The "productive scientific life" of UARS ended in 2005 when it ran out of fuel 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.
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Dr Stuart Eves Kettering GroupStatistically speaking, the most likely epitaph for the satellite is UARS R.I.P. (Remains In Pacific)”
The Upper Atmosphere Research Satellite (UARS) will start to tumble rapidly when it engages the top of the atmosphere, about 100km 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.
Astrophotographer Thierry Legault's video of the falling UARS
The group's latest estimates put a re-entry somewhere between 23:00 GMT and 15:00 GMT (00:00-16:00 BST).
"Statistically speaking, the most likely epitaph for the satellite is UARS R.I.P. (Remains In Pacific)," said group member Dr Stuart Eves.
"But there is still a chance that observers in the UK with clear skies could be treated to a 'fireball' display moving generally from north-west to south-east across the sky."
The UARS satellite was deployed in 1991 from the space shuttle Discovery on a mission to study the Earth's upper atmosphere.
It contributed important new understanding on subjects such as the chemistry of the protective ozone layer and the cooling effect volcanoes can exert on the global climate.
Liability question Nasa has warned members of the public not to touch any pieces of the spacecraft that may survive the re-entry, urging them to contact local law enforcement authorities.
"I've seen some things that have re-entered and they tend to have sharp edges, so there's a little concern that they might hurt themselves if they try to pick them up," said Dr Matney.
Experts have calculated the probability of anyone anywhere on Earth being injured by a falling UARS component as 1 in 3,200.
Under the terms of the Outer Space Treaty of 1967, the US government retains ownership of the debris and could, if it so wished, seek to take possession of any items found on the ground.
With those ownership rights also comes absolute liability if a piece of UARS were to damage property or injure someone.
"There is something called international responsibility; they're internationally liable," explained Joanne Wheeler of law firm CMS Cameron McKenna, and an expert representative for the UK on the UN Subcommittee for the Peaceful Uses of Outer Space.
"The Americans have to retain jurisdiction and control, and that pretty much can be interpreted as ownership. So they own it up there, they own it if it comes down to Earth and they're liable if it crashes into something."
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 UARS is seen 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.
- UARS orbits the Earth between 57 degrees North and South
- Nasa calculates some 26 components may survive the fall to Earth
- The largest is a moveable instrument platform weighing almost 160kg
- In total, about half a tonne may make it all the way to the surface
- The risk of any one of 7bn people being hit is 1 in 3,200, Nasa says
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