SPACE watchers are standing by for the big splash next week when Russia鈥檚 Mir
space station is due to plummet into the Pacific. But bringing down the most
massive man-made object in space will not be easy. The station has little power
for manoeuvring, could be thrown off course by solar activity, and its irregular
shape makes predicting its trajectory difficult.
Dave Mangus of NASA, who helped bring down the Compton Gamma Ray Observatory
last June, says such manoeuvres are nerve-racking. 鈥淲e lost a lot of sleep over
it,鈥� he says. Mir weighs 130 tonnes, nine times as much as Compton, so Russian
controllers are likely to be in a similar state. 鈥淭here鈥檒l be a lot of
practising and every team member will have a back-up,鈥� says Mangus.
A crewless Progress M1-5 spacecraft docked with Mir on 22 January to provide
the necessary thrust for re-entry. It then nudged Mir into an elliptical orbit
with an average altitude of 250 kilometres. Around 12 March, controllers in
Russia鈥檚 Mission Control Centre in Korolev near Moscow will start firing
Progress鈥檚 engines to trigger re-entry.
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These thrusts are designed to put Mir into a more elliptical orbit so that
the station enters the Earth鈥檚 atmosphere at a steep enough angle not to bounce
off鈥攁bout 1掳 from horizontal. If the angle is any less than this, it
will be 鈥渓ike skipping a stone on a lake鈥�, says Mangus.
The final thrust should bring Mir, travelling at about 8 kilometres per
second, into an orbit that is closest to the Earth over the Pacific, at a height
of less than 80 kilometres. At this point atmospheric drag will take over and
the station will go into a near-vertical dive. About half an hour later, the
debris should hit the sea at 47掳 south, 140掳 west鈥攁bout 4000
kilometres east of New Zealand. Travelling at 200 kilometres per hour, the
fragments could smash through 2 metres of concrete.
That鈥檚 the theory. But the time between the start of the final thrust and the
point of re-entry is full of risk. During this time, Mir will travel half way
around the planet, passing over central Asia and Japan before heading out over
the Pacific. Unlike all previous controlled re-entries, there is little power
available to correct mistakes: the Progress craft has to move an object almost
20 times its own mass.
So NASA鈥檚 Space Surveillance Network and Russian and European ground stations
will be tracking the station鈥檚 trajectory in case it is affected by ripples in
the Earth鈥檚 upper atmosphere or solar activity. 鈥淚n a [solar] flare you can see
the altitude take steps like a downward staircase and that just throws off any
calculation,鈥� says James Ryan of the University of New Hampshire, who worked on
the re-entry of the Solar Maximum Mission satellite in 1989. Others disagree.
鈥淚t won鈥檛 be a problem,鈥� says Walter Flury of the European Space Agency, who is
working with the Russian team.
The most likely source of trouble is Mir鈥檚 peculiar shape. To predict where
an object will re-enter and where the debris will scatter, scientists need to
know how much atmospheric resistance it will face. Mir, cobbled together over 15
years, has five modules attached to a central base station, a plethora of solar
arrays and antennas, and now the Progress attached.
During re-entry, the front end of Mir will set up shock waves that will heat,
bend and break up the station. 鈥淚t鈥檚 almost a chaotic process,鈥� says Ryan. In
the past, satellites that have broken up in an irregular way have caused
problems. In 1991, a 40-tonne Salyut-7 overshot the Pacific and landed in the
Chilean Andes because its solar panel became bent tens of kilometres before
re-entry, throwing out aerodynamic calculations.
Despite the possible pitfalls, most people are confident the Russians can
pull it off. 鈥淭he Russians have done this before,鈥� says a source within the
European Space Agency. 鈥淵ou have to think if anyone can bring Mir down safely,
they can.鈥�
