NASA is considering sending back two astronauts who flew to the International Space Station aboard the Boeing CST-100 Starliner spacecraft instead of the SpaceX Crew Dragon in early 2025 amid questions about the Starliner’s safety.

At a briefing on Aug. 7, NASA officials said they expect to decide by mid-August whether it would be too risky for astronauts Butch Wilmore and Suni Williams, who arrived at the station aboard the Starliner two months ago, to return on the same ship. To do that, they would need to remain on the station while the Starliner automatically docks and returns to Earth.

Under that scenario, NASA would have carried out the Crew-9 mission, which was postponed a day earlier from Aug. 18 to Sept. 24, with two people instead of four. Williams and Willmore would then return with Crew 9 at the end of the mission, which is planned for early 2025. Officials declined to say which of the current Crew 9 members would be retained in that scenario and which would be assigned to future missions.

NASA is considering this option due to ongoing concerns about the performance of the Reaction Control System (RCS) engines on Starliner. Several engines failed during the spacecraft’s approach to the station in June, but all but one were recovered.

The uncertainty about what caused the engine performance to degrade is the reason for the ongoing debate about the safety of the crew returning to Starliner, particularly how the engines will perform during the reentry or “descent” phase. “People really want to understand the physics of what’s happening with the engine,” said Steve Stich, head of NASA’s Commercial Crew Program, “and then look at the descent phase and the warm-up phase and see if we can model that. We’ll go with the descent phase and make sure we have good engines.”

RCS engine tests conducted last month at NASA’s White Sands Test Center showed that heating engines can cause the Teflon seal or canister to expand and bulge, restricting fuel flow. Heating the fuel can also cause it to vaporize, reducing engine performance.

The purpose of these engine tests was to provide confidence that the engines would perform properly during the reentry phase, but they may have done the opposite. “The testing at White Sands and after the test, it was a bit of a surprise to us to discover that the Teflon disk had been pushed out,” Stich said. “I would say it increased the level of discomfort.”

He said NASA has brought in propulsion experts from another part of the agency to examine the data while additional tests continue. The goal is to try to understand what’s happening in the engine engine and see if other failure modes are possible as the engines heat up.

Stitch described the worst-case scenario for Starliner as an “integrated failure mechanism” between engine issues and individual helium leaks in the propulsion system, which could make it difficult for the spacecraft to maintain orientation during deorbit. “The team wants to understand the worst-case effects of engine overheating,” he said. “They want to make sure that if the thruster gets too hot, there’s not a specific failure mode that we haven’t seen yet.”

That uncertainty contradicts Boeing’s comments on engine performance. Boeing said in a statement on Aug. 2 that the company had conducted extensive engine testing that led it to conclude the spacecraft maintained “safe docking and landing capabilities” for the mission. “Boeing remains confident in the Starliner spacecraft and its ability to safely return with a crew,” the statement said.

“Reasonable people can have different views on where we should go,” said Ken Bowersox, NASA’s deputy administrator for space operations, at a meeting that did not include a Boeing representative.

“The Boeing team is confident, because of their experience and their belief in their equipment, that the vehicle can get the crew home, even at this moment of uncertainty,” he said. “But we have other people who are probably a little bit more conservative. They are concerned that we don’t know for sure, so they assess the risk as higher.”

“Boeing said its vehicles were ready to come home on Friday,” Stich said, referring to Boeing’s announcement date. “I think your perspective on risk depends on your experience.”

Bowersox praised the teams working on the Starliner for discussing the vehicle’s safety, noting that people are speaking up and being heard. “I have to admit that sometimes when we have disagreements, it’s not fun. Those discussions can be painful,” he said. “That’s what makes our organization good, and that’s what’s going to help us make the right decision.”

He said a decision to return Williams and Wilmore to Starliner is “not too far away.” Later on the call, Stitch said NASA would need to make a decision by mid-August to allow enough time for training and other preparations if it decides to return Starliner without a crew.

That decision could come down to NASA’s top. Stich said the agency’s airworthiness review will make recommendations to Bowersox. “Depending on how that meeting goes, there could be other steps.” [помічником адміністратора] Along with Jim Free and eventually the manager.”