"Gary Morrison" wrote in message
...
An interesting thought just popped into mind.

The current going theory regarding the origin of the Moon appears to be
the Orpheus theory, in which a planetoid nearly the size of the Earth at
the time, impacted Earth and caused a ring of debris that later formed
the Moon.

What just popped into mind is this question: Wouldn't such an extreme
collision be likely to substantially purturb the Earth's orbit into
something substantially non-circular? Even if it wasn't circular
before, it seems unlikely that that impact would produce such a
nearly-circular orbit like we have now. Wouldn't it seem more likely
that it accreded into a nearly-circular orbit and stayed there?

Just a thought...


The current theory I know about (presumably the same one?) says the
collision was between something nearly the size of the present Earth and
another object about the size of Mars, which is about 10% of the mass of the
Earth.

The two objects would not have needed a large orbital velocity difference in
two different orbits to get close enough for a collision. The final speed
of the collision and its ability to spray out material to form a proto-Moon
would be largely due to the mutual gravity of the planets, a terminal
velocity approaching 11 km/sec just from falling together. Such a collision
would mean a difference in velocity of about 1 km/sec between the
proto-Earth and post-collision Earth, not enough to wildly perturb the
orbit. Even if the orbital velocities differed by 10 km/sec the worst case
would produce a velocity change of 2 km/sec. And it's always possible that
the directions were such that the eccentricity was reduced rather than
increased.

So it isn't an insuperable objection.

--
Mike Dworetsky

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