In the lead up to launch, McAullife was treated as a minor celebrity who seemed able to effortlessly charm talk-show hosts. And according to the documentary, this was exactly the effect that NASA officials hoped to achieve with the civilian astronaut program. They wanted to paint the space shuttle as a reliable mode of human space exploration that wasn’t much riskier than flying on a commercial airliner. If it was safe for a school teacher after only a few weeks of training, it was safe enough for everyone. But according to the testimony of several people featured in the doc, NASA’s public message conflicted with what many of its own engineers knew to be true: Every flight of the space shuttle was risky, and the circumstances surrounding this particular flight made it unsafe to launch.

“I think the most fundamental impact of the Challenger disaster was discarding the myth that the shuttle was safe enough to put ordinary citizens on,” says John Logsdon, a space historian at George Washington University who was not involved with the documentary. “There was a pervasive groupthink in the organization that this is what we’ve promised, and even though we know this vehicle isn’t capable of that, we’re not going to say so.”

The emotional rollercoaster of getting to know McAullife and the other astronauts who you know are doomed is a critical foil to the comparatively dry engineering drama that was simmering in the background. The cause of the Challenger disaster was ultimately determined to be a failed O-ring, a giant elastic band that was used to seal sections of the space shuttle’s two solid rocket boosters. Engineers at Morton-Thiokol, the contractor that manufactured the boosters for NASA, had noticed a disturbing tendency for the O-ring seals to fail during tests if temperatures were below about 50 degrees Fahrenheit. And when a cold snap hit Florida a few days before the Challenger mission, the weather was forecast to be in the low- to mid-30s during the launch.

“Our engineers were concerned that the O-rings were going to be colder than any we’d ever launched and that it might be worse this time than we’d ever seen,” Joseph Kilminster, the vice president of Morton-Thiokol’s solid rocket booster program, says in the film. Brian Russell, an engineer at the company, concurs. “We believed the risk was higher, but we didn’t know how much higher,” he says in the doc. “We didn’t know the point of failure.” But despite these concerns, managers at Morton-Thiokol and NASA decided to forge ahead anyway.

The question, of course, is why? Why would NASA and one of its contractors go against the advice of engineers who were concerned that the cold weather would cause a catastrophic failure? In the aftermath of the disaster, an investigation by a presidential commission found that managers at Morton-Thiokol “recommended the launch … contrary to the views of its engineers in order to accommodate a major customer.”

This is also the conclusion that Junge and Leckart arrive at in their film. “The ultimate deciders had pressures that probably had an undue effect on making what was, in the end, a terrible decision,” says Junge, speaking to WIRED.

NASA press representatives did not immediately respond to WIRED’s request for comment about this assessment. But in the documentary, William Lucas, the director of NASA’s Marshall Space Center, who received the brunt of the criticism for the disaster, says he would still make the same decision today with that data that he had received from Morton-Thiokol. “I did what I thought was right in light of the information I had,” he says in the documentary.