The small solid body, composed of frozen water and gas plus embedded dusty material, at the centre of a comet’s head. It is the source of activity in a comet, and contains essentially all its mass. The nucleus of Halley’s Comet, the first to be directly observed (by the Giotto space probe in 1986), is irregularly shaped, 15 × 8 km, with a dark crust; most nuclei are probably smaller. The nucleus of comet Churyumov–Gerasimenko has a twin-lobed structure, apparently resulting from the merger of two separate bodies. Many nuclei are probably aggregations of numerous smaller objects with voids between, resulting in average densities less than that of water (see rubble pile).

When very distant from the Sun, a comet’s nucleus is inert. Within 3–4 au, however, solar heating leads to gas sublimation, and production first of a coma, then a tail (see coma, cometary; tail, cometary). Near perihelion, surface temperatures may reach 25°C. Cometary nuclei have low densities (typically 0.2g/cm³), and are prone to fragmentation. Nuclei which have made several perihelion passages are thought to be covered by a dark crust, as Halley’s Comet. Gas emerges through fissures in the crust, carrying off dust which pursues its own orbit as a meteor stream. Perhaps only 10% of the nucleus consists of such active regions at any one time. Emerging jets are the cause of non-gravitational forces which affect the orbits of comets. Pristine nuclei from the Oort Cloud may lack a dark crust and experience less heating than those of well-evolved comets, as happened with Comet Kohoutek. Some Apollo asteroids may be old, outgassed nuclei.

http://photojournal.jpl.nasa.gov/catalog/PIA13629