The chances that your tombstone will read “Killed by Asteroid” are about the same as they’d be for “Killed in Airplane Crash.”
Solar System debris rains down on Earth in vast quantities — more than a hundred tons of it a day. Most of it vaporizes in our atmosphere, leaving stunning trails of light we call shooting stars. More hazardous are the billions, likely trillions, of leftover rocks — comets and asteroids — that wander interplanetary space in search of targets.
Most asteroids are made of rock. The rest are metal, mostly iron. Some are rubble piles — gravitationally bound collections of bits and pieces. Most live between the orbits of Mars and Jupiter and will never come near Earth.
But some do. Some will. More than a thousand known asteroids are classed as “potentially hazardous,” based on size and trajectory. Currently, it looks doable to develop an early-warning and defense system that could protect the human species from impactors larger than a kilometer wide. Smaller ones, which reflect much less light and are therefore much harder to detect at great distances, carry enough energy to incinerate entire nations, but they don’t put the human species at risk of extinction.
Every few decades, on average, house-sized impactors collide with Earth. Typically they explode in the atmosphere, leaving no trace of a crater. Once in about a hundred million years, though, Earth is visited by an impactor capable of annihilating all life-forms bigger than a carry-on suitcase.
One killer asteroid we’ve been monitoring is Apophis, which is large enough to fill the Rose Bowl. On Friday the 13th, April 2029, it will dip below the altitude of our communication satellites. If its trajectory on that day passes within a narrow range of altitudes called the “keyhole,” then the influence of Earth’s gravity on its orbit will guarantee that seven years later, in 2036, on its next trip around the Sun, the asteroid will hit Earth directly, likely slamming into the Pacific Ocean. The tsunami it creates will devastate all the coastlines of the Pacific Rim. If Apophis misses the keyhole in 2029, we’ll have nothing to worry about in 2036.
