Astronomers have identified numerous asteroids big enough to cause a catastrophe for the entire planet. None is on course to do so in our lifetimes, but there are many smaller 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 Tunguska, a remote part of Siberia. The object-an asteroid or a small comet-exploded several kilometers before impact, burning and blowing down trees across 2,000 square kilometers (80 square miles). Clouds of tiny particles of dust and ice filled the sky. The particles reflected the sun’s light onto the Earth, and for days people in Europe could read newspapers outdoors at night.
The next time a large object falls out of the sky, we may be taken by surprise-currently, there is no early warning system for near-Earth objects. However, over the next decade, sky surveys, like the one being done by Tholen, should begin filling that gap. Astronomers are compiling a list of thousands of asteroids to help us anticipate the next strike. “Every couple of weeks,” says former astronaut Lu, “we’re going to be finding another asteroid with, like, a one-in-thousand chance of hitting the Earth.”