S1, E10: Comets--these celestial travelers have forever filled us with fear and wonder. Lurking in the furthest reaches of our solar system, they come close to Earth as they orbit our Sun. Could something as destructive as comets hold the key to life? Are the building blocks of carbon-based life forms frozen inside? Might they contain information about the creation of our solar system? At the conclusion of two spectacular NASA missions that sent spaceships to rendezvous with these mysterious objects, we examine the scientific and historical record of comets, including man's reaction to them. Did a comet lead the Wise Men to Bethlehem? Did they foretell the death of kings, the destruction of civilizations? How did Halley's Comet provide Isaac Newton with the clues for his theories of gravity? Finally, what comprises this "dirty snowball" and how can we protect ourselves if headed on a collision-course with one?