Today is the 100th anniversary of the Tunguska Event, a massive explosion in the Siberian wilderness whose cause remained a mystery for most of the twentieth century. The explosion flattened eighty million trees over an 800-square-mile area, but left no discernible impact crater. The remoteness of the site impeded research into the origins of the explosion, and Soviet scientific findings diffused only gradually into the West. Consequently, astrophysicists, earth scientists, science fiction writers, and random crackpots developed a variety of hypotheses about the explosion - that it was caused by a comet, a chunk of interstellar antimatter, a quantum black hole (a popular hypothesis among science fiction writers), or a malfunctioning alien spacecraft. In the 1950s and '60s, however, Soviet researchers discovered microscopic glass nodules in soil samples from the site; the nodules contained high levels of nickel and iridium, both telltale markers of an asteroid. It seems likely, therefore, that the explosion was produced by a meteor - a small (20-30 meters) asteroid or a fragment of one - exploding in the air above the Tunguska River basin.
I don't usually direct readers to Wikipedia, but the entry on the Tunguska event appears judicious; it includes several firsthand accounts of the explosion from observers, a careful weighing of causal hypotheses, and the observation that these sorts of midair meteor explosions are actually rather common - but they generally occur over the ocean, where (until the advent of earth-observation satellites) there haven't been many witnesses.
While we're on the subject of asteroids intercepting the earth, we may note that Greg Easterbrook isn't the only person worried about the consequences of a large meteor impact in the near future. The U.S. House of Representatives just passed a bill (HR 6063) which, among other provisions, directs NASA to plan an unmanned monitoring expedition to the Apophis asteroid (which will pass uncomfortably near the Earth in 2029) and the White House Office of Science and Technology Policy to develop an asteroid-deflection strategy. The bill didn't estimate the cost of the program, but I can't imagine it will cost more than, say, a week or two of the Iraq War. No mention of any role for Bruce Willis, either, but he's still got 21 years to wait for the phone to ring.