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, 16 × 8 km, with a dark crust; most nuclei are probably smaller. 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.
Subjects: Astronomy and Astrophysics.