The Boeing CST-100 Starliner is a spacecraft designed to transport crew to and from the International Space Station (ISS) and other low-Earth-orbit destinations. Under development by Boeing under NASA's Commercial Crew Program (CCP), it consists of a reusable crew capsule and an expendable service module.
Slightly larger than the Apollo command module or SpaceX Crew Dragon, but smaller than the Orion capsule, the Starliner can accommodate a crew of up to seven, though NASA plans to fly no more than four. It can remain docked to the ISS for up to seven months and is launched on an Atlas V N22 rocket from Cape Canaveral Space Launch Complex 41 in Florida.
In 2014, NASA awarded Boeing a US$4.2 billion fixed-price contract to develop and operate Starliner, while SpaceX received $2.6 billion to develop and operate Crew Dragon. By February 2025, Boeing's effort had exceeded its budget by at least $2 billion.
Originally planned to be operational in 2017, Starliner has been repeatedly delayed by problems in management and engineering. The first uncrewed Orbital Flight Test in December 2019 was deemed a partial failure, leading to a second Orbital Flight Test in May 2022, which was also subsequently declared a partial failure.
In June 2024, during a Starliner Crew Flight Test, the capsule's thrusters malfunctioned on approach to the ISS, and NASA concluded that it was too risky to return its astronauts to Earth aboard the spacecraft. Thereafter, in September 2024, the capsule was returned uncrewed. The mission's astronauts, Sunita Williams and Barry Wilmore, eventually returned to Earth in March 2025 aboard a SpaceX Dragon capsule, which resulted in the Starliner mission being declared as a Type A mishap.
As the Space Shuttle program drew to a close, NASA sought to foster the development of new spaceflight capabilities. Departing from the traditional model of government-owned and operated spacecraft, NASA proposed a different approach: companies would own and operate spacecraft while NASA would act as a customer, purchasing flights as needed. NASA offered funding to support the development of these new vehicles, but unlike previous cost-plus contracts these new contracts would be fixed-price, placing the financial risk of cost overruns on the companies themselves.
Boeing had an extensive history of developing vehicles for space exploration, having built the first stage (S-IC) of the Saturn V rockets, assembling the Lunar Roving Vehicles, and serving as the prime contractor for the US Orbital Segment of the ISS since 1993. Hence, Boeing was suitable for the commercial spaceflight contracts.
In 2010, Boeing unveiled its entry into NASA's Commercial Crew Program competition: the CST-100. The company received initial funding of $18 million in the first round of the Commercial Crew Development (CCDev 1) program to support Starliner's development. Additionally, United Launch Alliance, a joint venture between Boeing and Lockheed Martin, secured $6.7 million to develop the Emergency Detection System to allow its Atlas V rocket to be human-rated to launch Starliner. At the time, Boeing expressed optimism that the Starliner could be operational as early as 2015, contingent upon timely approvals and funding.
In October 2011, NASA announced that the Orbiter Processing Facility-3 at Kennedy Space Center would be leased to Boeing for manufacture and test of Starliner, through a partnership with Space Florida.
Over the next three years, NASA held three more rounds of development funding, awarding Boeing $92.3 million under CCDev 2 in 2011, $460 million under the Commercial Crew integrated Capability (CCiCap) program in 2012, and $9.9 million under the Certification Products Contract (CPC) in 2013.
NASA was expected to announce its selection for the lucrative Commercial Crew Transportation Capability (CCtCap) contract in September 2014. Boeing had lobbied NASA for a sole-source contract, arguing that it needed the program's full budget for the successful development of Starliner. Within NASA, there was considerable support for this approach, with many decision-makers expressing confidence in Boeing's capabilities and safety record. In fact, NASA officials had even drafted a justification for selecting Boeing as the sole provider.
While William H. Gerstenmaier, NASA's human exploration lead, had considered the Starliner proposal as stronger, he was hesitant to award a sole-source contract. The multi-year Commercial Crew Program had been designed to foster competition and redundancy, and Gerstenmaier believed that selecting just one company would undermine these goals. Through his efforts, he successfully convinced NASA to delay the CCtCap announcement and secure additional funding to support two competing efforts.
On September 16, 2014, NASA announced that both Boeing and SpaceX would be awarded CCtCap contracts to develop crewed spacecraft. Boeing received a US$4.2 billion to complete and certify the Starliner, while SpaceX received a US$2.6 billion to complete and certify Crew Dragon. To receive the full contract amount, each company would need to successfully complete an abort test, an uncrewed orbital flight test, a crewed orbital flight test, and six crewed missions to the ISS. However, NASA would not need to pay for any failed tests and was only required to purchase two crewed missions to the ISS from each company. Following the initial guaranteed missions, the companies would compete for launch contracts on an ongoing basis.
In November 2015, NASA announced that it had dropped Boeing from consideration in the separate multibillion-dollar Commercial Resupply Services competition to fly cargo to the International Space Station.
The name CST-100 was first used when the capsule was unveiled to the public in June 2010. The acronym "CST" stands for Crew Space Transportation, while the number "100" represents to the Kármán line, the unofficial boundary of space located at an altitude of 100 kilometers (62 mi) above Earth. The design draws upon Boeing's experience with NASA's Apollo, Space Shuttle, and ISS programs, as well as the Orbital Express project.
The spacecraft consists of a reusable capsule and an expendable service module and is designed for missions to low Earth orbit. The capsule accommodates seven passengers, or a mix of crew and cargo. For NASA missions to the ISS it will carry four passengers and a small amount of cargo. The Starliner capsule uses a weldless, spun-formed structure and is reusable up to ten times with a six-month turnaround time. Boeing plans to alternate between two reusable crew modules for all planned Starliner missions. Each flight uses a new service module, which provides propulsion and power-generation capacity for the spacecraft. Starliner features wireless Internet and tablet technology for crew interfaces.
Starliner uses the NASA Docking System. Boeing modified the Starliner design prior to OFT-2, adding a hinged re-entry cover below its expendable nosecone for additional protection of the docking port during atmospheric entry. This was tested on the OFT-2 mission. By contrast, the reusable SpaceX Dragon 2 nosecone is hinged and protects its docking port during both launch and reentry.
The capsule uses the Boeing Lightweight Ablator for its re-entry heat shield.