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Newsgroups: sci.astro,alt.sci.planetary
From: (Henry Spencer)
Subject: Re: Lunar Data Support Idea That Collision Split Earth, Moon
Date: Mon, 5 Apr 1999 15:28:27 GMT

In article <>,
Peter Munn  <> wrote:
>>Their orbits intersected.  Nothing remarkable about that.
>Agreed, although it does get more difficult the lower the eccentricity
>is.  The approach velocity tends to become dominated by the
>gravitational attraction, and becomes off target because of Coriolis
>effects.  At very low eccentricities I think you need (bad) luck
>equivalent to rolling a ball along the top of a long cylinder (laid
>horizontally) without it falling off either side.

If you wait long enough, even relatively unlikely events can occur.  For
orbital periods of approximately a year, ten million years is enough time
for a lot of near misses leading up to one off-center hit.

>>Recent studies indicate that objects driven inward from the belt by
>>resonances with Jupiter are far more likely to go into the Sun than to
>>collide with the inner planets.
>Interesting... but unless they have one pretty close encounter with
>Jupiter to drive them inward to the Sun in one go (or perhaps into an
>eccentric retrograde orbit), aren't they going to need a few closish
>encounters with inner planets to help them on their way?

As I understand it, resonances, unlike close flybys of Jupiter itself, are
not one-shot things.  The object can stay in the resonance for a while,
getting its orbital eccentricity steadily pumped up.  Some resonances are
strong enough to be essentially a one-way ticket to the Sun; the pumping
is so quick that there is no significant chance of hitting an inner planet
in the process.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

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