# 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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