INVISIBLE COMETS ON EVOLUTIONARY TRACK OF SHORT-PERIOD COMETS

We systematically surveyed the orbits of short-period (SP) comets that show a large change of perihelion distance (q) between 1-2 AU (visible comets) and 4-5 AU (invisible comets) during 4400 years. The data are taken from Cosmo-DICE (Nakamura and Yoshikawa 1991a), which is a long-term orbital evolution project for SP comets. Recognizing that q is the most critical element for observability of comets, an invisibility factor (f), defined as the ratio of unobservable time span to observable span during 4400 years, is calculated for each of the large-q-change comets. A detection limit for each comet is obtained from the heliocentric distance at discovery and/or the absolute magnitude at recent apparitions. A mean f value for 35 SP comets with 2.9 <= J (J is the Tisserand's invariant) is found to be 19.8. This implies that for each visible SP comet of this J-range, at every' epoch of time, there exist about 20 invisible comets near the capture orbits by Jupiter, under the assumptions of steady-state flux and ergodicity for the SP-comet population.