Wednesday, October 06, 2010

Astronomical risk calculation

One of the greatest challenges facing media saturated democracies is the inability to correctly gauge the relative importance of catastrophic, visually impressive, rare events vs. common incremental ones. Here are some examples:

If, dear reader, you happen to die in transit, I personally guarantee that you won’t be killed in an airplane blown up by terrorists. The overwhelming odds are that you will die in your own car instead.

Similarly, if you are killed by our energy infrastructure, it won’t be nuclear meltdown that wipes you out. Chances are, you’ll have an asthma attack, or get emphysema from air pollution instead.

And as a geologist, I make the following prediction with even more confidence.

Nobody reading this blog will be killed by a meteorite impact.

It just isn’t going to happen. Fatalities from geohazards are a rare form of death anyway, but if you are unlucky enough to succumb to one, it will probably be a flood or some sort. If not, then an earthquake, or a debris flow. Asteroid impacts are far more spectacular than muddy rivers, but the big ones just don’t happen often enough to create any significant health risk. But that doesn’t stop bolide researchers from scare-mongering in order to get funding.

Just look at the second picture in this Planetary society post from last year. It is an overlay of the Tunguska event on the city of Los Angeles, showing how huge swaths of the city could be incinerated by this relatively modest sized impactor. No mention is given of the likelihood of this event. Luckily for us, though, it is easy to calculate.

Impacts are equally likely on any point on the globe. Thus, the likely hood of an impact in the city of Los Angeles is simply the area of LA divided by the surface area of the Earth. For those who don’t have the numbers on hand, this is about one in 400,000, or 2.5 out of a million. And LA is a very large city; the city limits are only slightly smaller than the state of Rhode Island. Assuming one Tunguska event every century, we’d expect to have to wait for something on the order of 20 million years for a 50% probability of hitting LA. And the rocks LA is built on aren’t even that old.

The most likely scenario for the next Tunguska-style event is for it to happen over the open ocean. And if that would occur, the effect we would see would be…
Absolutely nothing.
A satellite might see such an event. Otherwise, we wouldn’t even know it happened.

So, just for kicks, I pulled an equal area map off of the web, and hit it with 100 impacts, just to see what would happen. I’ll get into the gory details in another blog post, but just to whet your apatite, the picture is here.

The dots are about 130 km across. This is the approximate kill radius for a 400 meter rock under standard assumptions. A rock this big lands about once every 100,000 years, according to the theoreticians, so this is 10 million years worth of impacts.

In future posts I’ll look at some of the details, and what they might mean.


Brian Romans said...

This is cool, I like posts like this.

One question though -- wouldn't an ocean impact create a tsunami? I have no idea of the magnitude, perhaps it wouldn't be significant. Someone I'm sure has modeled that.

Also ... "whet your apatite" ... nice

C W Magee said...

Hi Brian,
The accompanying paper that is related to that disaster effects link talks about tsunamis, and basically the estimates are all over the place. One interesting hypothesis, which I haven't et dug into the links for, is that impact tsunamis break on the edge of the continental shelf, and have relatively little effect at shore. Presumably someone in your line of work would have to look for the depositional features related to such an effect.

Anonymous said...

So what were the odds of your experimental map landing a dot on the Yucatan peninsula?