Satgen 622  The Harder They Fall by GM4IHJ  (BID SGEN622)  2001-02-24

The programmed reentry of the MIR space station seems to have been
put back again. There is no harm in this provided control of MIR can
be retained for a few weeks longer. But MIR has misbehaved before,
and may do so again. So any delay is risky. Indeed the omens are not
good, if one checks the records of previous space station funerals.
Skylab set to descend into the Southern Ocean, decided instead to
deposit some large lumps onto the " happily deserted " centre of
Australia. While MIRs predecessor Salyut 7 , also destined for a dip
in the Southern Ocean managed to stay up long enough to put a large
object into a South American garden. 

So what is the problem ? In theory a  satellite needs to maintain a
speed of 7.7 kms/sec , to stay up at 200 kms altitude. But we have
just experienced solar cycle maximum. The Upper Atmosphere has
extended well above 200 kms, as witnessed by MIRs perigee height
decline of about 0.75 degrees/day , whilst its perigee has has
dropped from 308 kms altitude in mid December 2000, down to 270 kms
in mid February 2001. Which at first sight does not look too bad,
provided you do not look closely and see it was declining by 1 km/day
in mid February ie speeding up. A rate of descent which can be
expected to increase as we go into March 2001.

But provided MIR stays stable and does not start tumbling out of
control it should in theory be OK until its perigee falls to 150 Kms.
After which it would drop very quickly in a few days to about 110 kms
height , from which point it will fail to complete more than half an
orbit, before destruction. However, the plan is not to let MIR drift
down slowly, but to drive it downwards by rocket thrust in mid March.
Forcing it through the fatal 110 kms altitude point of no return, so
that its last partial orbit occurs over the Southern Ocean well
clear of land. A procedure which requires careful orientation of Mir,
prior to firing its attached descent motor. Then assuming all goes
well at that stage , Mir will enter a 30 km wide range of descending
altitude where the success or failure of its controlled plunge into
the ocean will be decided.

The problem is that the atmosphere at this altitude is getting quite
dense , and this will start to have a very marked effect on MIRs
progress. For unlike smaller uncluttered satellites which enter the
atmosphere smoothly , MIR has large extended solar panels which will
act as wings or air brakes depending on their orientation. Causing
Mir to fly aerodynamically for a brief period, which could take it
some way from its predicted path. Perhaps extending the station
trajectory forwards or sideways and additionally causing it to tumble
twist and turn. Twisting and turning, plus severe friction heating
will soon cause the station to lose its panels and start breaking up
into its separate compartments soon after it falls below 80 kms. Then
individual compartments will break up , exposing their contents to
the fierce heat. But previous station deorbits suggest not everything
will burn up. Heavy metal safes, electric motors, pumps and hatches
will survive much longer. Indeed this survival of the thickest, was
responsible for the dangerous earth impact debris from Skylab and
Salyut 7.