Re: Close Orbiting Planetoids Sharing Atmospheres

On Jan 16, 11:53 am, boomert...@xxxxxxxxx wrote:
On Jan 16, 2:18 pm, Luke Campbell <lwc...@xxxxxxxxx> wrote:



On Jan 16, 9:38 am, boomert...@xxxxxxxxx wrote:

Hey all.

One of the players in my game came up with this one. Their character's
homeworld was described thus, "The information scrolling across the
bottom of the screen revealing that it is in fact a binary planet with
several small moons orbiting it, making note that due to the proximity
of the two main planetoids and two of the moons that they all share
one atmosphere."

I noted this was impossible, atmospheric drag would bring the bodies
down into each other. The response, "Well the two planets orbit each
other with thier atmospheres colliding and the moons move in and out
of the atmospheres in kind of slingshoting manner (eliptical
orbits)".

Ok, the gravity of the moons diving in and leaving, like an unbalanced
and wobbly Kepler Rosette. I imagine it in my mind, like balls falling
in an invisible spirograph. Down towards the shared gravity of the two
worlds, through the la-grange point they share and out through the
balanced gravity of the two worlds, around, down to the shared gravity
of the two worlds, repeat. The worlds travel around each other, orbits
shifting from the tug of their moons, and their moons dive in and sail
out, dive in and sail out.

Something tells me the whole thing would crash...

Yah, this is not stable,  The moons would not have stable orbits even
neglecting the atmosphere, and would either be ejected or crash into
the planet.  With the atmosphere, the moons will experience drag every
time they pass through, losing energy in the process, and dropping
into a lower energy (closer) orbit with each pass, until they crash.

The only way I can think of to get an effect like this is to have the
moons in the L4/L5 points, and have a very thick atmosphere such that
it extends out into the two stable Lagrange points. This means a
crushing atmosphere on the major worlds, though - worse than Venus at
ground level.

Luke

Ok, next alternative is to just have the atmosphere around the two
worlds, instead of in the lagrange points. More feasible?

Yes, if they are orbiting very close together. Tidal forces will have
distorted both worlds into egg shapes, with the tips of the eggs
facing each other and almost touching. If they are close enough, they
can share an atmosphere. Gravitational influences of other bodies in
the system may cause a small amount of libration in the orbits. This
can lead to slight changes in the distances between the two planets.
Since they are already orbiting so closely, slight changes may mean
the egg-tips occasionally crash together, which would make things
*very* interesting for those living there. If they do not crash
together, you can still expect libration would produce significant
tidal heating, leading to vulcanism, earthquakes, and assorted
geological excitement. The amount of this fun stuff depends on the
amount of gravitational perturbation of other bodies in the solar
system - if the double planet is locked into an orbital resonance with
a gas giant, for example, you can expect fun stuff to happen.

If the planets are just a little bit closer, they will merge into a
single double lobed planet, sharing not just an atmosphere, but also a
rocky surface. Perturbations will heat the hourglass waist, leading
to lots of geologic activity there, perhaps even a molten surface at
the waist.

You might not be able to get stable orbits for the moons if they are
not at the Lagrange points. If they orbit too close, they will either
crash into the planets or get flung out into space. If they are far
enough away to avoid this egg-beater effect, they may be too far to be
effectively captured by the planets with respect to the gravitational
influence of the primary. Still, you can put moons in the Lagrange
points if you need them. This will mean the moons will never rise or
set, but stay at the same place in the sky to surface observers.
Clearly, this limits you to a maximum of two moons. If the orbital
dynamics do allow extra moons orbiting both planets, they will be far
away.

Luke
.

Relevant Pages

  • Re: Close Orbiting Planetoids Sharing Atmospheres
    ... several small moons orbiting it, making note that due to the proximity ... balanced gravity of the two worlds, around, down to the shared gravity ... neglecting the atmosphere, and would either be ejected or crash into ... can lead to slight changes in the distances between the two planets. ...
    (rec.arts.sf.science)
  • Re: Close Orbiting Planetoids Sharing Atmospheres
    ... several small moons orbiting it, making note that due to the proximity ... Down towards the shared gravity of the two ... balanced gravity of the two worlds, around, down to the shared gravity ... neglecting the atmosphere, and would either be ejected or crash into ...
    (rec.arts.sf.science)
  • Re: Close Orbiting Planetoids Sharing Atmospheres
    ... several small moons orbiting it, making note that due to the proximity ... balanced gravity of the two worlds, around, down to the shared gravity ... to share an atmosphere without further drag, maybe, possibly. ...
    (rec.arts.sf.science)
  • Re: Close Orbiting Planetoids Sharing Atmospheres
    ... several small moons orbiting it, making note that due to the proximity ... balanced gravity of the two worlds, around, down to the shared gravity ... neglecting the atmosphere, and would either be ejected or crash into ... the planet. ...
    (rec.arts.sf.science)
  • Re: Terraforming the Moon
    ... about 100 moons and planets ... dividing by 5b ... so just writing the names of these worlds would take ...
    (sci.physics)
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