How Afraid of Asteroids Should You Be?

> “Every man is free to rise as far as he’s able or willing, but the degree to which he thinks determines the degree to which he’ll rise.” -_Ayn Rand_

We’re all aware that one of the ways that human life on Earth could end, conceivably, is the same way that the dinosaurs went down.

And asteroid tracking and deflection technology is fast becoming one of the hot issues of the day. It appears so often in the news that you’d think we are at a high risk, any day, of being hit by a catastrophic asteroid.

But — and my opinion here definitely runs against the mainstream — I think this hysteria is absolutely ridiculous. One of the things you almost never hear about are the _frequency_ and the _odds_ of an asteroid strike harming you. If large asteroid strikes happened every few decades, we’d have something legitimate to prepare for and worry about. But if you’ve only got a one-in-a-million chance of an asteroid harming you over your lifetime — meaning you are over 100 times more likely to be struck by lightning than harmed by an asteroid — perhaps there are better ways to spend your resources.

Well, you know how we do things over here. Let’s talk about the science, and find out what the odds really are!

The Solar System is full of asteroids, mostly in a great belt between the orbits of Mars and Jupiter. A few of these asteroids are very large — 10 kilometers in size or larger — and could make the world uninhabitable for humans should one of them collide with our planet.

But the vast majority of asteroids that exist are much, _much_ smaller and less harmful than these. We can learn a lot about the history of impacts in our Solar System by studying, among other things, the craters on the Moon!

Take a look at the atmosphere-less Moon’s surface, above. Do you notice that there are a huge number of large craters with smaller craters atop them? This is because _most_ of the largest asteroids that collide with a world in our Solar System do so _rarely_, and they did so — mostly — a long time ago. But smaller, less destructive impacts happen more frequently.

But who cares about the Moon when we’re talking about saving the Earth? I bet you want to know what happens to our planet!

Fortunately for all of us, we have a wonderful defense shield against small asteroids: the _atmosphere_! An asteroid smaller than about 10 meters (33 feet) that hits the atmosphere will _not_ make it down to the Earth’s surface, nor will it affect anything that happens on the ground in any meaningful or destructive way.

All you get is a brilliant flash of light, known colloquially as a shooting star for tiny ones, or as a bolide (to astronomers, with apologies to geologists) for the larger ones!

How often do we get an asteroid larger than about 10 meters hitting the Earth? **About once every thousand years.** And we are “fortunate,” depending on your definition of fortunate, to have had one happen only about a century ago: the Tunguska event!

Now, this happened in a virtually uninhabited area in Russia, but the world is a much more populous place now. What would happen if something like the Tunguska event happened over a city like London?

The death toll could easily be in the millions. In addition, there are plenty of larger asteroids that — even though they hit Earth less frequently — cause greater amounts of damage. Once every 10,000 years, we get an asteroid that’s about 40 m in diameter hitting Earth. Once every 100,000 years, we’ll get an asteroid about 160 m in diameter (about one-and-a-half football fields) hitting the Earth. And about once every hundred million years, you’ll get that 10 km or greater “planet-killer” asteroid headed your way.

So when we take all of these numbers, consider the entire Earth, and calculate the probability of you being killed or injured by an asteroid, what do we get, and what do we learn?

First off, we learn that the Torino Scale — the scale that scientists have agreed upon for alerting the public about possible asteroid strikes — only matters if we consider numbers that are **eight or higher**. These are the asteroids _that will actually hit us_.

And second off, we find that _your_ odds of being killed or injured by an asteroid strike, in any given year, are about **one-in-70,000,000**. Which means, if you live to be 80, your _personal_ odds of being harmed by an asteroid strike in your lifetime are one-in-875,000. You are more than 100 times more likely to be struck by lightning, or die in other nasty ways.

Those are your odds. Those are your scientifically, number-crunched odds of being killed or injured by an asteroid here on Earth. If you’re terrified of those odds, so be it. But don’t let anyone exaggerate these odds to you, don’t let something with a Torino scale rating of 1 or 2 or 3 cause you to lose sleep at night, and please, if you’re a policymaker, **consider this reality** when you make your policy.

Thank you, and if you’re in the United States today, don’t forget to vote!

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URL: http://scienceblogs.com/startswithabang/2010/11/how_afraid_of_asteroids_should.php

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