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    Re: Not exactly Navigation, but close...
    From: Frank Reed
    Date: 2008 Apr 18, 03:28 -0400

    Robert, you wrote:
    "Regardless, a 38,000 km pass by something that poses such destructive
    potential still seems too close for comfort."
    Oh, it's definitely a close pass, and it will be quite fun to observe. I've
    seen two asteroids passing at the Moon's distance, and at a magnification of
    about 50x through a medium-sized telescope, their motion was apparent in
    real time. In the case of asteroid Apophis, you won't need a telescope!
    The article itself had a couple of internal hints suggesting trouble. First,
    the story of this asteroid has been around for quite a while. It's been
    well-analyzed by many different people skilled in celestial mechanics
    calculations (the idea that "some kid" is beating "NASA" is a hint there's
    something wrong --the science of astronomy is not equal to NASA). The flyby
    orbit was worked out in some detail and it's well above the plane of the
    equator, so there are very few satellites that it might potentially impact.
    In addition, though there are lots of satellites in geosynchronour orbit, to
    paraphrase the Hitchhiker's Guide to the Galaxy, "Space is big, REALLY big".
    It's just damn tough to run into something out there without trying.
    Additionally, if you do hit a satellite, so what? Their masses are puny
    compared to a solid mass a thousand feet across. The article said that the
    object would "become" a ball of iron a thousand feet across on impact. No,
    it IS a ball of iron (and/or rock). The article also mentioned that the
    asteroid would impact the Atlantic Ocean. Now if they are only estimating
    odds of impacts, and even those supposedly revised odds were 450-to-1, then
    the best you could do is say that it would impact one hemisphere of the
    Earth, along some band. It would not be possible until much closer to impact
    to narrow it down to the Atlantic (except in one special case).
    Finally, in the article that Peter referenced, I see that one of the
    celestial mechanics specialists identified as verifying NASA's calculations
    was Aldo Vitagliano. That brings us nicely full-circle back to NavList. His
    software, Solex, has come up on the list before, and I used it as a tool to
    generate some ephemeris data for my online lunar distance calculating tools.
    I recommend it for any of you interested in very high accuracy almanac
    calculations. Get it here: http://chemistry.unina.it/~alvitagl/solex/
    He also frequently collaborates on astronomical calculation projects with
    Jean Meeus, which is a familiar name to many folks on the list (try googling
    "Vitagliano" and "Meeus").
    Navigation List archive: www.fer3.com/arc
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