I don't disregard the possibility of collisions with co-orbiting
material. But the probability of colliding with something while passing
through the asteroid belt is still exceedingly small. That zone is still
basically empty space- very little material spread out in a massive
volume.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
----- Original Message -----
From: "Sterling K. Webb" <[EMAIL PROTECTED]>
To: "Chris Peterson" <[EMAIL PROTECTED]>;
<meteorite-list@meteoritecentral.com>
Sent: Monday, October 29, 2007 4:07 PM
Subject: Re: [meteorite-list] Comet Holmes
Hi, Chris, List
The best argument against a collision is the absurd
improbability of TWO collisions in the last century,
since this comet has a history of outbursts.
The problem with probability is the probability of the
assumptions that are applied. If 17P is an isolated object
and any impactor must come from another unrelated orbit,
the likelihood of any collision, ever, is very, very low.
Like all short period periodic comets, it is assumed
that 17P was perturbed into its present orbit, probably
by Jupiter. Since its orbit ranges from Jupiter to Mars
and is inclined to the solar system plane, 17P must transit
the Asteroid Zone twice every orbit (i.e., every 3.5 years).
One might pass harmlessly through the Zone at many
locations; at other places, you might not be so lucky.
If 17P is undergoing an on-going disintegration (from
a past major impact, perhaps very long ago), it may well
share its orbit with many smaller, darker (harder) fragments,
millennia-worth of its own "space-junk," a debris stream,
possibly arising from this ancient impact or partial breakup.
This would raise the probability of future "trouble" from
near zero to near 1.0. There may be more than one debris
stream accompanying it, braided around the principal orbit,
with objects distributed along the stream. Such streams
would be quite invisible to us. In the case of Holmes, the
odds of an outburst per orbit seem to be 12 to 1 against.
Collisions with co-orbiting objects occur at very small
velocity differentials (from the speed of a man walking
briskly up to that of a fast runner). Such collisions are not
catastrophic but damaging: gouging, ripping, crushing,
crust-breaking, volatile churning affairs. Once a century
is not that unlikely for such glancing impacts if there enough
co-orbiting fragments (especially the more silicate ones).
On the other hand, there may be no external impact event
responsible; it may be the result of some endogenous process
we do not understand. Whipple began the creation of models
that explain comet behavior and self-modification of their orbits,
the effects of thermal exposure, and so forth, and these models
have been greatly elaborated over the years, yet we cannot
explain much of comet behavior. Whipple suggested that Holmes
had been a "double" comet in which the pairs collided.
Holmes is a prime example of this. We think that it never gets
close enough to the Sun to explain the outbursts, but both the
discovery outburst and the present one occured after perihelion
passage with some delay. In both the discovery brightening and
the present one, the delay was five months! (June 16, 1892 to
November 6, 1892 -- 143 days; with a second outburst of equal
brilliance 60 days later. May 4, 2007 to October 24, 2007 --
173 days. A 60-day second outburst would make Holmes
a Christmas Comet.)
Does perihelion warming trigger some internal mechanism
that takes about five months to "boil up"? Or does Holmes catch
up with a stream of significant debris (a collisional association)
about five months after perihelion and sometimes interact
collisionally with it?
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