Tidal Disruption of Strengthless Rubble Piles--A Timescale Analysis
J. M. Hahn (LPI), T. W. Rettig (Notre Dame), W. R. Ward (JPL)
The brief and dramatic appearance of Comet Shoemaker-Levy 9 (S-L 9) has punctuated the notion that many small members of the solar system might be strengthless `rubble-piles'. Models of the S-L 9 encounter indicate that only a relatively strengthless rubble-pile is able to catastrophically disrupt into a cloud of debris that later condenses into twenty or so gravitating `clumps' having the S-L 9 `string-of-pearls' morphology (e.g., Asphaug and Benz 1996). These models show that gravitational condensation of the debris into distinct clumps depends sensitively on the progenitor's density as well as its orbit.
This phenomenon is re-examined
by tracking the orbital motion and
the local mass density of a tidally disrupted projectile
(see also Rettig et al. 1996). By employing
elementary orbit mechanics, it is shown that when the debris'
gravitational contraction timescale becomes shorter than its orbital
spreading timescale, the debris breaks up into 
distinct clumps, where L is the debris length and D is the
progenitor's diameter. Without appealing to a CPU-intensive
calculation, we extend the available N-body simulations to
unexplored regions of parameter space and reveal
how the number of clumps n depends upon the
progenitor's density, its periapse distance, and velocity
at infinity (the problem is generalized
to hyperbolic encounters as well).
These findings also provide an
additional constraint for the Galilean crater
chain problem. We find that the projectile's responsible for the Gomul
and Gipul crater chains on Callisto likely had comet-like densities
of 
gm/cm 
. However we are unable to distinguish
between cometary and asteroidal impactors
for the remaining chains which have fewer numbers of craters.