Scientists who study asteroids often say that Earth is drifting through a cosmic shooting gallery.
Some angry space rocks, like the recent Chelyabinsk meteorite that exploded over Russia, are big enough to shatter windows and crumble walls. Others, like the one that caused the Tunguska Event of 1908, can flatten entire cities. A handful can trigger global extinctions, like the asteroid that smashed into Mexico's Yucatan Peninsula 66 million years ago.
But how big of an asteroid does it take to damage cities, states, countries, and the planet?
On the fourth-annual World Asteroid Day (Saturday, June 30), we take a look at estimates from NASA, Purdue University's "Impact Earth" simulator, and other sources.
Images in this story come from a previously published video.
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Most of us go through life without thinking much about rocks from space. But even a small — and relatively frequent — asteroid can inflict great damage.
By numbers, most meteors are smaller than a car and burn up in the atmosphere as harmless meteors.
Any larger than that, and things start getting risky.
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