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Conglomeration of frozen water and gases (methane, ammonia, CO2) and silicates that that formed in the outer solar system and orbits the Sun. In recent years, the description of comets has shifted from dirty snowballs to snowy dirtballs with more dust than ice. However, the ratio is less than 10-to-1. Comets appear to contain a mixture of materials formed at all temperature ranges, at places very near the early sun and at places very remote from it. High-temperature silicates (e.g., olivine) formed near the protosun and were ejected, perhaps by strong bipolar jets, to the outer parts of the solar system. Recent data indicate that comets probably have porosities of ~75% and perhaps no solid core. It is likely that they consist of very loosely packed grains.

Comet orbits vary in eccentricity between an ellipse and a parabola and have known periods from 3 to 1000s of years. The names of such periodic comets are prefaced with “P/” (e.g., Comet P/Halley).



Comet Hale-Bopp. Image Credit: Unknown

Near the Sun, a comet’s materials are evaporated to produce a coma of gases and dust which may develop into a tail extending, in some cases, 80 million km. Comets have two types of tails, one comprised of dust and the other ionized gas, which point in different directions (white and blue, respectively, in the photograph; the red colors are of an emission nebula in the background).