acceptable level of risk.
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What failed—and what the system does
Starliner’s RCS uses small thrusters to control the spacecraft’s orientation and make precise maneuvers, including during rendezvous and station departure. The thrusters are part of a larger propulsion system in the service module. Helium pressurizes that system and helps operate its valves. A helium leak is therefore a pressure-management and reliability concern; an underperforming thruster is a maneuvering and attitude-control concern.
Those are related parts of the propulsion system, but the available findings do not establish that the helium leaks caused the thruster problems. Nor does the evidence support reducing the episode to one failed jet. NASA and Boeing needed to understand which thrusters were affected, how much capability remained for each flight phase, whether further degradation was likely, and how the leaks might affect the system over the full return sequence.
Spacecraft are built with redundancy, so one failed thruster does not automatically mean a vehicle cannot return. The risk depends on which units fail, whether the failures share a cause, how much backup capacity remains and whether engineers can predict what will happen next. A system may retain enough capability to perform a maneuver and still present too much uncertainty for a crewed flight.
Why the return kept getting delayed
NASA and Boeing extended the mission while they studied flight data, monitored the leaks, ran tests and assessed possible return profiles. In June, NASA discussed five small helium leaks in service-module manifolds and planned ground testing at the White Sands Test Facility. The teams also conducted thruster firings while Starliner remained docked. On July 30, a docked test fired 27 of the spacecraft’s 28 RCS jets in short bursts to gather more performance and helium-system data. NASA’s account of that test describes its purpose.
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Boeing reported that White Sands testing reproduced thruster degradation after aggressive firing sequences involving more than 1,000 pulses and simulated launch-to-docking and undocking-to-deorbit profiles. That was useful evidence, but it was not a complete explanation of what happened in space. NASA’s later inspector-general report said NASA and Boeing could not fully replicate the flight failures under ground-test conditions.
That distinction matters. A test can reproduce some symptoms without proving the full root cause or showing exactly how a vehicle will behave under different conditions. NASA needed evidence about the return as an integrated sequence—not just a demonstration that individual thrusters could fire. Engineers considered whether the remaining thrusters could support undocking and deorbit, whether helium pressure would remain adequate, what might happen if another thruster degraded, and how confidently those risks could be bounded.
Each additional test could narrow uncertainty without eliminating it. A successful firing or a stable leak rate was not, by itself, the same as a complete safety case for a crewed return.
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Why NASA chose an uncrewed return
On August 24, 2024, NASA announced that Starliner would return to Earth without Wilmore and Williams. The agency’s conclusion was not that the spacecraft was certain to fail or physically incapable of returning. It was that the available evidence and level of expert agreement did not give NASA sufficient confidence to accept the risk of flying the astronauts home in it. NASA’s announcement explains the decision and the planned alternatives.
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Starliner was designed to undock, deorbit, reenter and land autonomously. Sending it back empty let NASA and Boeing collect more flight data without exposing astronauts to unresolved propulsion risk. A successful uncrewed return could show that the vehicle completed that flight, but it could not by itself establish that every uncertainty relevant to a crewed return had been resolved or that Starliner was certified for regular crew service.
NASA instead planned for Wilmore and Williams to return aboard SpaceX’s Crew Dragon as part of Crew-9. They remained at the station, joined its expedition crew structure, and supported station research and operations during the extended stay. They returned to Earth in March 2025, not on Starliner.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the episode means for Boeing’s Starliner program
The crewed test flight did not immediately lead to certification for routine NASA crew missions. NASA’s Office of Inspector General reported in June 2026 that the helium leaks and propulsion-system failures remained unresolved as of March 2026. The root cause of the crew-module thruster failure during the test flight was still under investigation, and NASA and Boeing had not fully replicated the spaceflight failures in ground tests.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThe OIG also said Starliner-1 was planned as an uncrewed cargo mission, with no launch date scheduled at the time of its report, and that the timing of testing and human-rating certification remained uncertain. These findings do not mean the program was canceled; they do mean the 2024 flight had not closed the certification questions.
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The investigation also sits within a broader record of technical and program-management challenges. The OIG discussed earlier test-flight and certification-process shortcomings, including overconfidence in heritage systems, unrealistic scheduling, limited integrated testing and constraints on NASA’s access to contract data. The parachutes had anomalies during the Crew Flight Test, but performed within specified requirements; that is separate from the propulsion issues that drove the return decision. The OIG report is the most current source in this account for the program’s status and findings: NASA’s Management of Its Commercial Crew Program, IG-26-011.
The key distinction: capability versus confidence
NASA’s decision turned on the difference between asking whether Starliner could get home and whether the agency understood its behavior well enough to accept a crewed return. Redundancy and autonomous capability mattered, as did the option of using Crew Dragon. But neither removed the need to assess uncertain failures across the whole return sequence.
That is why the summer-long delay was more than a wait for one thruster to be fixed. NASA and Boeing gathered test and flight data, yet could not establish the level of confidence NASA required for the crewed trip home. Starliner returned on its own; the astronauts returned later on another spacecraft; and, according to the latest OIG status, resolving the propulsion and helium problems remained part of the work ahead.
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