Which is more dangerous: A shark or an asteroid? If the magical Risk Fairy fluttered over with her sparkly wand and offered you a chance to cancel out one of these two risks, which would you choose?
Let's run the numbers. In any given year, sharks kill about half a dozen people. Last year, the tally of asteroid-related deaths was zero. The year before that, it was also zero, as it was the year before that, and the one before that--and so on. In fact, the total number of human beings who have definitively fallen victim to asteroid strikes in all of recorded history is, you guessed it, zero.
But before you sic the Risk Fairy on those bloodthirsty sharks, consider this: If a killer asteroid were to strike the Earth, it wouldn't just pick off a couple of luckless surfers. An asteroid with a diameter of 5 km could kill a billion people. Not to get all 2012, but it could mean the end of civilization as we know it.
Freaky? Sure. But on the other hand, asteroids that big don't come along very often--only once every 10 million years or so. Ten million years is a lot of human lifetimes, especially when you consider that Homo sapiens has only been around for about 200,000 years. But if we're going to compare asteroids to sharks, we need an annual fatality rate, and that's the what Alan Harris was talking about at this morning's "Cosmic Disasters" session of the Division of Dynamical Astronomy. By combining what we know about the number, size, and trajectory of our neighborhood asteroids (and comets) with what we know about more familiar catastrophes (like nuclear bombs and tsunamis), scientists can project the number of fatalities expected each year from asteroids. And that number is 91.
Ninety-one! That's sky-high compared to the half-dozen shark fatalities. Just one more thing before you call in the Risk Fairy with your choice: Let's look at some other annual fatality rates. Take firearm accidents: 2,500 deaths per year. Malaria: 1 million deaths per year. Tobacco: 5 million deaths per year.
So, what would you do? Questions poured in from the scientists in Harris' audience: Is it even fair to rate the risk of low-probability, high-fatality events on the same numerical scale as, say, traffic accidents? Are these risks of a fundamentally different quality? And if they are, how can we begin to make informed policy decisions about investing in asteroid tracking and mitigation?
Facts and figures from Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies, a National Research Council report. Download the full report here.