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From: jay@npois.UUCP (Anton Winteroak)
Newsgroups: net.astro
Subject: re When did comet Halley get captured
Message-ID: <4@npois.UUCP>
Date: Tue, 7-Jan-86 13:07:53 EST
Article-I.D.: npois.4
Posted: Tue Jan  7 13:07:53 1986
Date-Received: Wed, 8-Jan-86 08:36:25 EST
Organization: ATTIS, Neptune, NJ
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	Someone asked if we can plot backward to find when the close
encounter that put Halley's Comet in approxomately it's modern orbit
was.
	The answer is that it is unlikely that we will ever know. Even
with all the very accurate data about the last several orbits, the time
of Halley's return this time was uncertain by more than a month.
	The further back we look at the records of Halley's Comet,
the worse the time and position data is. Also, if you note when the
closest approaches were, you'll see that the period has varied considerably.
	Most comets keep irregular orbits, since the jetting of material
towards the sun, and with the direction of rotation, causes a small but
signifigant change in momentum.
	Clearly a single event capture would require the comet to get
very close to a planet. This planet would most likely be Earth or Venus
since only those two planets are close to where Halley's comet crosses
the ecliptic.
	The Earth travels nearly two million miles a day, and Venus moves
even more quickly. The capture event would probably have brought the
Comet within a few hundred thousand miles of the Earth or Venus, perhaps
closer still. We can't know the position of the comet that well before
recorded astronomical history.
	If the Earth captured the comet, I'm sure it looked spectacular.