Space watchers are standing by for a big splash sometime after 12 March 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 and could be thrown off course by solar activity. Furthermore, its irregular shape makes predicting its trajectory difficult.
Dave Mangus of NASA, who helped bring down the Compton Gamma Ray Observatory in June 2000, says such manoeuvres are nerve-wracking. 鈥淲e lost a lot of sleep over it,鈥� he says.
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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.
Critical angle
An uncrewed 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.
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� about 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 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, travelling at about eight kilometres per second, 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� about 4000 kilometres east of New Zealand. Travelling at 200 kilometres per hour, the fragments could smash through two metres of concrete.
Solar flares
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.
Odd shape
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 was cobbled together over 15 years and has five modules attached to a central base station, a plethora of solar arrays and antennas, and now the Progress rocket is also 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.
But despite the possible pitfalls, most people are confident the Russians can pull it off. 鈥淭hey 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�