🚀 go-pugleaf

-From: morlan@afit.af.mil (Bruce W. Morlan)
-Subject: Re: Comet Collision
-Date: 28 Oct 92 18:17:45 GMT
-Organization: Air Force Institute of Technology

-rsb@mcc.com (Richard S. Brice) writes:

->Would anyone care to comment on how probability plays a role in the
->future interactions of earth and comet P/S-T and how much of the
->script is already written into the clock?

-Sure.  The script is nearly totally written.  The Heisenberg uncertainty
-associated with the objects in question (Earth, comet, Sun, other
-planets, etc.) is practically 0 over the time frame in question.  On the
-other hand, the _uncertainty_ in the measurements leave much room for
-surprises.  I cannot comment on the measurement errors.
 
-Bruce W. Morlan, Major, USAF             Air Force Institute of Technology
-Dept. Department Head                    AFIT/ENC
-Department of Mathematics                WPAFB  OH  45433

Chaos is the measure of the degree to which errors in the initial measurement
of a system will affect the accuracy of predictions of the future condition
of the system. I don't really know how chaos is quantified or used in
calculations. The degree of chaos varies widely from system to system, and
in some systems it varies widely over time. For instance, the orbits of
the inner planets are slightly chaotic, Pluto perhaps more so, and the 
degree of chaos in weather systems varies considerably over time (sometimes
very good predictions can be made several days ahead of time, and sometimes
the predictions for the next day are very uncertain). The net effect of
chaos is that the future state of a system is not a known quantity, but
a range of possible states, with a distribution of calculated probabilities.
(If some future event is considered a "sure thing" that means that almost all
of the probability distribution is within the range where the stated event
takes place.)

The long-term motion of Swift-Tuttle would have to be considered very 
chaotic, due to the velocity changes brought about by outgassing. As the
comet moves further from the sun, the amount of outgassing decreases, to
its trajectory becomes less chaotic. (That's why there's so much interest
in getting precise measurement some years from now, though accuracy is still
not guaranteed - I believe Halley's Comet had a major outgassing event several
months ago, though it was several years from the perigee (perihelion, for
the politically correct :-) of its orbit. Gravitational influences of the
bodies of the outer solar system will also affect the comet, and uncertainties
in those will affect the accuracy of the calculations.)

As the comet comes by on its next approach, solar heating and outgassing
will resume and affect the comet's path somewhat, but since those changes
will have much less time to affect the comet before Earth flyby, they are
of much less concern than the changes during this pass.

John Roberts
roberts@cmr.ncsl.nist.gov