Properties of Cometary Nuclei
Active long- and short-period comets contribute about 20 to 30 % of the major impactors on the Earth. Cometary nuclei are irregular bodies, typically a few to ten kilometers in diameter, with masses in the range 10(sup 15) to 10(sup 18) g. The nuclei are composed of an intimate mixture of volatile ices, mostly water ice and hydrocarbon and silicate grains. The composition is the closest to solar composition of any known bodies in the solar system. The nuclei appear to be weakly bonded agglomerations of smaller icy planetesimals, and material strengths estimated from observed tidal disruption events are fairly low, typically 10(sup 2) to 10(sup 4) N m(sup -2). Density estimates range between 0.2 and 1.2 g cm(sup -3) but are very poorly determined, if at all. As comets age they develop nonvolitile crusts on their surfaces which eventually render them inactive, similar in appearance to carbonaceous asteroids. However, dormant comets may continue to show sporadic activity and outbursts for some time before they become truly extinct. The source of the long-period comets is the Oort cloud, a vast spherical cloud of perhaps 10(sup 12) to 10(sup 13) comets surrounding the solar system and extending to interstellar distances. The likely source for short-period comets is the Kuiper belt. a ring of perhaps 10(sup 8) to 10(sup 10) remnant icy planetesimals beyond the orbit of Neptune, though some short-period comets may also be long-period comets from the Oort cloud which have been perturbed into short-period orbits.