Sooner or later, rather like you and me, all spacecraft come to the end of their useful lives. This point has been arrived at in the case of the Russian space station, Mir, and next week its remains will be disposed of.
Mir was launched in February 1986, and for the last 15 years it has been in low orbit around the Earth, circling the planet about every 90 minutes and playing host to over 60 visitors.
Mir's current orbit approaches Earth to within 160 miles and, left to its own devices, the spacecraft would be retarded by the very tenuous atmosphere existing even at that altitude; slowly losing height, it would eventually crash to ground.
Even now Mir is sinking at a rate of about a mile a day because of atmospheric drag, and as this drag increases with decreasing altitude it could be expected to plunge to Earth around the end of March. Some of its 130 tonnes would burn up by the friction generated in its passage through the atmosphere, but enough debris would survive potentially to cause a significant amount of damage in an uncontrolled descent.
The spacecraft, therefore, has to be "de-orbited". It must be lowered gingerly into the atmosphere in such a way that whatever remains of it will come to ground in some lonely, deserted, uninhabitable spot where is unlikely to cause injury to property or persons.
This would be relatively easy if Mir were spherical, and therefore its behaviour in the atmosphere predictable. But it is a gangly, irregular assembly of oddly-shaped modules, put together as if with a Meccano set, and not designed for aerodynamic flight.
The de-orbiting, therefore, currently scheduled for the early hours of next Thursday morning, and extending over several hours, is a very, very complex exercise.
The necessary thrust to control Mir in the early part of its descent will be provided by another spacecraft, specially dispatched for this purpose, and which attached itself to Mir about two months ago. It will ensure that Mir approaches at a steep enough angle to avoid its "skipping" along the top of the atmosphere like a stone thrown almost horizontally across a lake.
When the spacecraft is about 50 miles above the surface, atmospheric drag will be sufficient to put it into a near vertical dive, while at the same time much of its structure will burn up in the intense heat.
Then, if all goes well, the residue of about 30 tonnes of concentrated debris will land in the Pacific ocean, some 2,500 miles west of New Zealand, around 7.30 a.m. Irish time on Thursday morning.