Re: HST and STS-103

Ed Cannon (
Wed, 22 Dec 1999 00:56:19 -0600

Robin R. Wier ( wrote:

] National news reports Discovery visually sighting HST 
] at about 75 miles distance sometime earlier today, 
] with a join=up this evening.  Heavens-Above gives 
] visible passes (this evening) for the pair (Phoenix, 
] Arizona) separated by about 15 minutes. That figures 
] (in my head) to be about 4K miles apart.  Could 
] someone please make these numbers add up?

Belatedly, I think what was happening was that Heavens-
Above was, in this somewhat difficult situation for that
type of Web site, using obsolete orbital data.  HST was 
on a steady orbit, so predictions for it were reliable.  
However, the Shuttle was repeatedly maneuvering in order 
to catch up to HST.  If the site used orbital data from 
before a significant maneuver, its predictions for the 
location of the Shuttle would fairly rapidly diverge 
from reality.  I think that the problem is that for a 
site like Heavens-Above, keeping up with the data on an 
active mission is something of a challenge.

This is always an issue on missions where at least one 
object is being maneuvered.  For example, recently Alan 
Pickup sent a message about Mir being boosted.  Any 
predictions based on data from before the boost rapidly 
became grossly wrong.  I found a difference of 11 
minutes after just a few days.  So, anytime there's 
active maneuvering, it's very important to use the 
latest possible orbital data or be confident that your 
source is doing so.  One other lesson is for a docking
mission, if there's any doubt, the most reliable 
predictions are those for the target object -- the one 
that is not being maneuvered (very much).

The freshness of orbital data is also, by the way, very 
important if you are trying to see objects that are soon 
to re-enter due to natural decay of their orbits.

I hope that any errors that I've made will be corrected 
by the experts out there!

Robin, I hope that you saw them and get more opportunities!

Ed Cannon - - Austin, Texas, USA

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