“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.”