“How does a spacecraft, announced 15 years ago to carry astronauts by 2017, still have no completed crew rotation mission?” That question now looms over Boeing’s CST-100 Starliner after NASA confirmed the capsule will not carry humans on its next flight. The decision comes as Boeing is still recovering from a troubled first crewed mission and years of technical setbacks that have left its program far behind SpaceX’s proven Crew Dragon.
NASA has officially modified Boeing’s 2014 Commercial Crew Transportation Capability contract to reduce the number of operational missions from six to four. The first of those, Starliner-1, will be a cargo-only flight to the ISS, targeted for no earlier than April 2026. “NASA and Boeing are continuing to rigorously test the Starliner propulsion system in preparation for two potential flights next year,” said Steve Stich, NASA’s Commercial Crew Program manager. He added that the change “allows NASA and Boeing to focus on safely certifying the system in 2026, execute Starliner’s first crew rotation when ready, and align our ongoing flight planning for future Starliner missions based on station’s operational needs through 2030.”
The propulsion system remains the central obstacle. During the June 2024 Crew Flight Test, Starliner suffered cascading failures in its service module reaction control system thrusters. At one point, four thrusters were offline, threatening six-degrees-of-freedom control and forcing mission control to waive standard flight rules to enable docking. Compounding the crisis, engineers detected five helium leaks in the same propulsion system-one before launch and four more in orbit. According to root cause analysis, heat-induced deformation of Teflon seals inside thruster valves restricts propellant flow, while seal degradation from prolonged exposure to propellant vapor is also part of the problem.
All these issues have continued unabated despite the years of redesigns and lessons learned from previous flights. Software errors and command link outages saw Starliner’s first uncrewed test in December 2019 fail to reach the ISS, raising 80 corrective actions. The second uncrewed test back in May 2022 managed to dock but still produced anomalies in its thrusters. In 2021, a launch attempt was scrubbed when oxidizer valves corroded and required a major overhaul. Even as far as 2023, NASA flagged flammable tape on wiring harnesses and parachute strength margins as unacceptable, forcing further remediation.
The aftermath of the CFT mission underlined the stakes. NASA astronauts Butch Wilmore and Suni Williams, who were supposed to stay on the station for eight days, eventually spent almost nine months aboard the ISS while engineers assessed the safety of Starliner. Eventually, NASA ordered the capsule back to Earth uncrewed in September 2024. The astronauts returned home in March 2025, riding a SpaceX Crew Dragon back to Earth-the first time U.S. astronauts had launched into space on one commercial vehicle and returned on another.
The contrast is provided by the straight-edged track record of SpaceX: since its uncrewed Dragon test flight back in 2019, the company has conducted twelve crewed missions for NASA, ten of which have been successful crew rotations. Dragon’s reliability has made it NASA’s primary ride to the ISS with missions booked through the station’s planned retirement in 2030. In fact, it was a Dragon capsule that safely returned Wilmore and Williams when Starliner was deemed too risky.
The revised Starliner contract reflects not only the technical realities but also the narrowing operational window. With the ISS scheduled for decommissioning around 2030, even a fully certified Starliner will have limited time to serve NASA’s needs. The agency has long emphasized the importance of having two U.S. crew transport systems to avoid sole reliance on SpaceX or Russian Soyuz, but Starliner’s delays have eroded that redundancy.
Boeing’s financial exposure is huge: the company has absorbed more than $2 billion in losses on the fixed-price program, while the most recent contract modification reduced its value by $768 million, to $3.732 billion. However, according to Stich, Boeing’s leadership is very much still committed to Starliner-it continues to work to resolve propulsion anomalies, for one. The next mission’s cargo-only profile will be a proving ground for those fixes, which include redesigned seals and modified thruster housings. NASA’s Aerospace Safety Advisory Panel has agreed with the cautious approach. “Had the crew been aboard, this would have significantly increased the risk during reentry, confirming the wisdom of the decision,” the panel said in its February report on the uncrewed CFT return.
By flying Starliner-1 without astronauts, NASA will be able to validate the changes in actual mission conditions but with much less risk to human life. If Starliner-1 performs nominally, Starliner-2 could become Boeing’s first operational crewed mission before the end of 2026. But scheduling pressures are mounting. The ISS traffic plan is crowded with visiting vehicles, private astronaut missions, and SpaceX crew rotations. Finding a slot for Starliner will require both technical readiness and precise coordination. For now, the future of the program depends on solving the propulsion problems which have defined its troubled history.