Casualties Expected!

Did you know that the FAA measures the riskiness of private space operations in units of Expected Casualties?

Yes, that morbid unit is exactly what it sounds like. From the FAA document that shouldn’t be read unless you’re writing a blog post about it or love the potent boredom inducing combination of legal and engineering parlance:

 “Risk is defined by the safety community as the product of the probability of occurrence of an event and the consequences of that event.”

So, for example, the expected casualties for a SpaceX launch from Vandenburg Air Force Base is 0.0001 (one ten-thousandth.) Using expected casualties, as a unit of risk seems kind of ridiculous because it’s not the case that one ten-thousandth of a person will die every time there is a launch. It doesn’t even mean that one person will be killed every ten thousand launches.  It’s essentially useless except as a way to compare the risks of very improbable events – there’s a very small probability that the launch will fail and a bunch of people will die, and that failure will either be more disastrous or more probable than for a launch with 0.00001 (one one-hundred thousandth.)

Expected casualty numbers seem absurd even if you abstract them to just imply tiny relative probabilities because of the frequency of the launches they describe. Probabilities actually have meaning in frequent, repeated trials. You roll enough dice in Catan or Monopoly that you can make decisions based on the low probabilities of snake eyes or high probability of the dice summing to seven. However, a failure probability like 1/10,000 has almost no meaning when it applies to an event that will probably only happen ten or so times before changes in circumstance change the probability.

This is just one more area where the language of engineering can be improved. On the other hand, SpaceX has permission to launch from the West Coast at the bargain cost of only 1/10,000 of a life – sweet!


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