  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.
