re: Aerospace Corp. re-entry prediction for Long March

From: Ed Cannon (edcannonsat@yahoo.com)
Date: Thu Nov 25 2010 - 05:32:12 UTC

  • Next message: Tony Beresford: "Satobs nov 25 at 8597"

    Rob Dale wrote:
    
    "Pardon my stupid question - but how could it be 
    predicted to fall over one part of the world with 
    a 20 hour window? Wouldn't it be somewhere 
    completely different if it came down at 1820 UTC 
    instead of 0820? Would it still be in daylight no 
    matter when/where?"
    
    It's just a way of expressing their uncertainty.
    If you check out their graphic, it shows orbits 
    prior to and after their predicted time, with 
    the idea given that it could be anywhere on those
    tracks.  They do make a specific prediction, but
    they also communicate their uncertainty.  Their
    graphic also shows the terminator between daylight
    and darkness, so that you can see where the 
    possible re-entry orbits are relative to day and 
    night.
    
    Here's the link again, for convenience:
    
    http://reentrynews.aero.org/2010036b.html
    
    On space-track.org (registration required) there's 
    also a decay prediction for several hours earlier, 
    and its uncertainty is plus-or-minus twelve hours.
    
    So anyway, the meaning of Aerospace's graphic is
    to say, "If we manage to hit it exactly right, it
    will re-enter right there at that time.  But we 
    cannot be certain and express that uncertainty in 
    terms of it possibly decaying so many hours earlier 
    or later, somewhere else along the last few orbits."
    
    The orbital plane is known as well as its rate of
    precession (to within some measure of precision), 
    and those are what gives them the ground tracks of 
    the last few orbits.  Of course if it's much 
    earlier or later, it could re-enter in darkness 
    rather than daylight.
    
    Ed Cannon - Austin, Texas, USA
    
    
    
          
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