| Not so long ago, comets were considered dirty snowballs traveling on highly inclined and eccentric orbits about the sun and the rocky asteroids traveled in nearly circular, highly flattened orbits within the asteroid belt between the orbits of Mars and Jupiter. However, there are now so many exceptions to these perceptions that the line between comets and asteroids is no longer clearly drawn. Today we have asteroids traveling on comet-like orbits, comets embedded within the asteroid belt and some confused objects that are classified as both comets and asteroids. Ground-based and space mission results suggest the enormous physical diversity of comets and asteroids, a diversity that includes active comets, ex-cometary fluff balls, rubble piles, shattered rock and slabs of solid nickel-iron. Understanding the population of near-Earth comets and asteroids will inform studies on the origins of the solar system and life on Earth, the possibilities for future space resources, human exploration of the solar system and planetary defense. |