Physical evolution of comets

Mechanisms currently under consideration for the evolution of cometary nuclei are reviewed. Attention is paid to processes that should have occurred in the past history of observed comets, related to their origin or storage in the Oort cloud, but emphasis is placed on presently observed evolutionary effects and in particular the late stages characteristic of short-period comets. Evidence from brightnesses and nongravitational forces is discussed and support for a scenario of dust coverage coupled to the evolution of perihelion distance is found. However, for the cases of comets P/Halley and P/Encke where the perihelion distance is unusually small an ultimate fate of complete mass loss or disintegration can not be excluded. (author). 5 figs., 2 tabs., 102 refs