Why Hasn’t Starliner Returned

Critical Questions by Clayton Swope

Published August 19, 2024

On June 5, 2024, an Atlas V rocket launched Boeing's Starliner on its first crewed mission to the International Space Station (ISS). Though it successfully docked with the station, Starliner exhibited anomalies related to its propulsion systems, raising concerns about whether the spacecraft could safely return astronauts to Earth. At the time of writing, NASA has yet to decide whether to send its two-person crew home on the capsule or another spacecraft.

Q1: How did NASA get here?

A1: In the 1990s, five nations agreed to collaborate on building the ISS, the first component of which was launched in 1998. Two years later, the first ISS crew members arrived on a Russian Soyuz capsule. For the next 11 years, Soyuz and the U.S. Space Shuttle ferried ISS crew members from the United States and Russia, as well as other international partners, between Earth and the station. Anticipating the retirement of the Space Shuttle in 2011, NASA sought to develop a new U.S. capability to transport astronauts to the station, establishing the Commercial Crew Program (CCP) to provide commercial transportation services.

In 2014, NASA certified two operators-SpaceX and Boeing-for crewed missions. SpaceX carried out the first crewed CCP flight to the ISS in 2020. Before this crewed mission, SpaceX had completed an uncrewed test flight of its Dragon capsule in 2019. To date, SpaceX has successfully conducted eight crewed missions to the station. Boeing's first crewed Starliner capsule launched to the ISS on June 5, 2024, having previously conducted two uncrewed test flights-the second flight being necessary because the first was only a partial success.

Currently, the two astronauts and Starliner launched on June 5 remain at the space station, though the mission was originally planned to last around 10 days. Due to a number of technical issues, at least one of which was known while Starliner sat on the launch pad, NASA has repeatedly extended the mission as it has tried to gain a better understanding of the risk facing the astronauts and the spacecraft on Earth reentry.

Q2: What are the issues affecting Starliner?

A2: In late May 2024, while United Launch Alliance (ULA) addressed a malfunctioning valve in the Atlas V rocket intended to carry Starliner into space, Boeing discovered a helium leak in 1 of the 28 reaction control system (RCS) thrusters on Starliner's service module. After additional analysis, NASA, Boeing, and ULA ultimately agreed to proceed with the launch, concluding that the leak did not pose an unacceptable risk to the mission. On June 5, Starliner successfully launched into orbit.

Once in space, engineers detected additional helium leaks in the service module's RCS thruster system, bringing the total number of helium leaks to five. On top of the helium leaks, Starliner experienced other on-orbit propulsion anomalies, with 5 of the 28 service module's RCS thrusters failing at various times as the spacecraft prepared to dock with the ISS. Eventually, engineers were able to reset and restart 4 of the 5 malfunctioning thrusters. With all but one of the service module's RCS thrusters operational, NASA gave approval for Starliner to dock with the ISS on June 6.

Once docked with the station, per standard procedures for Starliner, engineers closed off the valves to the helium tanks, preventing further leaks. Since June 6, NASA and Boeing have attempted to isolate, replicate, and understand the issues affecting the thruster system, conducting ground testing with an RCS thruster at NASA's White Sands Test Facility. Two separate rounds of on-orbit firings for Starliner's 27 operational service module's RCS thrusters, checking performance and helium leak rates, have also occurred since the spacecraft docked with the station.

Even with the leaks, NASA and Boeing assess that Starliner would have enough helium to return to Earth. Additionally, officials note that the 27 working RCS thrusters on the service module operated within expected parameters during the most recent on-orbit testing on July 27. However, NASA officials have yet to make a final decision on when-and how-Starliner's astronauts will return home. NASA has stated that it is considering returning the crew as part of the next SpaceX Dragon mission to the ISS, which would likely not return until February 2025.

Q3: How do these malfunctioning components fit together?

A3: Starliner is made up of two main components: a crew module, which returns to Earth, and a service module, which is only used in space and burns up upon reentry into Earth's atmosphere. The crew module has 12 RCS thrusters used for orientation during reentry. The service module has 28 RCS thrusters-a different model from the RCS thrusters used on the crew module-as well as 20 orbital maneuvering and attitude control (OMAC) thrusters and four launch abort engines.

Under normal operations, Starliner is designed to use both the service module's RCS thrusters and OMAC engines at different phases of the undocking and maneuvering sequence to position the spacecraft on a trajectory for the crew module to reenter the atmosphere and safely return to Earth.

Only the service module's RCS thrusters, used for maneuvering in orbit and during a high-altitude abort, have experienced anomalies during Starliner's current mission. While it remains unclear how these thruster malfunctions relate to the helium leaks, both issues are specific to the service module, not the crew module. Coincidentally, two of the service module's RCS thrusters as well as two OMAC engines and one of the crew module's RCS thrusters malfunctioned during the second uncrewed test flight of Starliner in May 2022. NASA has stated that it worked with the RCS thruster manufacturer to develop a contingency plan requiring only four of the service module's RCS thrusters for a successful reentry.

Q4: What options does NASA have for Starliner?

A4: Senior NASA officials do not currently agree on next steps, with some uncomfortable with the risks associated with sending astronauts home on Starliner. To date, NASA has had time to make its assessment and decision. The long-term performance of Starliner's batteries was originally expected to limit the length of time the spacecraft could operate, but so far the batteries have been successfully recharged by the space station, buying more time for analysis. At this point, NASA has not made clear what else it can do to gain more confidence, as it has already spent weeks trying to replicate what's happening on Starliner with the test RCS thrusters at the White Sands Test Facility. However, NASA has noted it will decide how to proceed by the end of August 2024 to prepare for upcoming missions to the ISS.

Ultimately, NASA's decision will reflect its assessment of the safest way to return the two Starliner astronauts to Earth, though it will also take into account concerns about the ISS itself should the spacecraft experience certain thruster malfunctions as it moves away from the station. A number of officials who will make key decisions on Starliner, both at NASA and Boeing, had some role in the events leading up to the disintegration of Columbia and loss of her crew in 2003. As NASA contemplates how to send the two Starliner astronauts currently on the ISS, Butch Wilmore and Suni Williams, back home, those events in February 2003 likely cast a heavy shadow on decisionmakers. This leaves NASA with a difficult question: Should it send the astronauts home on Starliner or save two seats for them on the return flight of the next Dragon mission, scheduled to launch in September 2024 and return in February 2025?

If NASA decides to send Wilmore and Williams back on a SpaceX capsule, it could send Starliner home uncrewed, assuming Boeing is able to upload software to facilitate an autonomous undocking from the space station. This approach would allow NASA and Boeing to validate the results of their testing and assumptions made over the last two months, since they have been assessing Starliner's leak and thruster issues. From a system-testing standpoint, a successful return of the capsule without a crew gets NASA and Boeing the same data it would get from a successful return with a crew. While inconvenient, an uncrewed Starliner return would minimize risk to human life, using the Dragon capsule-with an established safety record under its belt-to bring Wilmore and Williams safely home. But no option carries zero risk-the backup plan would leave the two astronauts for a few days without a way to evacuate the station in an emergency after Starliner undocks and before the next Dragon arrives.

In 1965, astronaut Gus Grissom, who lost his life in the Apollo 1 tragedy, said, "the conquest of space is worth the risk of life." Today, however, there is no reason to take that risk. With Columbia, there was no viable backup option to transport the Space Shuttle crew home. The orbiter did not have enough fuel to reach the ISS. The dynamics are entirely different today with Starliner. Though no option is risk free, there is another way to send the two astronauts home, with the main downside being a possible reputational blemish to Starliner's manufacturer, Boeing. Arguably, for the future successes of Starliner and American crewed spaceflight, as well as for Wilmore, Williams, and their families, that is a price worth paying.

Clayton Swope is the deputy director of the Aerospace Security Project and a senior fellow in the International Security Program at the Center for Strategic and International Studies (CSIS) in Washington, D.C.

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