asteroid belt demonstrate that there can be a fine line separating the two categories of objects. Origin Comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation. How and when comets formed is debated, with distinct implications for Solar System formation, dynamics, and geology. Three-dimensional computer simulations indicate the major structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals. The currently favored creation mechanism is that of the nebular hypothesis, which states that comets are probably a remnant of the original planetesimal "building blocks" from which the planets grew. Astronomers think that comets originate in the Oort cloud, the scattered disk, and the outer Main Belt. Size Most cometary nuclei are thought to be no more than about 16 kilometers (10 miles) across. The largest comets that have come inside the orbit of Saturn are 95P/Chiron (≈200 km), C/2002 VQ94 (LINEAR) (≈100 km), Comet of 1729 (≈100 km), Hale–Bopp (≈60 km), 29P (≈60 km), 109P/Swift–Tuttle (≈26 km), and 28P/Neujmin (≈21 km). The potato-shaped nucleus of Halley's comet (15 × 8 × 8 km) contains equal amounts of ice and dust. During a flyby in September 2001, the Deep Space 1 spacecraft observed the nucleus of Comet Borrelly and found it to be about half the size (8×4×4 km) of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight. The nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter. Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas. The nucleus of P/2007 R5 is probably only 100–200 meters in diameter. The largest centaurs (unstable, planet crossing, icy asteroids) are estimated to be 250 km to 300 km in diameter. Three of the largest would include 10199 Chariklo (258 km), 2060 Chiron (230 km), and (≈220 km). Known comets have been estimated to have an average density of 0.6 g/cm3. Below is a list of comets that have had estimated sizes, densities, and masses. Composition It was once thought that water-ice was the predominant constituent of the nucleus. In the dirty snowball model, dust is ejected when the ice retreats. Based on