Re: FengYun 1C debris

From: John A. Dormer 2 (jad@texas.net)
Date: Sun Jan 28 2007 - 12:25:58 EST

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    Greg Williams wrote:
    > Is there any way to determine the mass of the debris by the 2-line
    > elements?
    
    I don't think that the publically-available ones will be accurate enough
    to calculate even the ballistic coefficient. If you knew how large the
    object is (need multiband or monopulse radar to nail it, with many
    observations of the same piece because it's probably tumbling), had a
    decent atmospheric model, the object was dipping far enough into the
    atmosphere (at perigee or otherwise) for the model to matter, and
    assumed some density for the material (successively refined), I'd be
    willing to guess it would be possible to tell within 10%. How's that for
    hedging? :)
    
    This Wikipedia article relates to bullets, but it can be applied:
    
    	http://en.wikipedia.org/wiki/Ballistic_coefficient
    
    > I know there's a decay rate, but can that be converted into mass to give
    > the size of the bigger objects?
    
    Unfortunately the only thing you could readily calculate would be the
    ratio of mass to projected area (ballistic coefficient). This would come
    from the changes in position on each subsequent orbit. The rather large
    and tedious measurements I outlined above would probably do alright, but
    observation with a laser/LIDAR system would probably be best. It may be
    necessary if it were a small object.
    
    And then there's the point of knowing that you're really looking at the
    same object over and over in the case of a swarm like Fengyun's debris.
    It is possible that even a ground-based LIDAR wouldn't have sufficient
    resolution in position and time to get the really small stuff. I've come
    across items which give me confidence that there exists the ability to
    get position and velocity to well with 1m and 1m/s with LIDAR systems.
    But if what such a thing was looking at was like a cat's litter box
    (various sizes of small objects surrounded by dust), it would come down
    to the limits of the optics and signal processing in the system.
    
    Properties of the target materials would really matter to any system
    which tried to track the cloud as well.
    
    	John
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