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The prime objective is to settle a contradiction between a highnongravitational acceleration affecting the orbital motion of comet C/1995 O1and its enormous nucleus by modeling it as a compact cluster of boulder-sizedfragments held together by its own gravity. The nongravitational effect isinterpreted as a perturbation of the cluster's principal, most massivefragment. This and other constraints suggest that the principal fragment wasprobably 8-9 km across and the entire cluster ~150 times less massive than asingle-body nucleus of an equal cross-sectional area derived from the Herschelfar-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 ofmeters across under a steady-state distribution. The cluster was at most ~200km in diameter, subject to frequent collisions and significant perturbations bythe Sun near perihelion, and apparently a product of tidal fragmentation of theoriginal nucleus, more than 20 km across, at the time of close encounter withJupiter 4 millennia ago, if the comet's tensile strength was then as low asseveral Pa. Published for the first time are the results of a search forcompanion nuclei in three post-perihelion images taken with the HST's STISinstrument in 1997-1998. At least 29 such objects <1200 km (projected) from theprimary were detected, with their signals from 2.3% to 25% of the primary's andthe signal-to-noise ratios between 5:1 and 29:1, apparently also cluster-likein nature.

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