the story would have made for a much inferior movie. In real accidents, people rant and rave and hunt down the culprit. They do, in short, what people in Hollywood thrillers always do. But what made Apollo 13 unusual was that the dominant emotion was not anger but bafflement — bafflement that so much could go wrong for so little apparent reason. There was no one to blame, no dark secret to unearth, no recourse but to re-create an entire system in place of one that had inexplicably failed. In the end, the normal accident was the more terrifying one.
3.
Was the Challenger explosion a normal accident? In a narrow sense, the answer is no. Unlike what happened at TMI, its explosion was caused by a single, catastrophic malfunction: the so-called O-rings that were supposed to prevent hot gases from leaking out of the rocket boosters didn’t do their job. But Vaughan argues that the O-ring problem was really just a symptom. The cause of the accident was the culture of NASA, she says, and that culture led to a series of decisions about the Challenger that very much followed the contours of a normal accident.
The heart of the question is how NASA chose to evaluate the problems it had been having with the rocket boosters’ O-rings. These are the thin rubber bands that run around the lips of each of the rocket’s four segments, and each O-ring was meant to work like the rubber seal on the top of a bottle of preserves, making the fit between each part of the rocket snug and airtight. But from as far back as 1981, on one shuttle flight after another, the O-rings had shown increasing problems. In a number of instances, the rubber seal had been dangerously eroded — a condition suggesting that hot gases had almost escaped. What’s more, O-rings were strongly suspected to be less effective in cold weather, when the rubber would harden and not give as tight a seal. On the morning of January 28, 1986, the shuttle launchpad was encased in ice, and the temperature at liftoff was just above freezing. Anticipating these low temperatures, engineers at Morton Thiokol, the manufacturer of the shuttle’s rockets, recommended that the launch be delayed. Morton Thiokol brass and NASA, however, overruled the recommendation, and that decision led both the president’s commission and numerous critics since to accuse NASA of egregious — if not criminal — misjudgment.
Vaughan doesn’t dispute that the decision was fatally flawed. But, after reviewing thousands of pages of transcripts and internal NASA documents, she can’t find any evidence of people acting negligently, or nakedly sacrificing safety in the name of politics or expediency. The mistakes that NASA made, she says, were made in the normal course of operation. For example, in retrospect it may seem obvious that cold weather impaired O-ring performance. But it wasn’t obvious at the time. A previous shuttle flight that had suffered worse O-ring damage had been launched in 75-degree heat. And on a series of previous occasions when NASA had proposed — but eventually scrubbed for other reasons — shuttle launches in weather as cold as 41 degrees, Morton Thiokol had not said a word about the potential threat posed by the cold, so its pre-Challenger objection had seemed to NASA not reasonable but arbitrary. Vaughan confirms that there was a dispute between managers and engineers on the eve of the launch but points out that in the shuttle program, disputes of this sort were commonplace. And, while the president’s commission was astonished by NASA’s repeated use of the phrases acceptable risk and acceptable erosion in internal discussion of the rocket-booster joints, Vaughan shows that flying with acceptable risks was a standard part of NASA culture. The lists of acceptable risks on the space shuttle, in fact, filled six volumes. “Although [O-ring] erosion itself had not been predicted, its occurrence conformed to engineering expectations about large-scale technical systems,” she writes. “At NASA, problems were the norm. The word anomaly was part of everyday talk.…The whole shuttle system operated on the assumption that deviation could be controlled but not eliminated.”
What NASA had created was a closed culture that, in her words, “normalized deviance” so that to the outside world, decisions that were obviously questionable were seen by NASA’s management as prudent and reasonable. It is her depiction of this internal world that makes her book so disquieting: when she lays out the sequence of decisions that led to the launch — each decision