Re: [meteorite-list] Slightly OT: UARS decay strategy

[Marco Langbroek]

Rob Matson wrote:


A third alternative would have been to let the orbit naturally decay to
a lower altitude before doing that burn. The advantage of this approach is
that once the orbit is very low (as it is now), that final burn can push
perigee so low that reentry is guaranteed half an orbit after the burn.
This allows spacecraft controllers to choose the reentry location
judiciously (e.g. over the South Pacific Ocean). By burning years early
as they did, they sacrificed the ability to choose the reentry location.


The problem here is, that it would have taken 30+ to 40+ years for UARS to 
naturally decay to a suitable altitude for such a burn.


This would have meant they would be obliged to keep the satellite operational 
for 40+ years. Apart from questions whether the satellite will not fail before 
that time (I am quiet sure it wasn't designed with 40+ years in mind), for a 
science satellite that is financially impossible to do.


Why does it need to be kept operational for these 40+ years in this case? The 
point is that without active maintenance, the satellite will start to lose 
attitude control quickly, pointing wildly to all places as it starts to tumble. 
Which means its receiver antennas could no longer point to earth at the moment, 
40 years ahead, you want to contact it. But a bigger issue is the fuel in the 
tanks. Without active maintenance, the fuel will freeze in the tanks. Once that 
happens, it becomes impossible to use the engines for a deorbit boost.


UARS is not the only thing currently coming down by the way. In about a year, a 
1.5 tons malfunctioned Japanese spy satellite (IGS 1B) will come down in an 
uncontrolled re-entry as well. That sat does still have some fuel onboard, 
unlike UARS, which is an added risk.


UARS is a nice bright object easily seen by the naked eye during a pass. Here is 
a picture I shot last year:


http://sattrackcam.blogspot.com/2011/09/watch-uars-its-dropping.html

- Marco

-
Dr Marco (asteroid 183294) Langbroek
Dutch Meteor Society (DMS)

e-mail: d...@marcolangbroek.nl
http://www.dmsweb.org
http://www.marcolangbroek.nl
-
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[meteorite-list] Slightly OT: UARS decay strategy

[Matson, Robert D.]

Hi Jim,

Doug has already provided most of the answer to your question. The
bottom line
is that for objects in low earth orbit, it is far less expensive
fuel-wise to
force early reentry than it is to move the satellite to a (MUCH) higher
orbit
that won't decay for hundreds or thousands of years. UARS did not have
sufficient fuel remaining to achieve anything useful in terms of raising
its
orbit, so they made the smarter decision to expedite its reentry by
lowering
the orbit.

A third alternative would have been to let the orbit naturally decay to
a
lower altitude before doing that burn. The advantage of this approach is
that once the orbit is very low (as it is now), that final burn can push
perigee so low that reentry is guaranteed half an orbit after the burn.
This allows spacecraft controllers to choose the reentry location
judiciously (e.g. over the South Pacific Ocean). By burning years early
as
they did, they sacrificed the ability to choose the reentry location.

In essence, they balanced two risks: the risk of death, injury or
property
damage on the ground associated with an uncontrolled reentry vs. the
increased risk of collision between UARS and another satellite or piece
of space debris during the added years or decades that it would have
taken UARS to naturally decay to its current altitude.  --Rob

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