Comets are dirty snowballs of ice, gas, and dust that may hold the keys to answering how our solar system formed from a giant cloud of gas and dust some 4.5 billion years ago.

Photograph courtesy NASA, ESA, H. H. Weaver (JHU/APL)

MOST VIEWED NEWS "Lost" Amazon Complex Found; Shapes Seen by Satellite Earliest Known Galaxies Spied in Deep Hubble Picture PICTURES: Tigers Butchered for Trade at "Zoos" in China NEWS FEEDS      Get our news delivered directly to your desktop—free. How to Use XML or RSS