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    Re: Cel nav in space
    From: Frank Reed CT
    Date: 2005 Jan 4, 18:12 EST
    "So the trick is do time/distance calculations and fire
    13+ days at the spot I calculate the Moon will be when my projectile reaches
    that spot."
    Yes. Point and shoot, but be very, very careful about where you point that thing! :->
    The idea I was getting at with respect to navigation in space is that because gravity is the overwhelming force out there and because gravity is so simple, predictable, and non-dissipative, if you point in the right direction and fire your projectile at the right speed, you *will* get where you intend to go with very little need for navigation en route. So orientation becomes a critical factor. And measuring the effectiveness of a change in velocity (e.g. a rocket burn) is equally critical. Get those right and Isaac Newton takes care of the rest.
    Do you know the story of the manually-oriented rocket burn on the flight of Apollo 13? It was depicted in the movie. That spacecraft had a "sextant" which was intended to be used in emergency circumstances. This sextant was really intended for orienting the spacecraft, not so much for fixing position. In a real emergency, it proved to be useless because the vehicle was surrounded by a field of artificial stars --little bits of debris from the explosion-- which made it impossible to pick out the real stars. Fortunately, in training someone had thought of using the Earth for orientation, so that's what they did.
    "Point and shoot at a star 100 light years away?  That has to be the great
    circle route. <G>  Where I perceive it to be is where it was 100 light years
    ago.  So I need to determine where it is now and where it will be in the
    time it takes the projectile to bridge the gap.  At least at the
    100-light-year distance it should be swirling around in the galactic center
    of the Milky Way, a break as I see it."
    This perhaps wouldn't be as big a problem as it sounds. Even today we know where the stars are at present (within some range, to certain levels of accuracy) and we know where they're going (within limits). Outside of dense star clusters and the obvious case of binary stars, the motions of the stars in the Milky Way are much simpler over human time scales (even hundreds of thousands of years) than the motions of the planets, by comparison. The stars are basically free particles travelling in straight lines over such time scales. An orbit around the center of the Milky Way takes roughly 225 million years.
    42.0N 87.7W, or 41.4N 72.1W.
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