Abstract
Mean rates of the decrease in secular brightness of periodic comets are determined in two different ways: (1) from the statistics on their maximum absolute magnitudes reached at individual apparitions, and (2) from the statistics on their capture into and ejection from the inner planetary system, assuming a state of equilibrium on the time scale of a century. The results of both independent approaches are in very good agreement, indicating mean fading rates of about +0.01 magnitude per revolution at perihelion distances q < 1.5 AU and +0.04 magnitude per revolution at q > 1.5 AU. This progressive decrease in brightness is overwhelmed by much larger irregular fluctuations, which can even change its sign for a number of revolutions of each individual object. Implications for the total active lifetimes of periodic comets are also discussed.
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