The remainder of the crippled Phobos-Grunt spacecraft is set to crash to Earth on Sunday following its botched mission to Mars, space experts have said.
The minibus-sized Russian craft has been in a low orbit around Earth since losing contact with engineers shortly after its launch on November 8.
It had been intended to explore Phobos, one of Mars"s two moons, but became stranded while still orbiting Earth and attempts to put it back on its original course failed.
Most of its mass is expected to burn up as the craft re-enters the atmosphere but 20 or 30 pieces of small debris collectively weighing about 200kg could reach Earth.
In a normal re-entry about 20 per cent of the space junk"s mass would be likely to reach Earth, but in the case of Phobos-Grunt it could be even less because it contains large quantities of unused fuel which will burn or dissipate in the atmosphere.
In theory the remains could land anywhere south of Watford or north of the Falkland Islands, with a sea landing most likely due to the size of the oceans relative to the continents.
But scientists said observers would be unlikely to even see the debris crash to Earth unless it flew directly overhead in a clear sky, and that the chances of anyone being injured were absolutely minute.
Prof Richard Crowther of the UK Space Agency said: "The chances are so low – it certainly doesn"t keep me awake at night worrying about the probability of a piece of space debris coming through my roof."
He added, however, that international space agencies need to discuss the amount of disused equipment being left in Earth"s orbit amid fears space junk could reach a "critical mass" where damaging collisions with active satellites and space craft become inevitable.
The craft will explode on its descent meaning any material reaching the ground could be spread over an area measuring 200km long and 20km wide, and many pieces will be so small they would be hard to spot on the ground.
British experts based at RAF Fylingdales in North Yorkshire will monitor the craft as it descends but even as it begins its last orbit, 90 minutes before it arrives, they will only be able to predict its landing spot with an uncertainty of 4,000km.