Is it time to start worrying about killer meteors?

February 15, 2013

A 10-ton meteor tore through the sky over the Ural Mountain city of Chelyabinsk this morning, exploding with the force of a nuclear blast. Thanks to Russia's penchant for dashboard cameras, it was recorded for posterity:

The meteor wreaked plenty of havoc. After breaking apart some 18 to 32 miles in the air, the fragments hit the surface at an estimated 10 to 12 miles per second. The resulting sonic boom and falling debris smashed windows and roofs and injured more than 900 people. Scientists never saw it coming.

But do we really need to add tiny killer meteors to the list of things to fret about? Probably not.

Meteoroids are generally defined as less than 10 meters in diameter. About 40,000 tons of these rocks fall to the Earth every year, much of it minuscule space dust. The vast, vast majority of smaller meteors burn up harmlessly in the atmosphere.

It's only about once or twice a decade that a meteor the size of the one in Chelyabinsk actually makes impact. These are difficult for space agencies to track, because they're relatively small. And yet, as Stuart Clark of Across the Universe explains, they also almost always hit uninhabited areas or plop into the ocean:

No one is previously recorded to have been killed by a meteorite falling from the sky. There are stories of a dog being killed in Egypt in 1911 and a boy being hit but not seriously injured in Uganda in 1992. Most of the Earth's surface is uninhabited by humans, so meteorites usually fall over desolate areas or the oceans.

For example: Back in 2008, a meteor similar in size to the Chelyabinsk one was spotted heading toward Africa. It ended up crashing in the Nubian Desert in Sudan, far away from any humans.


Now this is something to fret about! (Space.com)

So that's the good news. The bad news is that there are also much, much bigger space rocks floating out there — asteroids. These can range anywhere from 10 meters across to 1,000 kilometers across. If a very large one fell to the Earth, it wouldn't burn up in the atmosphere. And depending on where it hit, an asteroid could wreak an enormous amount of havoc. In 1908, an asteroid about 50 meters across hit the Tunguska region of Siberia and leveled hundreds of miles of forest.

These larger asteroids often do wander close. On Friday, asteroid DA14 will come within 17,000 miles of us (this is a separate and unrelated event). That may sound like plenty of breathing room, but the asteroid will actually sweep closer than some weather satellites do.

Fortunately, scientists and space agencies tend to keep a closer eye on these bigger asteroids, though it's still not entirely clear what's out there: As my colleague Brian Vastag reports, NASA has identified about 95 percent of the large asteroids lying in our path but only about 1 percent of the smaller asteroids, like DA14, that could conceivably hit Earth.

Further reading: 

--Nature has a nice scientific breakdown of the Chelyabinsk meteor.

--Scientists at Purdue have created a handy asteroid calculator lets you see what would happen if various asteroids hit the Earth. An asteroid 400 meters in diameter, for instance, would create tsunami waves 60 feet high if it landed in the ocean.

--We previously looked at why so many Russian drivers have dashboard cameras. It's to provide evidence in courts of law in case of an accident. It's also handy for recording meteor strikes.

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Dylan Matthews · February 15, 2013