Comets with hyperbolic or parabolic orbits appear only once, after which they are flung outside the solar system. Lastly, main-belt comets, a group consisting of only three members, reside in the main asteroid belt. Similar objects, with their circular orbits, may have deposited water on Earth billions of years ago. This class, now consisting of 133P/Elst-Pizarro, 118401 1999 RE70, and P/2005 U1, exists at the crossroads between comets and asteroids.
By contrast, scientists know of over 642,000 minor planets. Of these, nearly 400,000 have been studied well enough to warrant permanent numerical designations, and more than 16,000 asteroids have names. Astronomers believe the total number of asteroids in the solar system larger than 1 mile (1.6km) across is about 1 million.
The largest asteroid in the main belt is Ceres, which has a diameter of about 600 miles (970km). Altogether, the mass of main-belt asteroids adds up to only about 4 percent of the Moon’s mass.
During the 1980s and '90s, scientists’ neat distinction between comets and asteroids began to blur. With the discovery of the Kuiper Belt, it became clear that a vast population of icy bodies existed on the solar system’s fringe, much closer than the Oort Cloud.