(meteorobs) Why so much uncertainty about re-entry?

[Ed Cannon]

For Paul and Dan and others questioning the large uncertainty 
in the re-entry predictions, here's a relevant paragraph from 
the Aerospace Corporation:

"It is very difficult to predict where debris from a randomly 
reentering satellite will hit Earth, primarily because drag 
on the object is directly proportional to atmospheric density, 
and atmospheric density varies greatly at high altitudes. In 
general, we can predict the time that reentry will begin to 
within 10 percent of the actual time. Unfortunately, reentering 
objects travel so fast that a minute of error in the time is 
equivalent to many miles on the ground."

http://www.aerospace.org/capabilities/cords/reentry-overview.html

They are experts on this topic.  This is one of their specialty
centers:

http://www.aerospace.org/capabilities/cords/index.html

Here's their latest (last?) prediction for UARS:

http://reentrynews.aero.org/1991063b.html

Note that the predicted location would have occurred at about
noon local time, somewhere north of Antarctica.

The actual reentry was no doubt observed by one or more of 
the Defense Satellite Program assets:

http://www.losangeles.af.mil/library/factsheets/factsheet.asp?id=5323

Ed Cannon - Texas, USA
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