[Omah]

http://www.bbc.co.uk/news/science-environment-15034073

US officials now say the fall to Earth of Nasa's six-tonne UARS satellite could occur early on Saturday (GMT).

Estimates of where debris might fall will be narrowed hours before impact.

And a UK team studying the trajectory says the most likely time for re-entry could be after 23:00 GMT Friday, and as late as 03:00 GMT on Saturday.

Most of the decommissioned and now unpowered spacecraft should simply burn up, but modelling work suggests perhaps 500kg could survive to the surface.

UARS is the largest American space agency satellite to return uncontrolled into the atmosphere in about 30 years.

As of 15:30 GMT on Friday, the satellite was orbiting at an altitude between 160km and 170km (100 miles by 105 miles).

If the estimates for its re-entry are correct, it means the spacecraft will not come in over North America.

"The spacecraft orbits the Earth in 90 minutes, so even if we're off by a few minutes in the prediction - that's thousands of kilometres down range," said Mark Matney, an orbital debris scientist from Nasa's Johnson Space Center.

"We'll be able to know generally a few hours before, but we'll only get a final report after it re-enters. Even then, we won't know where the pieces fall because they'll be scattered over a 500-mile path," he told BBC News.

The Upper Atmosphere Research Satellite (UARS) will start to tumble rapidly when it engages the top of the atmosphere, about 80km up.

Mechanical forces will rip off less robust structures such as the solar array and antennas.

The heating that the satellite then experiences as it plunges deeper into the atmosphere will start to deform and melt low-temperature materials and then vaporise them.

Components expected to survive are made from high-temperature metals such as stainless steel, titanium and beryllium.

Some 26 items have been suggested as impactors. The largest is one of UARS' instrument tables and weighs over 150kg.

With more than 70% of the Earth's surface covered by water, the chances are that any debris will fall into the ocean.

But if the re-entry does occur over populated areas of land, it should make for a spectacular streak across the sky, even in daylight.