Abstract
Observational evidence for the aging processes in periodic comets is reviewed. This includes progressive changes of the comets’ absolute brightness, sudden destructive events (outbursts and splitting of cometary nuclei), temporary intermissions in activity, total disappearance, and existence of asteroidal objects moving in cometary orbits. Indirect statistical evidence is provided by the equilibrium between the aging rate and the net injection of comets into the inner planetary system. All of this information is consistent with typical active lifetimes of 200 to 500 revolutions at small perihelion distances (q < 3 AU). The active lifetimes are sometimes intermitted by dormant periods, which tend to occur especially during the latest phases of evolution. Splits and outbursts do not seem to have a decisive statistical effect on the survival time. Some periodic comets evolve into inactive asteroid-like objects, but the question of whether these still contain some supply of volatiles, and thus can renew their activity, remains open.
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