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    Re: Cel nav in space
    From: Frank Reed CT
    Date: 2005 Jan 4, 06:39 EST
    Bill wrote:
    "What reference points and coordinate system(s) are used for navigation of
    spacecraft within our solar system, e.g. Earth to Moon, Earth to Mars, or
    potentially planet-to-planet, not including Earth?"
    If we're talking about autonomous navigation, meaning things that you --well, actually, you and your computer-- can figure out without calling home, then you need two things: an extensive "background starfield database" (BSD) and a good "nearby object almanac" (NOA). Let's say you're going to the asteroid Vesta. You select minor asteroids in the main belt from your NOA and photograph their expected locations based on your best estimated position. Using your BSD, you do a best fit of the coordinates of those nearby objects relative to the background database. This gives you a precise RA and Dec (or any equivalent) of each object, and each such set implies that you are somewhere along a 3-d line of position pointing in that particular direction. Those lines cross in one point in space. That's where you are.
    Generalizing this procedure to interstellar space or any larger scale is no problem. It's all about selecting very distant objects for the BSD and reasonably near objects for the NOA. If you were flying to a star a hundred lightyears away, you could use distant galaxies and quasars tens to hundreds of millions of lightyears away as your background, and you could use the myriad red dwarf stars scattered across nearby space at 4 to 5 lightyear intervals as your local buoys. Of course, there's nothing much to this navigation really. It's more or less "point and shoot". ;->
    Wanna see the local stars in 3d? Check out some stereograms of the neighborhood that I put online some seven years ago here: www.historicalatlas.com/stars
    If you've ever wondered how Altair and Procyon and Sirius etc. are arranged around our Solar System, this might help you visualize it.
    By the way, the coordinate system you choose makes little difference so long as you're consistent.
    42.0N 87.7W, or 41.4N 72.1W.
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