> The point of all this is that it is a whole lot
> easier to get a chunk of ice (comet) to explode in mid air than anything
> else. My guess on Tunguska is that it was a small comet rather than a
> carbonaceous chondrite.
>
Just as an aside, the same article ended on a note of warning. The authors
believed that carbonaceous chondrites posed a significant threat to human
civilization, since the effect of of a CC airburst is almost
indistinguishable from a nuclear airburst (or so they claim). The authors
believed that a CC airburst over New York, Moscow or Peking could trigger a
nuclear war since (at the time that article was written) no one would suspect
or believe that a meteorite could do the same kind and amount of damage as a
thermonuclear warhead.
[snip]
>
> Kevin is correct that a Tunguska style event is a possible explanation. But
> I don't think it will work for the following reasons. First there was not
> much of a dust cloud.
There is a way this might work. Carl Sagan has claimed that it would only
take about a dozen one megaton explosions to initiate a nuclear winter (I
don't know if he was referring to airbursts or ground detonations.) If this
is correct, then a large comet that broke up before hitting the earth (like
Shoemaker-Levy 9) could supply enough material to create anywhere from one to
two dozen airburst explosions of sufficient magnitude to put enough dust and
soot into the air to lower the temperature for a year (which is just about
all that would be needed to collapse most ancient civilizations).
The problem with this explanation is that, while I can imagine it happening
once, I cannot see it happening more than twice in the past six thousand
years; in other words, I doubt we can invoke a "Nemesis affect" to explain
the collapse of ancient civilizations.
Kevin L. O'Brien