Satellite's crash back to Earth expected this evening
IT weighs six tonnes, is hurtling out of control in our direction and should arrive with a bang some time this evening.
But US space agency (NASA) can not say precisely when or where its climate satellite will crash back to earth.
Experts say they will get a more precise idea in the last 12 hours before the satellite is due to return.
NASA says the risk to life from the Upper Atmosphere Research Satellite (UARS) is one in 3,200.
Satellite and space expert Dr Stuart Eves stressed the large uncertainties involved in tracking the "decay" of satellite orbits (their slow fall back into the Earth's atmosphere).
Using the most recent determination of UARS' orbit -- taken in the early hours of yesterday morning -- Dr Eves and a colleague have come up with their own projections of the satellite's final descent.
But he explained that a spacecraft's orbit lifetime could only be estimated to about 10pc accuracy.
This translates to a six-hour window either side of the expected decay which is based on a range of probabilities.
Calculations using the data suggested a splash down in the Southern Ocean. But this is likely to change as tracking specialists get a better fix closer to this evening.
Given the 10pc accuracy figure, and the fact that UARS takes about one-and-a-half hours to complete an orbit of Earth, the satellite could come down during one of four possible orbits of Earth this evening/Saturday morning.
And a number of different estimates could be produced depending what software is used to model the satellite's decay, explained Dr Eves, who is lead mission concept engineer at Surrey Satellite Technology Limited (SSTL).
Satellite decays can be affected by a number of different factors, such as the shape of the spacecraft and its unpredictable tumbling, as well as heating of the Earth's atmosphere by ultraviolet radiation from the Sun.
This can make the atmosphere expand, causing UARS to fall to Earth faster than expected. But other conditions could see the satellite stay in space for longer than anticipated.
Nasa says that debris could fall across an area 400-500km long. In its latest update, the space agency said only that the spacecraft would "not be passing over North America" when it is expected to be pulled through the atmosphere to Earth.