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Then on December 26, 2004, a real catastrophe struck: the Indian Ocean tsunami, which claimed hundreds of thousands of lives. The public forgot about Apophis. In the meantime, astronomers had dug out earlier images of the asteroid. The extra data enabled the scientists to calculate its orbit, and they discovered that it would actually whiz safely by Earth in 2029. But they could not rule out a slender chance that Apophis would strike with catastrophic effect its next time around, on Easter Sunday, 2036.

An estimated ten million rocky asteroids and ice-and-dirt comets pirouette in outer space, and once in a while their paths fatefully intersect our planet’s. One such encounter took place a hundred miles from present-day Washington, D.C., where a 53-mile-wide crater lies buried beneath Chesapeake Bay—the scar left when a two-mile-wide rock smashed into the seafloor 35 million years ago. More notorious is the titan, six miles in diameter, that barreled into the Gulf of Mexico around 65 million years ago, releasing thousands of times more energy than all the nuclear weapons on the planet combined. “The whole Earth burned that day,” says Ed Lu, a physicist and former astronaut. Three-quarters of all life-forms, including the dinosaurs, went extinct.

Astronomers have identified several hundred asteroids big enough to cause a planet­wide disaster. None is on course to do so in our lifetimes. But the heavens teem with smaller, far more numerous asteroids that could strike in the near future, with devastating effects. On June 30, 1908, an object the size of a 15-story building fell in a remote part of Siberia called Tunguska. The object—an asteroid or a small comet—exploded a few miles before impact, scorching and blowing down trees across 800 square miles. The night sky was so bright with dust from the explosion, or icy clouds from the water vapor it blasted into the upper atmosphere, that for days people in Europe could read newspapers outdoors at night. On Tunguska’s hundredth anniversary, it’s unsettling to note that objects this size crash into Earth every few centuries or so.

The next time the sky falls, we may be taken by surprise. The vast majority of these smallish bodies, capable of wiping a city off the map, are not yet on our radar screens. “Ignorance is bliss, in that if you don’t know about these things, you just go about your merry way,” says Lu. Over the next decade, however, sky surveys like Tholen’s should begin filling that gap, cataloging asteroids by the thousands. “Every couple of weeks,” says Lu, “we’re going to be finding another asteroid with like a one-in-a-thousand chance of hitting the Earth.”

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