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From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.astro
Subject: Halley's Doomsday
Message-ID: <646@petrus.UUCP>
Date: Tue, 15-Oct-85 18:43:16 EDT
Article-I.D.: petrus.646
Posted: Tue Oct 15 18:43:16 1985
Date-Received: Thu, 17-Oct-85 01:08:26 EDT
Distribution: net
Organization: Bell Communications Research, Inc
Lines: 32

The current theme of the Bloom County comic strip (the discovery by
Oliver Wendell Jones that Halley's comet will in fact hit the earth
on April 11, 1986) got me thinking.

The November issue of Scientific American gives the mass of Halley's
Comet as 2e14 kg. Now assuming that the impact velocity would be about
70 km/sec (30 km/sec for the earth's orbital motion + guesstimated 40 km/sec
for the comet, since it is in a retrograde orbit), the released energy
on impact would be about 5e23 J.  A kilogram of TNT releases about 4.6e6J;
a megaton (English ton) about 4.17e15 J.  The collision would therefore
be equivalent to about 100 million megatons, in round numbers.  This
number dwarfs by far even our combined nuclear weapons arsenals.

Here's another way to look at it.  The energy released by the detonation of
1 kg of TNT is enough to accelerate a 1 kg mass to a velocity of 3,033 m/sec
(this is the same as the ideal exhaust velocity were it possible to use TNT
in a rocket engine, and real rocket propellants have similar or even higher
values).  Therefore, a mass traveling at 3 km/s has kinetic energy
equivalent to an equal mass of TNT. Kinetic energy goes up as the square of
velocity, so Halley's comet at a closing velocity of 70 km/s has 544 times
the energy of an equal mass of TNT, and the thing weighs 200 metric
gigatons!

If anyone has more accurate figures (especially the relative orbital
velocities) please let me know. It seems rather unlikely that such a
collision would cause the Earth to plummet into the center of the sun, as
OWJ predicts, but it would still do a pretty effective job on its life
forms. Would anything larger than a microbe survive?

Kind of humbling, isn't it?

Phil