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From: Michael Andre-Driussi <mantis@sirius.com>
Subject: (whorl) Freaky Orbits
Date: Mon, 3 Apr 2000 15:54:46 

Adam Stephanides and Kieran Mullen,

I am interested in your notes--of course you know I've been puzzling over
the orbital details, too.

Lately I've been thinking along the lines of those freaky orbits of
Saturn's moons Janus and Epithemis  (sparked into posting this by the refs
to shepherd moons).  Took me a few minutes just now, but I found a clean
and easy reference:

'Co-orbiting satellites are another weird example of a resonance.  Saturn's
tiny moons Janus and Epithemis nearly share an orbit; but one never passes
the other.  They're forever going faster and slower in the orbit,
approaching as though to pass a baton in a relay, only to drop back before
getting close enough to touch.  As seen from Saturn, they'd get closer,
slow down, and then separate again as though they'd "bounced off" each
other.  Things would look even more bizarre from one of the moons
themselves; you'd see the other moon approaching, but you'd slow down and
it would speed up as you got closer, until finally it would flee out of
sight.  Then the whole pattern would repeat half an orbit later--but you'd
have to go around to the other side of the satellite to watch it again.'

'The interaction is like that between a ring and a shepherd . . . '

'The co-orbiters in Saturn's system are stable, and perhaps co-orbiting
planets occur somewhere, though I don't know that anyone's looked at the
dynamics.  Two Earthlike worlds, forever chasing each other around a star,
is not a setting anyone's used, so far as I know!' (Gillett,

So I think we have a third model to consider.

Model 1: Two Planets (Earth and Venus), with standard Bode-Titus style
orbits and a comparatively wide separation (significant fraction of an
Astronomical Unit).  (Problems: to see one as a looming disk from the
surface of the other, the world has to be much closer than this suggests.
Possible solution: ditch Bode-Titus! <g>)

Model 2: Double Planet (Sainte Anne/Sainte Croix; Urth/Lune) quite close,
celestially speaking (which makes it hard to work in that "six year" bit),
orbiting around a common center between them (as well as going around the
primary star).

Model 3: Co-orbiting Planets.  Freaky orbit.

Granted, there are immediate problems: if the model is exactly like that of
Saturn's moons, then it couldn't be once every "six local years," it would
have to be "twice every local year."  (Where local year = orbit.)

The Double Planet mode would work if you can have a wildly eliptic orbit,
such that the two bodies get close every six local years--I haven't thought
about this particular angle long enough to form an opinion up or down!
(Every six years!  Holy cow, that's like a comet or something!  Or maybe
the freaky orbits of Pluto and Neptune--whoops, there's a fourth
model--where sometimes Pluto, the "furthest planet from Sol," is sometimes
inside the orbit of Neptune.)

(Doh!  Adam already mentioned something about the wildly eliptic orbit for
Double Planet approach.)

Model 4: Crossing Orbits (like Pluto and Neptune).  Only slightly less
Freaky.  (Mercury's got a Freaky Thang, too, but not germane to my rambling
here.)  But making the Crossing Point a Close Encounter every six years,
hwoof!  That's nigh impossible, even with short "inner world" orbits.
Scratch this model in favor of number 3.

Anyway, just throwing out some wild thoughts.  Keep working, guys; I'm
reading your notes and enjoying it.  Just a little dizzy right now, I'll
try to stablize and contribute more asap.


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