Orpheus Theory -> Non-Circular Orbit?

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...

Another concern about the Orpheus theory just popped into mind: Is it
really feasible, in an accreding planetary system, to ever have a
collision of two roughly-Earth-sized objects? There would seem to be
two possibilities: Either:

* The "planetessimals" that, conceptually speaking, formed Orpheus
and those that formed Earth were in mutually colliding orbits to
start with, or
* They accreded into fully-formed planets while in mutually
exclusive orbits, and then something deflected one or both formed
planets' orbits to where they collided afterward.

In the first case, I'd think it impossible to form two separate planets.
The planetessimals would be colliding with one another, which I'd think
would be more likely to form a single planet, or perhaps just scatter
each other so that nothing is able to form. It seems odd that two
separate planets would form from debris in intersecting orbits.

In the second case, what energy source (or gravity source at least)
would be able to deflect an object of planetary mass into another?

Gary Morrison wrote:

The planetessimals would be colliding with one another...

In saying that, I'm assuming that planetary accresion is a greadual
process, at least on a time scale of hundreds of thousands of years,
giving plenty of time for the planetessimals in the two orbits to
collide with one another. Perhaps the answer is that planetary
accresion happened a lot faster than I suspect.

Or perhaps the orbit of Orpheus was a comet-like, extremely non-circular
one, thereby giving it a very long orbital period thereby making the
opportinities for collision comparatively rare - long enough for planets
to form. If that were the case, though, I'd go back to my earlier
concern, that such an impact would seem highly likely to throw the
resulting Earth into a very non-circular orbit - in essence, sort of the
average of the two planets' orbits.

"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

(Remove "pants" spamblock to send e-mail)

Thanks for the reply. I'll have to absorb it a bit - consider the
orbits that could lead to such a collision and so forth.