K. Cordes, Space Telescope Science Institute, Graphics Dept.
Short-period comets take less than 200 years to orbit the Sun and originate in the Kuiper Belt. Since their paths are deflected inward by the giant planets, most short-period comets orbit in the same region as the asteroid belt, located between the orbits of Mars and Jupiter. The relatively flat Kuiper Belt lies at the outer edge of the planets, beyond Neptune's orbit. Its radius is between 30 and 50 astronomical units (AU). There may be as many as 100 million objects in the Kuiper Belt.
Long-period comets require more than 200 years to orbit the Sun, generally taking many thousands to a few millions of years. They are usually only observed once. They come from the vast region in the outer reaches of our solar system known as the Oort Cloud. The Oort Cloud is a huge sphere of ancient, icy objects that envelops the region of the planets. The Oort Cloud extends a thousand times farther than the Kuiper Belt. It is 50,000 AU in radius.