NASA administrator Bill Nelson announced today that US astronauts Sunita Williams and Barry Wilmore will return next February with the SpaceX Crew-9 mission after spending more than 80 days aboard the International Space Station (ISS).

According to NASA Commercial Crew Program manager Steve Stich, “As we got more and more data over the summer and understood the uncertainty of that data, it became very clear to us that the best course of action was to return Starliner uncrewed.” He said NASA found “there was just just too much uncertainty in the prediction of the thrusters.”

“If we had a way to actually predict what the thrusters would do, for the undock, and all the way through the de-orbit burn, and through the separation sequence, I think we would have taken a different course of action. But when we looked at the data and looked at the potential for thruster failures with the crew on board ... it was just too much risk for the crew, and so we decided to pursue the uncrewed testflight. 

Responding to a press question about how NASA can trust Boeing again, NASA Associate Administrator Ken Bowersox said, “We’ve had a lot of tense discussions, right? Because the call was close, and so people have a lot of emotional investment in either option, and that gives you a healthy discourse. But after that, you have to do some work to keep your team together, right?”

“And I’ll acknowledge that we have some work to do there. It’s pretty natural when you’ve had a difficult decision to make.” Bowersox said that NASA remains “committed to working with Boeing.”

Stich weighed in, saying, “Boeing did a great job building a model. The question is, ‘Is that model good enough to predict performance for a crew?’” He added later, “There was just a little disagreement in terms of the level of risk. And that’s kind of where it got down to, and I would say it’s close. It’s very close; it just depends on how you evaluate the risk. We do it a little differently with our crew than Boeing did.”

The two astronauts were originally scheduled to spend just eight days aboard the ISS following a successful launch of Boeing’s Starliner on June 5th before parachuting back to Earth aboard the same spacecraft. Those plans changed after thruster failures, helium leaks, and valve issues plagued the already long-delayed Starliner while it was docking with the ISS.

With limited access to the spacecraft docked with the ISS, tests at NASA’s White Sands Test Facility indicated that deformed Teflon seals may have been one of the reasons the spacecraft’s thrusters failed. But without conclusive answers, NASA waited to decide between returning the astronauts to Earth aboard the Starliner or working with SpaceX to bring them home early next year aboard the Crew-9 mission, which is planned to launch to the ISS in late September.