The brilliant trailing beauty of fiery comets has inspired fear, wonder, and awe since the dawn of human history. Brighter than stars, moving and disappearing in their own singular orbits, comets have been among the most mysterious elements in the sky, eluding our understanding until very recently. With the aid of space probes, scientists have discovered that these swiftly moving chunks of ice and carbon are more plentiful and far more dangerous than suspected. Scientists are also beginning to realize the monumental role played by comets in the development of the Earth and solar system. David...