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Re: asteroid capture AND ring material escape?
> For being captured by a planet, Jupiter has 4 outer satellites that
> appear to be captured asteroids. And both Phobos and Deimos also
> appear to have been captured by Mars. This is much easier if the
> orbit of the asteroid is close to circular. If it is too elliptical,
> it velocity as it passes a planet will be too great to allow
> it to be captured by that planet. This is the case for most
> interplanetary space probes, which is why they are required to
> be slowed down by either a rocket burn or aerobraking.
But still, at least Phobos and Deimos must have been knocked towards
Mars by some collision, and decelerated (relative to Mars) sufficiently
in that same (or some other) collision.. All that because the
escape/capture velocity of the planet is so much smaller than the
heliospheric orbit velocity. In the case of Jupiter, the speed of the
heliospheric orbit is much lower, and the escape velocity is much
higher, so there an asteroid-capture would be much more "common", maybe
even a decelerating collision would not be needed.. the nearer the Sun,
the harder it is to capture a small body into a planet-circling orbit,
right? Is my understanding of celestial mechanics right?
And btw, I must add: Doesn't Jupiter have 8 (not four) outer satellites
which seem to be remnants of two separate asteroids (4 each)? At 11 and
22 million km.
As for the elliptical or circle-shaped orbit, I don't think it matters
that much. If the far (and slower) end of the highly elliptical orbit
happens to be sufficientlyclose to the planet, a capture could occur.
That is, as easily as in a similar circular orbit. Consider the Hohmann
transfer ellipse, for example.. (Actuallly, I have no clue what I'm
talking about. :)
Back to the idea of an ejecta ring around Earth. Has anybody ever made
any simulations of such a dust/ejecta ring around Earth? How long would
it last, and how far away would it have been? (Of course, this depends
on the size/velocity/material/etc of the impacting k/t body.)
As far as I remember, the gas giants' rings would "vanish" unless there
was some supply of new material (moons inside/near rings). Therefore, a
ring caused by impact ejecta should have vanished *quite* quickly...
Not counting the Van Allen or other belts caused by the magnetic field,
or the human made debris, is there *any* dust/rocks circling Earth? Say
at Moon's distance? I was just thinking of "regular" meteorite bombing
on the Moon, and since it doesn't take a *big* explosion to eject debris
off the gravity well there...
--
| Jarmo Korteniemi http://www.student.oulu.fi/~jkorteni/
| student
| Dept. of Astronomy "Science is hard. Wishful thinking is easy."
| Oulu University -P.Stephens, The South Shore Skeptic,May'98
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