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From: jeff@utastro.UUCP (Jeff Brown the Scumbag)
Newsgroups: net.astro
Subject: Re: Comet lifespans
Message-ID: <207@utastro.UUCP>
Date: Tue, 24-Dec-85 17:17:39 EST
Article-I.D.: utastro.207
Posted: Tue Dec 24 17:17:39 1985
Date-Received: Sat, 28-Dec-85 12:24:45 EST
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Organization: U. Texas, Astronomy, Austin, TX
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As our local comet people are away for the holidays and the news
retention time on our system is so short they may not see this,
I will run the gauntlet of trying to answer these.  Nearly all
my information is from a paper in the latest (October '85) 
Publications of the Astronomical Society of the Pacific by Armand
Delsemme and is based on a review talk he gave at last summer's
A. S. P. meeting.  It is direct enough so that I think most
reasonably literate people (and most people on the net qualify,
I think :-) could read most of it; it's not as bad as a lot of
technical literature.

> 	a) initial comet mass and size
We wish we knew.  All we have now is *present* cometary measures, of
couse, though periodic comets are supposed to be old and parabolic
ones new.  Delsemme says most periodic comets are 0.6 < (radius
in kilometers) < 4.  Parabolic comets are larger by about a factor
of two.  Radar measurements of comet IRAS/Araki/Alcock (which came
within .03 AU of Earth in 1983) indicated a diameter of about 7 km.
Masses are even more uncertain; the lower limits are based on
estimates of the masses of meteor streams and are about 10 to the
16 or 17 power grams.  The upper limits are so large as to be useless.
I suspect that from the passage of the ICE spacecraft past Comet
G-Z a couple of months ago that a good estimate of that comet's mass
will appear very soon (it may be out already and I didn't see it).
The Halley probes should give similar information.

> 	b) loss of mass and size at each orbit
For Halley, the estimate (averaged over one whole orbit) is 113 kilograms
per second; that is, 2.7 times 10(14) {10 to the 14} grams per orbit.

> 	c) number of orbits with "significant" tail
Observations show that Halley has been going for 2200 years.  Delsemme
says "...Halley has a present mass of 5 times 10(16) grams
and its half-life is 13 centuries"  and later says the uncertainty
is more than a factor of 2.

> 	d) final mass and size, assuming a "dead" core exists (does it?)
Here you're asking about Apollo-Amor objects, another class of objects
we wish we knew more about.  Unfortunately, I wasn't able to find a 
value for the size of these, other than a table entry of 0.5 km in a
source I'm not confident about.  I suspect a real planetary science
person would know this and I encourage any of them to sneer at my ignorance
and wave the number in my face.
 
> Or to be more specific: when is Comet Halley likely to wear out, presuming
> that we don't accidentally take it out with a probe first?
From everything above and other stuff I've heard/read, on the order of
a thousand years, give or take a factor of two, I guess.

Jeff Brown the Scumbag
		{allegra,ihnp4}!{noao,ut-sally}!utastro!jeff
		jeff@astro.UTEXAS.EDU
Astronomy Department, U. of Texas, Austin