Satgen 482  The Descent of Mir  by GM4IHJ   (BID SGEN482)  1998-06-20


Recent completion of Shuttle mission STS 91 to Mir, signals the start
of the final act for the aging space station. When the last Russian
cosmonauts leave, the station will be abandonned and allowed to re
enter the lower atmosphere and self destruct.


Predicting exactly when Mir will die, is not going to be easy, for the
following reasons :-
Firstly, the timing of Mir entry into its final orbit depends on how
quickly the atmosphere drags it down. In normal operation , Mir was
maintained at an altitude between 350 and 400 kms . It had to stay
this low in order that the Soyuz and Progress transport craft could
reach it when carrying useful loads, and it was important to be above
350 kms because a lower height would have required many more thruster
or Progress burns to keep it up. Noting that its orbit at even that
height range was not stable , requiring regular push ups on an average
of once every one or two months.


Once these regular orbit adjustments cease , atmospheric drag will
start to decrease Mirs orbital height. How quickly this will happen
depends on several random factors. One of these factors is the
attitude of the spacecraft, which may no longer be controllable. Nose
first progress experiences minimal drag, whereas bottom or top first
subjects a much greater surface area to the effects of drag. 


Another pair of factor affecting Mirs orbit are the long term increase
now occuring in upper atmosphere density and hence drag, as solar
activity maximum approaches , and even more difficult to predict, the
effects of solar mass expulsions which briefly enhance the solar wind
enormously , producing a temporary rise in the density of the upper
atmosphere which can cause a dramatic reduction in Mir or any other
space crafts orbital height. 


In several instances in the past, spacecraft thought to be safe have
suddenly begun to descend very rapidly. Skylab was planned to be
visited again for a fourth time but it soon became obvious that a
mission planned to do this would be far too late. Equally unexpected,
Salyut 7 was deliberately boosted up to an orbit well above 400 kms "
for safe keeping. Unfortunately it too , proceeded to drop out of
orbit much faster than expected . So plans to reuse it , had to be
abandoned.


Mir is somewhat like Skylab and Salyut, in that once it is no longer
stabilized it may well take up an attitude producing maximum drag,
with the added problem that solar max with its high atmospheric
density is rapidly approaching. So predicting the actual date of Mirs
final departure is very problematic, as is the problem of where it
will come down and what if anything will survive re entry. Going by
the examples of Skylab and Salyut, several big bits of metal , such as
hatches , heavy safes and storage units , plus bulky machinery such as
inertial wheels, will probably make it back to earth. Hopefully over
the ocean, somewhere between latitudes 52N and 52S ie well away from
Scotland.