Re: [Fwd: Earth Battered Through History By Comets]
Thu, 26 Aug 1999 19:58:34 +0000

At 09:27 AM 08/26/1999 EDT, wrote:
>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
>or believe that a meteorite could do the same kind and amount of damage as a
>thermonuclear warhead.

I think it would still start a nuclear war even in today's peaceful times.
>> Kevin is correct that a Tunguska style event is a possible explanation.
>> 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
>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).

I read all of Sagan's Nuclear winter articles back when they proposed it.
As I recall most of the worst particulates were from the fires that the
nukes started, not really the dust. The fires would burn for days or weeks
burning all the petroleum rich materials like plastics, clothing, etc. The
heat plumes would lift the particulates from the fires into the

The difference between the airburst and the nukes is the higher temperature
and lower altitude of the nukes, although airbursts would start fires under
certain circumstances.

I wish I had my old program that was stolen, but as I recall, any object
greater than about 10 feet in diameter or so will crash into the earth.
Anything larger than a mile (or something like that) won't even begin to
slow down due to atmospheric friction.

>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.

The probabilities are higher than one might imagine but not as often as
that book cited earlier in this thread wants. THis is part of a 1994 post
to usenet by jon Leech:
Asteroid Number of objects Impact probability Impact energy as
diameter (km) (impacts/year) multiple of
Hiroshima bomb

10 10 10^-8 10^9
1 1 000 10^-6 10^6
0.1 100 000 10^-4 10^3

assuming simple scaling laws. The Hiroshima explosion is assumed to be
.013 MT TNT equivalent, or about 5*10^13 joules.

Gehrels, T. 1985 Asteroids and comets. _Physics Today_ 38, 32-41. [an
excellent general overview of the subject for the layman]


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