Astronomers have separated the small objects in the solar system into two main categories based on their telescopic appearance. Known as the comets and asteroids, they differ in composition, orbits, and beginnings. Most comets are dirty balls of ice with very elongated orbits that take them out to the distant reaches of the solar system, moving at every possible angle to the Earth’s orbital planet. The asteroids are lumps of rock confined mostly to nearly circular orbits relatively close to the Sun, in the main belt between Mars and Jupiter, moving in the same plane and direction as the planets. Comets sublimate surface ice when they travel close to the Sun, emitting gas and dust, but asteroids do not emit anything wherever they are.
Recent discoveries have blurred the boundaries between these two classes of small objects. There are comets that behave like asteroids, and asteroids that act like comets. A handful of bodies have a dual personality, behaving like both a comet and an asteroid. So there is some overlap between the two categories, and our strict definitions have to be relaxed to take the anomalies into account.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University