Feynman was requested to serve on the Presidential
Rogers Commission which investigated the
Challenger disaster of 1986. Feynman devoted the latter half of his book
What Do You Care What Other People Think? to his experience on the Rogers Commission, straying from his usual convention of brief, light-hearted anecdotes to deliver an extended and sober narrative. Feynman's account reveals a disconnect between NASA's engineers and executives that was far more striking than he expected. His interviews of NASA's high-ranking managers revealed startling misunderstandings of elementary concepts.
In one example, early tests resulted in some of the booster rocket's
o-rings burning a third of the way through. These o-rings provided the gas-tight seal needed between the vertically stacked cylindical sections that made up the solid fuel booster. NASA managers recorded this result as demonstrating that the o-rings had a "safety factor" of 3. Feynman incredulously explains the magnitude of this error: a "safety factor" refers to the practice of building an object to be capable of withstanding more force than it will ever conceivably be subjected to. To paraphrase Feynman's example, if engineers built a bridge that could bear 3,000 pounds without any damage, even though it was never expected to bear more than 1,000 pounds in practice, the safety factor would be 3. If, however, a truck drove across the bridge and it cracked at all, the safety factor is now zero: the bridge is defective.
Feynman was clearly disturbed by the fact that NASA management not only misunderstood this concept, but in fact inverted it by using a term denoting an extra level of safety to describe a part that was actually defective and unsafe. Feynman continued to investigate the lack of communication between NASA's management and its engineers and was struck by the management's claim that the risk of catastrophic malfunction on the shuttle was 1 in 10
5; i.e., 1 in 100,000. Feynman immediately realized that this claim was
risible on its face; as he described, this assessment of risk would entail that we could launch a shuttle every day for the next 274 years without an accident. Investigating the claim further, Feynman discovered that the 1 in 10
5 figure was reached by the highly dubious method of attempting to calculate the probability of failure of every individual part of the shuttle, and then adding these estimates together. This method is erroneous by standard probability theory: the correct way to calculate such risk is to subtract each individual factor's failure risk from unity and then multiply all differences. The product will be the net safety factor and the difference between it and unity, the net risk factor.
Feynman was disturbed by two aspects of this practice. First, NASA management assigned a probability of failure to each individual bolt, sometimes claiming a probability of 1 in 10
8; that is, one in one hundred million. Feynman pointed out that it is impossible to calculate such a remote possibility with any scientific rigor. Secondly, Feynman was bothered not just by this sloppy science but by the fact that NASA claimed that the risk of catastrophic failure was "necessarily" 1 in 10
5. As the figure itself was beyond belief, Feynman questioned exactly what "necessarily" meant in this context—did it mean that the figure followed logically from other calculations, or did it reflect NASA management's desire to make the numbers fit?
Feynman suspected that the 1/100,000 figure was wildly fantastical, and made a rough estimate that the true likelihood of shuttle disaster was closer to 1 in 100. He then decided to poll the engineers themselves, asking them to write down an anonymous estimate of the odds of shuttle explosion. Feynman found that the bulk of the engineers' estimates fell between 1 in 50 and 1 in 100. Not only did this confirm that NASA management had clearly failed to communicate with their own engineers, but the disparity engaged Feynman's emotions. When describing these wildly differing estimates, Feynman briefly lapses from his damaging but dispassionate detailing of NASA's flaws to recognize the moral failing that resulted from a scientific failing: he was clearly upset that NASA presented its clearly fantastical figures as fact to convince a member of the public, schoolteacher
Christa McAuliffe, to join the crew. Feynman was not uncomfortable with the concept of a 1/100 risk, but felt strongly that the recruitment of laypeople required an honest portrayal of the real risk involved.
Feynman's investigation eventually suggested to him that the cause of the
Challenger explosion was the very part to which NASA management so mistakenly assigned a safety factor. The o-rings were rubber rings designed to form a seal in the shuttle's solid rocket boosters, preventing the rockets' super-heated gas from escaping and damaging other parts of the vehicle. Feynman suspected that despite NASA's claims, the o-rings were unsuitable at low temperatures and lost their resilience when cold, thus failing to expand and maintain a tight seal when rocket pressure distorted the structure of the solid fuel booster. Feynman's suspicions were corroborated by
General Kutyna also on the commission who cunningly provided Feynman with a broad hint by asking about the effect of cold on o-ring seals after mentioning that the temperature on the day of the launch was far lower than had been the case with previous launches: below freezing at 28 or 29
Fahrenheit (-2.2 to -1.6
°C); previously, the coldest launch had been at 53 °F (12 °C).
Feynman obtained samples of the seals used on the Challenger by dismantling a model supplied to the commission intending to test the resilience of the seals at low temperature in front of the TV cameras, but in an act that he claims to have been ashamed of, ran the test first in private to ensure that it was indeed the case that low temperature reduced the resilience of the rubber as he suspected.
When testifying before Congress, Feynman questioned a NASA manager with seeming innocence, focusing on the cold temperatures that the o-rings could be subjected to while remaining resilient (i.e., effective). The NASA manager insisted that o-rings would retain their resilience even in extreme cold. But Feynman managed to obtain a glass of iced water, and used it to cool a section of o-ring seal clamped flat with a small clamp he had purchased earlier at a hardware store.
After receiving repeated assurances that the o-rings would remain resilient at subzero temperatures, and at an opportune moment selected by Kutyna during a particular NASA slide-show, Feynman took the o-ring out of the water and removed the vise, revealing that the o-ring remained flattened, demonstrating a lack of resilience at 32 °F (0 °C), warmer than the launch temperature. While Feynman worried that the audience did not realize the importance of his action,
The New York Times picked the story up, crediting Feynman for his ruse, and earning him a small measure of fame.
Feynman's investigations also revealed that there had been many serious doubts raised about the o-ring seals by engineers at
Morton Thiokol, which made the solid fuel boosters, but communication failures had led to their concerns being ignored by NASA management. He found similar failures in procedure in many other areas at NASA, but singled out its software development for praise due to its rigorous and highly effective quality procedures which were under threat from NASA management which wished to reduce testing to save money since the tests were always passed.
Based on his experiences with NASA's
management and
engineers, Feynman concluded that the serious deficiencies in NASA management's scientific understanding, the lack of
communication between the two camps, and the gross misrepresentation of the shuttle's dangers required that NASA take a hiatus from shuttle launches until it could resolve its internal inconsistencies and present an honest picture of the shuttle's
reliability. Feynman soon found that, while he respected the
intellects of his fellow Commission
members, they universally finished their criticisms of NASA with clear affirmations that the
Challenger disaster should be addressed by NASA internally, but that there was no need for NASA to suspend its
operations or to receive less funding. Feynman felt that the Commission's conclusions were not compatible with its findings, and could not in good
conscience recommend that such a deeply flawed
organization should continue without a suspension of operations and a major
overhaul. His fellow commission members were alarmed by Feynman's dissension, and it was only after much petitioning that Feynman's minority report was included at all: as an appendix to the official document. Feynman's book
What Do You Care What Other People Think? included a
copyedited version of the appendix in addition to his narrative account.
It was announced in May 2006 that a movie would be made about the disaster.
Challenger is to be directed by
Philip Kaufman—whose 1983 film
The Right Stuff chronicled the early history of the space program—and would focus on the role of Feynman in the ensuing investigation.
David Strathairn will play Dr. Feynman.