Until the emergence of modern astronomy in the 1500s and 1600s, comets remained mysterious and frightening signs of disastrous or extraordinary events. The noted Protestant leader Martin Luther, who died in 1546, stated the common view. He was aware of the writings of Greeks like Aristotle and Romans like Seneca, who had held that comets result from natural processes. Luther flat out rejected the ideas of these "heathens," or nonbelievers in Christianity, saying, "The heathen writes that the comet may arise from natural causes; but God creates not one [comet] that does not foretoken a sure calamity."8

In the two centuries following Luther's passing, how- ever, a number of pioneering astronomers demonstrated that comets are indeed natural, rather than supernatural, bodies. Moreover, these scientists showed, comets orbit the sun. And following their measurable, predictable orbits, some comets return to Earth's neighborhood on a...