Comets have fascinated people for millions of years, but until 1994 no one had ever had the chance to witness firsthand the destructive power of these wanderers. From July 16 to 22, fragments of Comet Shoemaker-Levy 9 smashed into Jupiter, creating a series of fireballs in the giant planet’s atmosphere. The string of explosions was awesome, yet the discovery that comets can wreak havoc would not have come as a surprise to our distant ancestors. People of widely different cultures have long regarded comets with fear and dread. The unheralded, unpredictable appearances of these ephemeral celestial visitors were believed to be omens of disaster for rulers, realms, and entire populations. Comets, it was thought, signaled or caused wars, revolutions, plagues, and other calamities. Haley’s Comet made an appearance in 1066. Modern scientists may not believe in omens, but they have come to realize that comets, which are only mountain-sized bodies, are capable of causing worldwide disasters as terrible as any that were imagined of old—and paradoxically might also be responsible for our very existence.