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(CNN) — Butch Wilmore and Suni Williams — two veteran NASA astronauts piloting the first crewed test flight of Boeing’s Starliner spacecraft — have now been in space for 63 days, roughly seven weeks longer than initially expected.

There is still no clear return date in sight, and NASA is now making clear that the astronauts may not come home on Starliner at all.

SpaceX, Boeing’s rival under NASA’s Commercial Crew Program, may be tapped to bring Williams and Wilmore home instead. The move could potentially extend the astronauts’ stay on the International Space Station by another six months, pushing their return into 2025, agency officials said in a news conference Wednesday.

The comments from NASA leaders indicate a stunning change of tone for the space agency. Until now, officials have repeatedly indicated that Williams and Wilmore were likely to return home on Starliner, and a backup scenario involving Crew Dragon was mentioned as a mere possibility. Wednesday’s update, however, suggests the SpaceX vehicle is rapidly becoming a serious option.

“I would say that our chances of an uncrewed Starliner return have increased a little bit based on where things have gone over the last week or two,” said Ken Bowersox, associate administrator for NASA’s Space Operations Mission Directorate, referring to NASA’s internal review processes that needs to be completed before a return date for Starliner is set. “But again, new data coming in, new analysis, different discussion — we could find ourselves shift in another way.”

Officials said the space agency has until roughly “mid-August” to make a final decision.

Boeing’s propulsion woes

CNN confirmed on Tuesday that NASA has not yet started a “flight readiness review” for the Starliner crew’s return from the space station. The agency had said on July 26 that it would begin that process in the first couple days of August.

But Boeing and NASA teams are still working on a potential return date for the mission that launched June 5, as officials evaluate testing data and conduct analyses about the propulsion issues and helium leaks that hampered the first leg of the Starliner capsule’s flight.

The ground testing that mission teams carried out in New Mexico as they worked to understand the problems led to surprising results, said Steve Stich, NASA’s Commercial Crew Program manager, on Wednesday.

The space agency had previously said it found excess heat around some of the Starliner thrusters was causing teflon seals to bulge, restricting the flow of propellant and triggering the thruster issues.

Uncertainty about whether those bulging seals are indeed the root cause of the issue — and how the problem might affect the Starliner vehicle in space — are the basis for disagreements within NASA about how safe it is for crew to return on the Starliner, officials revealed at the news conference.

NASA officials “got more and more uncomfortable with the uncertainty around the thrusters,” Stich said.

As NASA works to come to a consensus on the likely root cause of those issues and how much danger it might pose to a crewed Starliner return, the space agency has more seriously considered alternative routes home for Williams and Wilmore.

Neither Boeing nor SpaceX officials were available for comment during the Wednesday briefing. Boeing maintains that, in its view, Starliner is safe for the astronauts’ return, according to a Friday statement from Boeing.

Starliner contingency planning

Officials said they had several return scenarios they are considering if Starliner is deemed unsafe to bring Williams and Wilmore home.

One option is to launch a planned SpaceX Crew Dragon mission, Crew-9, with two astronauts on board instead of four. That would leave two seats empty for Williams and Wilmore to occupy on the Crew-9 flight home, but that would also make the astronauts part of the overall Crew-9 rotation on the International Space Station. That means Williams and Wilmore would remain on the station for an additional six months — the length of a routine mission to the ISS — pushing their return to at least February 2025.

In that case, Starliner would be left to fly home empty. And NASA would then have to decide whether the data the mission gathered on this flight is enough to give the space agency the confidence to officially certify Starliner to make routine trips to orbit.

Still, NASA noted, it is not certain that the space agency will need a contingency plan at all.

“We haven’t approved this this plan,” Stich said. “In other words, we’ve done all the work to make sure this plan is there; we have the suits identified to fly up on Crew-9 (for Williams and Wilmore to wear aboard Crew Dragon).  … But we have not turned that on formally, as that’s the path that we’re going to go down.”

Stich added that returning Williams and Wilmore aboard Starliner remains the “prime” option.

The odds of a backup plan

Notably, however, NASA announced Tuesday that it was delaying the launch of SpaceX’s Crew-9 mission.

Crew-9 had been slated to take off as soon as August 18 — with the expectation that the Starliner capsule would have returned home with its astronauts before then. Now, Crew-9 will not take off before September 24, NASA said, allowing more flexibility for the agency to make a decision on use of a Starliner contingency plan.

“This adjustment allows more time for mission managers to finalize return planning for the agency’s Boeing Crew Flight Test,” NASA said in a Tuesday news release.

Some complications remain, however, whether NASA chooses to fly Williams and Wilmore home on Crew Dragon or not.

“Switching off of our primary path of bringing Butch and Suni home on Starliner causes us to take additional risk in the rest of our mission profile — and so we have to compare all those risks,” Bowersox said.

If Starliner is made to fly home empty, for example, the spacecraft will require a software update to configure the vehicle for an autonomous flight home. That would essentially revert the Starliner’s computers to run on code the company originally wrote for a 2022 uncrewed test flight — and NASA and Boeing haven’t revisited such software changes in a couple years, according to Stich.

Engineers then would need to run the software update through its paces at what’s called a “integrated test facility” on the ground, Stich added.

“That’s really what would take time if we were to pivot” to returning Williams and Wilmore home aboard the SpaceX Crew-9 mission, he said.

It is not currently clear whether NASA is leaning toward returning the astronauts on Starliner or making use of its contingency plan.

When asked, Bowersox said Wednesday that he could not offer a sense of the likelihood of either scenario.

“It could change drastically one way or another, depending on new data,” he said.

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