The solid, centrally located part of a comet. The nucleus is a mass of dust and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the
coma, which is later swept into an elongated tail. Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of Comet Hale-Bopp is one of the largest (perhaps 40 km). The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko). The dominant volatile is water, followed by carbon dioxide, carbon monoxide, formaldehyde, methanol, methane at a few percent level (with respect to water) and many other molecules at a lower level.
A small body of gas and dust which revolves around the Sun in a usually very elliptical or even parabolic orbit. It is seen to be composed of a head and often with a spectacular gaseous tail extending
a great distance from the head. The rocky-icy head is called the comet nucleus. As the comet nears the Sun, the increased temperature causes the ice in the nucleus to sublimate and form a gaseous halo around the nucleus, called the coma. Comets often possess two tails, a dust tail that lies in the orbit behind the comet generated by surface activity, and a brighter, ionized gas tail, that points away from the Sun, driven by solar wind. Cometary reservoirs are thought to represent primordial solar system material. A comet with a dust coating on its surface that inhibits gas production might be classified as an asteroid. Because of this ambiguity, objects such as Chiron, Centaur asteroid, have been reclassified as comets. Comets are primarily composed of amorphous water ice, but also contain carbon dioxide, carbon monoxide, formaldehyde, methanol, methane at a few percent level (with respect to water), and many other molecules at a lower level.