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    Re: Delta-T
    From: Frank Reed CT
    Date: 2004 Dec 4, 00:29 EST
    George H wrote:
    "Not to us,
    nor to many generations ahead. But the time-scales involved in significant
    changes to planetary orbits, such as that of the Earth, are not long in
    comparison with the time-scales for biological evolutionary changes here on
    Earth. Such orbital changes may well have a long-term effect on the future
    viability of our species, and may have made a significant impact on
    evolution in the past."
     
    It's a funny thing about those time scales. Basically, the way the astronomical experts see it these days, there are few short-term instabilities in the Solar System for the simple reason that anything that would be likely to happen quickly (involving the major planets at least) already did happen --and quickly-- a long, long time ago. Here's an abstract from a review article that addresses the long-term stability of planetary orbits:
     
      "The physical basis of chaos in the solar system is now better understood: In all cases investigated so far, chaotic orbits result from overlapping resonances. Perhaps the clearest examples are found in the asteroid belt. Overlapping resonances account for its Kirkwood gaps and were used to predict and find evidence for very narrow gaps in the outer belt. Further afield, about one new "short-period" comet is discovered each year. They are believed to come from the "Kuiper Belt" (at 40 AU or more) via chaotic orbits produced by mean-motion and secular resonances with Neptune. Finally, the planetary system itself is not immune from chaos. In the inner solar system, overlapping secular resonances have been identified as the possible source of chaos. For example, Mercury, in 10^12 years, may suffer a close encounter with Venus or plunge into the Sun. In the outer solar system, three-body resonances have been identified as a source of chaos, but on an even longer time scale of 10^9 times the age of the solar system. On the human time scale, the planets do follow their orbits in a stately procession, and we can predict their trajectories for hundreds of thousands of years. That is because the mavericks, with shorter instability times, have long since been ejected. The solar system is not stable; it is just old!"
     
    (from Ann. Rev. Astro., Sep 2001)
     
    For a recent example of the manner in which chaotic dynamics can still yield interesting information about what happened in the Solar System even BILLIONS of years ago, take a look at this article from "Nature":
     
    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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