How it could happen: Objects from space impact Earth all the time, generally burning up in the atmosphere. Occasionally, a large object makes it through, resulting in a massive impact. The most recent major impact was the 1908 Tunguska event that flattened a 2,000-square-mile area of Siberian forest with an explosion about 1,000 times more powerful than the Hiroshima bomb. The object involved, which was likely only a few dozen meters in diameter, could have wiped out a major metropolitan area.
The real danger would come from an object more than a kilometer in diameter, which could kick up enough sediment to cause environmental damage and crop failures worldwide. A rock the size of the 15-kilometer object that is thought to have killed the dinosaurs 65 million years ago would probably wipe us out too.
How likely is it? Another large object is sure to hit the earth eventually, but almost certainly not during our lifetimes. An object big enough to kill off a substantial portion of the earth's population only hits earth about twice every million years. None of the objects yet discovered by NASA's Near Earth Objects program have a high probability of hitting the earth -- though one known as 1950 DA will come extremely close in 2880. Given the relatively small effort devoted to identifying near-earth objects, there's no guarantee that the earth would have much warning time before one hit. A previously unknown seven-meter asteroid passed just 14,000 kilometers from the earth's surface on Nov. 6 and was noticed by NASA only 15 hours before what counts as an entirely too close encounter.