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

[Ed Cannon]

The US military were getting data at least once per orbit,
but their data are classified.  They are available via the
www.space-track.org website, but it requires registration
and agreement with rules against re-release of their data.
In this case NASA and Aerospace Corp. and maybe others 
(e.g., celestrak.org) were releasing information, but I 
would think that they were all based on the original data 
from the US military.  

Here are the epoch dates of the last four element sets for
it (catalog number 21701, designation 91-063B):

21701U 91063B   11266.86654902
21701U 91063B   11266.92730638
21701U 91063B   11267.04854895
21701U 91063B   11267.10904230

I queried the last five elsets, but the last two were 
identical except for the orbit number and checksum.  The
epochs are about 90 minutes apart except for about twice 
that between 11266.93 and 11267.048.

So I guess the primary answer to your question is that the
original data that everyone uses are classified.

Amateur satellite trackers are very good but have to have
opportunities to spot objects, and the closer an object is
to the Earth's surface, the more limited the spotting 
opportunities become.  If we had more expert satellite 
spotters in more widely scattered places...

Ed Cannon - Texas, USA


----- Original Message -----
From: Paul Goelz <pgoelz at comcast.net>
Sent: Saturday, September 24, 2011 10:47 AM
Subject: Re: (meteorobs) Why so much uncertainty about re-entry?

At 11:30 AM 9/24/2011, Ed Cannon wrote:
>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."

Just to be clear.... I am not questioning the difficulty of 
PREDICTING the reentry or impact point.  I fully understand those 
difficulties.  What I am questioning is the lack of any specific 
information in the hours/minutes leading up to the event and 
especially many hours afterwards.

Yes, I am sure the event was observed by some entity.  I find it odd 
that those observations have not been made available.

Paul

Paul Goelz
Rochester Hills, Michigan USA
pgoelz at comcast.net
www.pgoelz.com