A big German spacecraft is about to make an uncontrolled fall from the sky.
The Roentgen Satellite (Rosat) is due to come back to Earth at some stage over the weekend - possibly Sunday.
Just as for Nasa's UARS satellite, which plunged into the atmosphere in September, no-one can say precisely when and where Rosat will come in.
What makes the redundant German craft's return interesting is that much more debris this time is likely to survive all the way to the Earth's surface.
Experts calculate that perhaps as much as 1.6 tonnes of wreckage - more than half the spacecraft's launch mass - could ride out the destructive forces of re-entry and hit the planet.
In the case of UARS, the probable mass of surviving material was put at only half a tonne (out of a launch mass of more than six tonnes).
The difference is due to some more robust components on the German space agency (DLR) satellite.
Rosat was an X-ray telescope mission and had a mirror system made of a reinforced carbon composite material. This mirror complex and its support structure are expected to form the largest single fragment in what could be a shower of some 30 pieces of debris to make it through to the surface.
But again, as was the case with UARS, any Rosat wreckage is strongly tipped to hit the ocean, given that so much of the Earth's surface is covered by water.
Destructive designUARS' final resting place was tracked to a remote region of the Pacific, north-east of the Samoan islands.
Rosat could come down anywhere between 53 degrees North and South latitude - a zone that encompasses the UK in the north and the tip of South America in the south.
Future spacecraft sent into orbit may have to meet stricter guidelines that limit the amount of debris likely to fall back on to the planet, but these rules are still some way from being introduced said Prof Richard Crowther, an expert on space debris and adviser to the UK Space Agency.
"Up until now we've designed satellites to survive the harsh environment of space, and we haven't given much thought to designing them for destructive re-entry," he told BBC News.
"But in future, we will have to consider whether we have got this balance right, and perhaps satellites should be designed in such a way that we can ensure more of what comes down is destroyed in the atmosphere and doesn't hit the surface.
"Unfortunately, there is a whole legacy of spacecraft - 50 years of satellites - and we are going to have to put up with this situation for quite some time, I'm afraid."
Science successRosat was launched in 1990 to survey the X-ray sky. It mapped more than 100,000 sources of this high-energy radiation. X-rays tend to come from the hottest and most violent parts of the cosmos, such as the regions around exploded stars and the "edges" of black holes.
The spacecraft worked for eight-and-a-half years before its star tracker failed and it could no-longer work out its position and point correctly. It was shut down in February 1999, and has been in descent ever since. Controllers do not have any contact with the craft; all they know is its altitude and path across the sky through radar tracking.
The fall to Earth has accelerated in recent months and weeks as the spacecraft has experienced increased drag as a result of its passage through residual air molecules still found more than 200km above the planet.
The deeper it reaches, the faster Rosat will be pulled in. But without a propulsion system, the precise timing and location of its impact cannot be influenced by controllers.
Tough materialsRosat will start to tumble rapidly when it engages the thicker parts of the atmosphere, about 80km up.
Mechanical forces will first rip off its flimsiest structures, such as its solar arrays and antennas.
The heating the satellite then experiences as it plunges downwards will deform and melt low-temperature materials and vaporise them.
Only high-temperature metals such as stainless steel and titanium will put up much resistance.
Tracking stations will typically witness the uncontrolled return of at least one piece of space debris every day; and on average, one intact defunct spacecraft or old rocket body will come back into the atmosphere every week.
Something the size of Nasa's UARS satellite is seen to enter uncontrolled perhaps once a year.
Much larger objects such as space station cargo ships return from orbit several times a year, but they are equipped with thrusters capable of guiding their dive into a remote part of the Southern Ocean.
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