Orbital Evolution, Activity, and Mass Loss of Comet C/1995 O1 (Hale-Bopp): II. Nucleus and Companions as Compact Clusters of Massive Fragments

The prime objective is to settle a contradiction between a high nongravitational acceleration affecting the orbital motion of comet C/1995 O1 and its enormous nucleus by modeling it as a compact cluster of boulder-sized fragments held together by its own gravity. The nongravitational effect is interpreted as a perturbation of the cluster's principal, most massive fragment. This and other constraints suggest that the principal fragment was probably 8-9 km across and the entire cluster ~150 times less massive than a single-body nucleus of an equal cross-sectional area derived from the Herschel far-infrared photometry of the inactive comet detected near 30 AU from the Sun. The cross-sectional area required the smallest fragments to be a few tens of meters across under a steady-state distribution. The cluster was at most ~200 km in diameter, subject to frequent collisions and significant perturbations by the Sun near perihelion, and apparently a product of tidal fragmentation of the original nucleus, more than 20 km across, at the time of close encounter with Jupiter 4 millennia ago, if the comet's tensile strength was then as low as several Pa. Published for the first time are the results of a search for companion nuclei in three post-perihelion images taken with the HST's STIS instrument in 1997-1998. At least 29 such objects <1200 km (projected) from the primary were detected, with their signals from 2.3% to 25% of the primary's and the signal-to-noise ratios between 5:1 and 29:1, apparently also cluster-like in nature.