NASA engineers were concerned about what happened when they tested the Starliner's engines

Later this week, Boeing's damaged Starliner is scheduled to begin its return journey from the International Space Station.

But instead of bringing NASA astronauts Butch Wilmore and Suni Williams back to Earth, the satellite will undock and re-enter the Earth's atmosphere without a crew on board – that is, after a software update, since it was originally unable to fly without astronauts on board.

Even before the ill-fated capsule's launch in early June, engineers noticed several helium leaks. During the Starliner docking process, the leaks quickly became a real problem. The spacecraft missed its first attempt to dock with the space station.

Since then, engineers at Boeing and NASA have been trying to determine the root cause of the problem.

At first, NASA insisted that it was just a routine measure to investigate the accident before Wilmore and Williams returned to the Starliner immediately afterward. The agency repeatedly dismissed reports that the two astronauts were “stranded” in space, arguing that engineers simply needed a little more time to resolve the problem.

But it didn't take long for NASA to change its mind. While attempting to replicate the problem at NASA's White Sands Test Facility in New Mexico, engineers finally found obvious proof. SpaceNews“Jeff Foust explains this in detail in a new, detailed breakdown of the timeline.”

A Teflon seal in a valve known as a “poppet” expanded when heated by neighboring engines, significantly restricting the flow of oxidizer – a worrying finding because it severely affected the engines' performance.

Worse still, engineers were unable to accurately reproduce and analyze the problem in a near vacuum, and therefore were not entirely sure how the problem actually played out in orbit.

During a press conference in late August announcing the decision to send the Starliner back empty, Steve Stich, NASA's commercial crew program manager, admitted that “there were simply too many uncertainties in the predictions regarding the engines.”

“People really want to understand what's going on physically with Teflon, what's heating it up and what's contracting it,” he admitted. “That's what the team is really trying to understand. I think the NASA community in general would like to know a little bit more about the root cause.”

Although engineers found that the engines had returned to a more regular shape after firing in space, they feared that similar deformations could occur during longer firings outside of orbit.

The stakes were high. Without perfect control of the engines, NASA feared the spacecraft could spiral out of control.

“One of the really important factors for me is that we simply don't know how many times we can use the engines on the way back before we run into a problem,” said Ken Bowersox, NASA's deputy director for space operations, according to SpaceNews.

“If we had a way to predict exactly what the engines would do throughout the burn and separation sequence, I think we would have acted differently,” Stich said during the conference call last month. “But when we looked at the data and looked at the potential for engine failures with a crew on board … the risk was just too great.”

That's a polite way of saying that NASA had very serious concerns. According to Faust's report, the saga developed into “NASA's biggest human spaceflight safety crisis since the Columbia disaster more than two decades ago.”

Earlier this week, NASA announced that the unmanned undocking of the Starliner would take place as early as Friday evening.

Wilmore and Williams will stay behind and probably watch their flight into space depart without them.

The two astronauts will have to be patient, as their replacement shuttle, SpaceX's Crew 9 mission, will not arrive until September 24 at the earliest. Even then, the pair will have to wait until the Crew Dragon spacecraft returns to Earth in February, turning the planned eight-day mission into an eight-day one.Month Affair.

More about the Starliner: Astronauts hear strange noises from Boeing's cursed Starliner