The majority of the asteroids stay within the asteroid belt, a band of chunks of rock between the orbits of Mars and Jupiter. Short-period comets, or those with orbits from a few to 200 years, are thought to have originated in the Kuiper Belt, a fat disk of comet-like objects beyond the orbit of Neptune and Pluto. Long-term comets, or those that travel into the solar system every few thousand years (or may never return at all), are thought to originate in the Oort Cloud, a theoretical cloud of comets proposed by Dutch astronomer Jan Oort (1909–1992). The cloud surrounds the solar system about 100,000 astronomical units from the Sun (one astronomical unit is equal to about 93 million miles [149,637,000 kilometers], the average distance between Earth and the Sun).