RE: An elementary, but puzzling question

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Fri May 19 2000 - 10:11:47 PDT

  • Next message: Henney, Paul J: "RE: Question on Cosmos 2370"

    Roger Curry asked:
    
    > When a rocket is launched from, say, KSC, is it programmed 
    > to fly south of east so that it crosses the equator somewhere 
    > over Africa if it is in a 28.5 deg. inclination orbit, or does it just 
    > travel east and somehow the axial tilt of the Earth does the 
    > job of making it cross the equator twice?
    
    They just fly east.  Spherical geometry is all that's
    involved (earth's axial tilt is unrelated).  Here's one
    way of thinking about it that may help you conceptually.
    Satellites fly in geometric planes, and those planes must
    pass through the center of the earth (due to gravity).
    If the Shuttle's orbital plane is at 28.5 degrees north
    when it's over the Cape (roughly longitude 80 west), then
    the orbital plane must be at 28.5 degrees south when it's
    at longitude 100 east (180 degrees around the globe).  By
    the same reasoning, the orbit must cross the equator at
    10 east (descending node) and 170 west (ascending node).
    
    Of course, the earth is rotating to the east at roughly
    15 degrees per hour, so these longitudes are constantly
    shifting westward by ~22.5 degrees per orbit for a satellite
    with a 90-minute orbital period.  (The orbital planes also
    precess due to the earth's equatorial bulge, but this is
    a smaller effect.)
    
    Cheers,
    Rob
    
    
    
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