NASA prepares for Starliner return from ISS

Accelerated departure profile reduces thruster demands

The breakout burn allows Starliner to exit the station's vicinity more rapidly than a standard crewed departure, which would include a flyaround for photographic documentation and manual control contingencies. Anthony Vareha, lead NASA space station flight director for the undocking, explained that without astronauts on board to assume manual control if needed, there are significantly fewer variables to manage. This trajectory places less demand on the reaction control system (RCS) thrusters, whose overheating issues during the June approach ultimately drove NASA's decision to return the spacecraft empty.

Steve Stich, NASA commercial crew programme manager, noted the breakout profile involves "a lot fewer thruster firings" compared with the original plan. Once clear of the station, controllers intend to test-fire several thrusters with short 0.1-second pulses to gather additional performance data. Engineers are still selecting which thrusters to test, but the candidates likely include aft thrusters that experienced problems, among them one that appeared to lose all thrust on approach and failed to recover in subsequent docked tests. These brief pulses will help engineers understand how the thrusters perform after undocking and what thrust levels they produce, supplementing data collected while Starliner was docked.

Helium leaks traced to seal degradation from oxidiser exposure

Beyond the thruster anomalies, Starliner experienced multiple helium leaks during its mission. Stich indicated the leading explanation is that exposure to oxidiser vapours is causing a seal to degrade over time. The helium system is critical for pressurising the propulsion system, and any persistent leakage would complicate future crewed missions where margins are tighter. NASA is evaluating a different seal material and a slightly enlarged seal design as potential remedies. This seal degradation issue, while distinct from the thruster overheating, compounds the certification challenge because both affect the same propulsion subsystem.

Operational changes preferred over hardware redesign for thrusters

For the thruster overheating problem, NASA appears to be leaning towards operational and thermal management changes rather than replacing or fundamentally redesigning the thrusters themselves. The root cause involves a Teflon poppet component that swells when the thruster overheats, constraining propellant flow. Stich emphasised that "the thrusters are working well when we don't command them in a manner that overheats them," noting most thrusters performed without issue during the mission. The overheating occurs when specific pulse patterns cause the poppet to swell, restricting propellant flow and further degrading performance.

Potential solutions include modifying firing sequences to avoid pulse patterns that cause overheating, and structural changes to the service module's "doghouse" enclosures that house the thrusters to improve heat dissipation. Nothing has been ruled out, but the preference for operational fixes suggests a faster and less costly path to certification, provided the approach satisfies NASA's safety requirements. Stich added that the team needs to understand "what kinds of pulses in particular cause that swelling, the number of pulses" to define safe operating envelopes.

Certification timeline and potential second test flight remain unclear

NASA did not provide a timeline for post-flight reviews, corrective action implementation, or a decision on whether Boeing must conduct another test flight before Starliner can be certified for regular crew rotation missions. Stich acknowledged the agency has been "entirely focused this summer on understanding what is happening on orbit" and now needs to "lay out the overall plan, which we have not had time to do." Notably, Boeing did not participate in the 4 September briefing, continuing a pattern from previous updates.

The absence of a clear certification pathway introduces schedule risk for NASA's commercial crew programme, which relies on having two certified providers — SpaceX's Crew Dragon and Boeing's Starliner — for redundancy and competition. Any requirement for a second uncrewed or crewed test flight would push operational missions further into the future, potentially leaving NASA dependent on a single provider for crew transport to the International Space Station.

Tense technical discussions but no shouting matches

NASA officials pushed back against media reports that earlier internal meetings about whether to allow astronauts Butch Wilmore and Suni Williams to return on Starliner descended into yelling. Stich characterised the discussions as "a tense technical discussion" rather than a shouting match, acknowledging "some tension in the room" as Boeing argued the spacecraft was safe enough for crew return while NASA judged the thruster performance models too uncertain for crewed re-entry. The disagreement highlights the inherent tension between a contractor advocating for its vehicle and a customer responsible for astronaut safety. NASA's decision to return Starliner uncrewed reflects its conservative posture when safety models carry high uncertainty.

What this means

Starliner's uncrewed return will close a difficult chapter in Boeing's commercial crew journey, but the harder work lies ahead. NASA's preference for operational fixes over hardware redesign offers a potentially faster route to certification, yet the agency's refusal to commit to timelines — or rule out a second test flight — underscores the technical and bureaucratic hurdles remaining. For NASA, the priority remains astronaut safety and programme redundancy; for Boeing, the stakes involve reputation, contract value, and its future in human spaceflight. The coming months will reveal whether operational adjustments can tame Starliner's thruster thermals and seal degradation sufficiently to satisfy NASA's exacting standards, or whether a more fundamental redesign — and another costly test flight — becomes unavoidable.