On Jan 9, 5:47 am, Phil Hays <[email protected]> wrote:
> On Thu, 2010-01-07 at 13:48 -0800, Alastair wrote:
> > In general, any existing asteroid with and Earth-crossing orbit is not
> > going to hit the earth now, since it has been missing us for 4.600
> > billion years! In other words, Earth crossing asteroids are the the
> > exceptions that prove the rule.
>
> Ever been to Meteor (Barringer) Crater, in Arizona USA? It is an
> impressive sight. Or hear anything about the strange geology under the
> Yucatan? Or under Chesapeake Bay? There are more than 150 identified
> craters on the Earth's surface, and most of them are less than a billion
> years old. So why didn't these hit the Earth during the previous 3.600+
> billion years? In the case of the Chesapeake crater, how did it miss for
> 4.565 billion years?
>
> Orbits change. Asteroid orbits can change slowly, by perturbation and
> more subtle processes, and rapidly with a near passage to a planet, or
> even near passage to one of the more massive asteroids (Ceres, Pallas,
> Vesta, and a dozen others). Planet orbits change as well.
I probably did not explain that very well :-) I think what I should
have said is that using Earth crossing asteroids (i.e. ones that miss
the Earth) as examples of ones that collide is fraught with logical
difficulties. No, maybe I should not have said that either :-
( Forget I ever mentioned the matter, please.
> > I've runhttp://www.orbitsimulator.com/(Thanks Phil :-) with one
> > asteroid that had an orbit greater than any at present but less than
> > that of Jupiter. Its orbits slowly filled the space between Mars and
> > Jupiter, and probably would have continued to fill the space as far as
> > Earth. Left longer yet it could have reached Venus, but if it had
> > already collided with Mars or Earth that would not happen.
>
> Glad you like it. Running one orbit is interesting, but if you would
> like to get a better understanding, try some close passages, both of
> Jupiter and other planets. These might be even more interesting. Don't
> use just one asteroid, run 10 or more with very slightly different
> starting parameters. If they stay clumped, try again, getting closer to
> the planet, or perhaps just simulate for more time.
Thanks again, it is great, but for one thing:
I did run another, with an orbit closer to Jupiter, but after a few
cycles it was ejected. I didn't like to mention that <grin>.
I then thought I would try ten asteroids, and clicked on new. That
wiped out all the data for all solar system planets, and restrarting
the system did not get them back. I have now installed the system
again but not tried any new experiments.
However "There are more things in heaven and earth, Horatio, than are
dreamt of in your philosophy."
William Shakespeare, "Hamlet", Act 1 scene 5
We now have three types of comet, Kuiper Belt, Oort Cloud, and Main
Belt. It is suspected that comets, when all their ice has been
sublimed away turn into earth crossing asteroids. We have asteroid,
which if they hit the earth are called meterorotes, and have dozens to
types http://www.meteoritemarket.com/type.htm
The thing is that the outer asteroids that are supposed to have
supplied Earth's oceans, are yet another type of asteroid. But they
may no longer exists since they crashed into Earth. This means we
cannot run the planetary simulator backwards and expect them to
reappear. We could run the simulator backwards and see where Jupiter
and Saturn were. But since the three body system is chaotic, then
further back we run it the less reliable would be the results. Since
we have to run it back to the beginning, then it is better to leave
that job to the professionals.
What they are saying is that about 3,800 million years ago Jupiter and
Saturn were in synchronous orbits which cause severe disruption to the
orbits of the outer asteroids. Their orbits would have changed from
simple ellipses to strange attractors. When the strange attractors
reached the Earth's orbit, the asteroids would be flying in parallel
with the Earth, and so a collision would be more likely than with an
earth crossing orbit. Of course for the asteroid to get there it would
have to miss Mars, and all the other asteroids, but this is not too
difficult because space vehicles have managed it.
You may have noticed that this is not what I said originally. I have
learnt a bit since then. Thanks!
Cheers, Alastair.
downloade t
>
> --
> Phil Hays <[email protected]>
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