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Oh no, not another impact thread...
Since some "astrophyics types" have actually been asked to comment on some
of the assertions in this week's Chicxulub discussion--
V.S. is quite correct that the tidal disruption of Shoemaker-Levy 9 occurred
far outside the atmosphere. The comet (or pedantically former Jovian
satellite) broke apart during a 1.5-hour passage inside Jupiter's
Roche limit (the radius for tidal disruption of an object bound only
by its own gravity), two years before impact. The tida stresses would
be even stronger right before impact, and in fact the debris clouds around
each fragment were seen to be radially stretching in the last pre-impact
The Earth could certainly cause similar fragmentation, but the cross-section for
a close enough passage is correspondingly smaller than for Jupiter. Breakup
on the final approach doesn't make much difference to the impact, as the same
amount of kinetic energy as carried in in a small volume whether in one
clump or several.
More to the point, there are some hints that the parent bodies can fragment
and produce sets of objects that might impact over thousands of years.
S.V.M. Clube in the UK has made this argument (in his inimitable and
provocative style); the events that replenish the supply of potential
Earth-crossers would be expected to yield several objects that have
high probability of impact over this timespan.
The atmopsheric detonations of SL9 in Jupiter do not necessarily scale to
similar events on Earth, as well. The pressure at the explosion level
is several times our surface pressure (i.e. if we'd been the victims,
they would have hit the surface first).
Astronomy, University of Alabama