For a decade, NASA has been busy trying to identify what else is headed this way, particularly those potential “civilization killers” of 1 kilometer (.62 miles) or more in diameter that have orbits coming within 30 million miles of the Earth’s — too close for comfort by space standards.

But the big ones are, in many ways, the easy part. Smaller rocks matter, too. Perhaps nowhere is that so evident as in central Siberia, where 100 years ago last week, something — presumably a meteoroid, most experts say — streaked across the sky and exploded at an estimated height of 28,000 feet with a force equivalent to 185 Hiroshima bombs, leveling some 800 square miles of forest. Simulations by the Sandia National Laboratories showed that object could have been just 90 feet across.

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