Back to Top

Ask OCE — February 8, 2006 — Vol. 1, Issue 3


January 28, 1986 is a day etched in the memories of everyone associated with NASA at the time. The impact of the Challenger disaster was not confined to NASA, of course; it was a national tragedy. The loss of the seven-person crew on Mission STS-51-L, including the first school teacher to fly in space, shocked an Agency as well as a world accustomed to an unbroken string of Space Shuttle successes into remembering that human space flight is an inherently dangerous undertaking.

Twenty years later, NASA is a different organization. A generation of successes, failures, organizational reforms, and developments in government and business and have profoundly changed the way we operate. But one thing remains the same: NASA still depends on people to assess and manage risks. And research has shown time after time that people have a hard time weighing risks objectively, whether they’re rocket scientists or individual stock market investors.

One of the most common and costly errors we can make in judging risks is to “normalize deviance.” It’s not a problem that’s specific to NASA; it’s a human problem. Homeowners, for example, may normalize deviance by believing that a few years of exponential growth in housing prices indicate a trend that will continue unabated in the future. As the standard disclaimer says, past performance is no guarantee of future results.

With spaceflight projects, normalizing deviance means that we downgrade known technical problems to “acceptable risks” when we shouldn’t. Rather than hit the brakes until we find a solution that mitigates a problem, we begin to think that we overestimated the importance of our initial engineering analysis because the risk in question didn’t cause a major problem the first, tenth, or fiftieth time that we executed a procedure. In the case of Challenger, the Solid Rocket Booster joint, a known risk, was downgraded from Criticality 1 to Criticality 1-R (redundant) to “closed” in the problem-reporting system shortly before the launch of STS-51-L. A high risk became an acceptable risk. The consequences were tragic.

Challenger is far from the only example of normalizing deviance and downgrading risks in NASA’s history. As the Columbia Accident Investigation Board pointed out, a variation on the same phenomenon occurred with Columbia. (For more details, see Chapter Eight of Columbia Accident Investigation Board Report, Vol. 1,.) When we normalize deviance from procedures, the result can be costly even when it’s not deadly, as with NOAA A-Prime.

As we pause to honor the memory of the men and women of STS-L-51, we should renew our efforts to remain ever vigilant against normalizing deviance, against treating risks as business as usual. Complacency is our enemy. We know all too well the cost of losing sight of this.


Learn more about the Challenger accident from the NASA History Office.


More Events: This Week in NASA History

January 31, 1958 Explorer I: First U.S. satellite

January 31, 1961 Mercury MR-2: An American chimp goes to space and back.

January 31, 1971 Apollo 14 Launched

January 28, 1986 Space Shuttle Challenger Mission STS-51-L

In This Issue

Message from the Chief Engineer

NASA in Washington: John Marburger’s Call for a Single LANDSAT Spacecraft (PDF)

This Week in NASA History: Space Shuttle Challenger

James Barrowman on Defining Requirements

Modeling the Way We Work

Stardust Returns, New Horizons Takes Off

About the Author

Share With Your Colleagues