Comets; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

The word "comet," from the Greek, means "long--haired," an apt description for what may appear to be a blur or smudge in the heavens. Visitors from the farthest reaches of the solar system, comets consist of a solid nucleus of dust and ice, which has led them to be called "dirty snowballs." Interactions with the sun produce the nebulous coma and one or more tails that smear the comet against the sky. It was most likely a comet (although an asteroid remains a candidate) that smashed into Earth 65 million years ago, causing the mass extinction that killed the dinosaurs and paved the way for our own evolution.