Anniversary of Chelyabinsk asteroid impact: We need to test a deflector mission.
What Are the Odds an Asteroid Will Kill Us All?
What are astronomy's most intriguing puzzles?
Feb. 13 2014 11:47 PM

When Will the Earth Get Hit by Another Asteroid?

It’s time to test a deflector mission. It could save us all.

A meteorite trail is seen above a residential apartment block in Chelyabinsk, Russia, on Feb. 15, 2013.
A meteorite trail is seen above an apartment block in Chelyabinsk, Russia, on Feb. 15, 2013.

Photo by Oleg Kargopolov/AFP/Getty Images

Your odds of dying in an asteroid impact are about one in 700,000.

Surprising, isn’t it? That’s about the same chance you have of dying in a flood or a fireworks accident over your lifetime. It may be even more surprising when you consider that there has never even been a confirmed human death resulting from an impact. But this number involves something of a trick: A big enough impact will kill everyone on Earth. A smaller impact might devastate a local region on Earth, but a big one can wipe out entire species. Just ask the dinosaurs …

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

For a global event, you get these odds roughly by dividing the time between impacts by the average human lifespan. But it’s still a little misleading because it’s similar to the lottery: The chance is 100 percent that someone will win the lottery, but the chances are extremely low that you specifically will. Your odds of dying in an impact event are pretty low, but the odds of some random person somewhere getting killed are higher.


Of course, asteroid impacts are a lottery you get to play whether you want to or not.

Today is a good day to think about all this: It’s the first anniversary of the Chelyabinsk impact over Russia. On Feb. 15, 2013 (it was still Feb. 14 in U.S. time), a rock the size of a house came screaming in from space. In a single moment, its huge energy of motion was converted into light and heat. The resulting explosion was the equivalent of a half-million tons of TNT detonating all at once. Even though it exploded dozens of kilometers above the Earth’s surface, the shock wave shook the ground, set off car alarms, and shattered windows. More than 1,000 people were injured, some seriously, by flying glass.

Amazingly, no one was killed, but it shows quite vividly that the threat of asteroid impacts is quite real.

So when will the Earth get hit again? And what can we do about it?

* * *

The Earth gets hit by about 100 tons of material every day, but that’s in the form of tiny pebbles that burn up high in the atmosphere and produce shooting stars.

Big impacts are rare. The Chelyabinsk asteroid was 19 meters (62 feet) in diameter, and, on average, we should expect an impact from an object that size somewhere on Earth about once every 25 years. (Because most of the planet is covered in water, many of these go unnoticed.)

A small chunk of Chelyabinsk meteorite, given to me by a friend.
A small chunk of Chelyabinsk meteorite, given to me by a friend.

Photo by Phil Plait

Bigger impacts are more rare. In 1908 an object 30 meters or so in diameter came across the Earth, exploding high over a swampy region of the Russian countryside near the Tunguska River. The yield was equivalent to a 15 megaton nuclear bomb! Something like this Tunguska event (as it’s now called) happens every few centuries on average.

You probably know that the dinosaurs were taken out by an asteroid or comet about 10 kilometers wide. Happily, those events are extremely rare, occurring on a timescale of tens of millions of years. As it happens, we’re pretty sure there’s no dinosaur-killer on its way to Earth for the next few centuries. But the lesson of Chelyabinsk and Tunguska is that it doesn’t take a flying mountain to ruin your whole day. A hill will do nicely.

A hole in the Chebarkul Lake made by meteor fragments. A meteor shower hit Russia's Chelyabinsk region on February 15, 2013.
A hole in the Lake Chebarkul made by meteor fragments from the Chelyabinsk meteor shower on Feb. 15, 2013.

Photo ITAR-TASS Itar-Tass Photos/Newscom

If we want to prevent asteroid impacts from happening, the first thing we need to do is spot these threats. And we’re working pretty hard on that.

Astronomers have built quite a few observatories dedicated to patiently scanning the heavens looking for blips of light. Thousands of near-Earth asteroids have been found this way, their orbits meticulously calculated, projected into the future, and determined to be potentially threatening or not.

As things stand now, we don’t have the capability to find them all. But we will, soon. The huge Pan-STARRS telescope is looking deep for threats and is already producing data. LSST is a planned monster 8-meter telescope specifically designed to look for near-Earth objects and is expected to catalog hundreds of thousands of them.

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