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deorbiting (was Re: Iridium)



At 10:41 PM 3/11/00 +1000, Matt Donohoe wrote:
>Why would they force the satellites into Decaying orbits ?

Generally, at end of life, one puts satellites into a decaying orbit so 
that they will burn up.  This gets rid of them.  Otherwise, a satellite in 
a stable orbit will stay there for a very very long time, and add to the 
ever increasing debris in earth orbit.  If (when?) one of these objects 
collides with another, the resulting smaller pieces of garbage that result 
will mostly also be in stable orbits, and many of these pieces will be too 
small to track, resulting in an increased hazard of space 
navigation.  Collision with one of these small fragments is much like 
collision with a bullet.  The velocities are high.

An excellent book on this subject:
Artificial Space Debris
by Johnson & McKnight
Orbit Book Company
Malabar, FL, 1987
ISBN 0-89464-012-7

This book has a beautiful plot of the positions of all the cataloged 
satellites in July 1987.  They surround the earth in a way that looks very 
much like a swarm of bees.  Of course there are many more smaller objects 
(many of them fragments from breakup of rocket bodies, etc) which are too 
small to be cataloged.  According to this book, only 5% of the 7,000 
trackable objects are functioning satellites.  The remainder is an ever 
growing swarm of "junk".

>What costs would be incurred keeping the satellites up?

In the case of Iridium specifically, I have no idea.  However, clearly one 
would need to retain some highly skilled staff to operate and maintain a 
ground control facility to keep the satellites "under control" so that one 
could de-orbit them later if and when appropriate.  Perhaps a shoestring 
operation could be run for a few million $ a year.

Of course there are risks associated with waiting.  As time goes on, 
failures will occur in various satellite subsystems.  Some of these 
failures may make deorbiting impossible.  Several Iridium satellites are 
known to be spinning now.  (We know this because for these satellites the 
Iridium flares show up as a series of flashes instead of one long 
flare.  Several web sites contain detailed observations of this.)  I wonder 
if those satellites can be deorbited now.  If attitude control has been 
lost, it might be impossible to point the nozzle the correct direction for 
a de-orbit burn.  This is all just speculation of course.  These are 
complex machines, and there are surely many more tricks than I (or other 
outside folks) know about.

I have a hard time believing that they would deorbit the Iridium satellites 
any time soon.  One would imagine that the bankruptcy judge, with the 
creditors interest in mind, would allow the most time possible for someone 
to come up with something useful to do with one or more of them.  On the 
other hand, from what I know about them, these satellites are extremely 
specialized, and it is therefore difficult to imagine other uses.  Perhaps 
there is equipment on board beyond the basic payload that is used for 
Iridium service, and perhaps that equipment may have some use.  I don't 
know.  Again the disclaimer: This is speculation.

The failure of Iridium is terribly damaging to the credibility of the whole 
satellite industry.  This is unfortunate.  Even tho some products I've 
worked on compete with these folks, I'm sorry to see this happen.


PS: This message is being sent via Globalstar packet data service.  (Cable 
from my computer to a Globalstar portable phone is providing my internet 
connection at the moment.)





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