Boeing’s CST-100 Starliner will return from the International Space Station in September without two astronauts after it launched in June after NASA concluded that engine problems posed too much of a risk.

NASA announced on August 24 that NASA astronauts Butch Wilmore and Suni Williams, who flew to the ISS in June as part of the Starliner crewed flight test mission, will remain on the station until February of next year, with the Starliner set to return to Earth in 2015 without a crew in early September.

The agency made the decision after concluding they didn’t have a good enough understanding of the reaction control engine that failed during Starliner’s approach to the station in June, agency officials said in a briefing. Since then, NASA and Boeing have been working to determine what caused the engine performance degradation and whether it could happen again during Starliner’s docking and return to Earth.

But they say they haven’t fully resolved all the uncertainties about the engines. “That uncertainty persists in our understanding of the physics of what’s going on in the engines,” said Jim Free, NASA’s deputy administrator.

Engine tests at NASA’s White Sands Proving Ground in New Mexico were able to mimic the engine’s performance loss, and inspections revealed that the Teflon seal was heating and expanding, restricting the flow of oxidizer into the engine. But officials said they didn’t know enough about how this happened to be sure there would be no problems during the time-critical burns during Starliner’s separation from the station and deorbit.

“There was too much uncertainty in the engine performance estimate,” said Steve Stich, head of NASA’s Commercial Crew Program. “There was too much risk to the crew, so we decided to do an uncrewed test flight.”

“To me, one of the biggest factors is that we don’t know how long we can use the engines before we have a problem on our journey home,” said Ken Bowersox, NASA’s associate administrator for space operations.

Both Bowersox and Stitch said they were very close to making the decision to return the Starliner without the crew, with Bowersox saying it was only last week that he realized that returning Wilmore and Williams with the Starliner was not possible. “We could have been a lot smarter with more time, but we’re at the point now where we need to get the Starliner home,” he said.

Bowersox added that the decision to return without a crew was unanimous among NASA officials who participated in the review. He said Boeing was prepared to support both unmanned and manned returns. “They believe in their vehicle and they are ready to get the crew home.”

NASA Administrator Bill Nelson said at a briefing that there was no political influence on the decision, despite speculation about any role that the office of Vice President Kamala Harris, the Democratic presidential candidate and chair of the National Space Council, might play. “I can tell you very clearly from a personal standpoint, politics played no role in this decision,” he said. “She had absolutely nothing to do with it.”

The way forward

NASA now plans to move forward with an alternative strategy it outlined earlier this month. Starliner will depart the station without a crew and return to Earth for a landing at White Sands or another location in the southwestern United States. Crew-9 will not launch until September 24 with two of the four crew members on board to make room for Williams and Wilmore. That Crew Dragon spacecraft will return as planned in February 2024.

At the briefing, NASA did not say which of the four Crew 9 members assigned to the mission — NASA astronauts Zena Cardman, Nick Hague and Stephanie Wilson, and Roscosmos cosmonaut Oleksandr Gorbunov — would remain on board. “We are working to finalize this crew and update the training plan,” said Norm Knight, NASA’s director of flight management. “We are working to finalize this crew and update the training plan.” “Those decisions will be made public as soon as they are finalized.”

Both he and Dana Weigel, NASA’s ISS program manager, said Wilmore and Williams are prepared to stay on the station for much longer. Both have previously completed long-duration flights to the ISS and have retrained in ISS operations as part of contingency planning in case their stays are extended.

“They have become an integral part of the Expedition 71 crew,” Knight said, referring to the long-term crew currently aboard the station who “contribute to the important work on the International Space Station.”

Starliner launches will also change, Stich said. “We’ll be changing the separation sequence a little bit to get off the space station a little bit quicker,” he said. Software configurations on the spacecraft will also be changed to support a docking without astronauts on board to monitor operations and make changes. NASA has not announced a specific date for Starliner’s return, other than early September.

The future of Starliner

It is not known when Starliner will fly again. Officials said they will wait until Starliner returns without a crew to see what changes are needed to the spacecraft and how they will be made. That includes whether another crewed test flight is needed before NASA certifies the spacecraft for crewed rotation missions to the ISS.

“We haven’t finalized what our goals are or what we’ve accomplished yet,” Stich said. “I don’t think we’ve decided yet how to do another crewed flight test.”

“I wouldn’t rule anything out,” Bowersox added. “We have a few options moving forward.”

There has been speculation that Boeing, which has already reported a $1.6 billion loss on Starliner, including a $125 million loss it announced in July, could abandon the fixed-price contract rather than take on additional losses. Nelson said Boeing’s new CEO, Kelly Ortberg, had assured him the company would continue with the Starliner program.

“He expressed to me that they would continue to work on the issues once Starliner returned safely,” Nelson said. “We would have backup crew access to the space station.” But he later said he did not ask Ortberg in a conversation whether the company would be willing to take on costs such as additional test flights and that it “would not have been appropriate” to discuss the issue in that conversation.

Boeing representatives were not present at the briefing. Bowersox said that because the briefing was about NASA’s decision, the agency felt it was appropriate for only its own officials to speak to the media about its decision.

“Boeing continues to focus on crew and spacecraft safety first and foremost,” the company said in a brief statement. “We are fulfilling the mission defined by NASA and preparing the spacecraft for a safe and successful return without a crew.”

Nelson said he expects Starliner to fly with astronauts again. When asked how confident he is about that, he simply said, “100%.”

From eight days to eight months

After multiple delays, Wilmore and Williams boarded the Starliner on June 5 and docked with the station the next day. At the time of the mission’s launch, NASA had planned to keep the Starliner at the ISS for only eight days, even giving a specific docking and landing time for the spacecraft’s return on June 14 during the post-launch press conference.

However, Starliner’s engine problems as it approached the station and the discovery of several additional helium leaks forced NASA to extend the spacecraft’s stay by days, then weeks.

In late June, NASA delayed Starliner’s return until at least the second half of July to conduct ground tests of Starliner’s engines in an attempt to replicate the problems Starliner’s engines encountered during the approach to the station. However, in late July, NASA delayed Starliner’s launch until mid-August and continued engine research, including testing the engines on the spacecraft while docked to the ISS.

As of late July, there was no public indication that NASA was considering an alternative to returning Williams and Wilmore to Starliner. “I am very confident that we have a good vehicle to bring the crew back,” Mark Nappi, Boeing’s vice president and commercial crew program manager, said at a July 25 briefing.

Less than two weeks later, NASA officials acknowledged that they were considering scenarios where the two astronauts would remain on the ISS and the Starliner would return without a crew. Under that scenario, Williams and Wilmore would remain on the ISS until February 2025 and return on the Crew-9 Crew Dragon spacecraft, whose launch NASA delayed from Aug. 18 to Sept. 24.

At NASA’s pre-Starliner briefing on August 14, the agency said it needed to make a decision on Starliner by the last week of August to plan for the spacecraft’s launch with or without astronauts, and to allow for the launch of Crew-9 and the return of Crew-8. At that briefing, officials were still working to model and understand the behavior of the engines, but there was no consensus on whether it was safe for Wilmore and Williams to return to Starliner.

With NASA’s decision to return Starliner without a crew, Willmore and Williams’ stay at the ISS will be extended from eight days to more than eight months.

“All the astronauts on the station are professionals,” Knight said. He said Wilmore and Williams were briefed on the decision to return Starliner without them and supported it. “It’s very disappointing that they won’t be returning home with Starliner, but that’s OK. This is a test flight. That’s what they’re doing, and they understand the risks.”

“We keep them very busy on the ISS,” he added. “They are part of the team and they feel good.”