By Jonathan Amos Science correspondent, BBC News The BBC's Andy Moore says where debris came down is "a bit of a mystery"Nasa says its six-tonne UARS satellite plunged to Earth over the Pacific Ocean, off the US west coast.It appears likely the decommissioned craft came down between 03:23 and 05:09 GMT - with a best estimate of 04:16.
If correct, this means any debris that survived to the surface probably went into water and not on land.
The Upper Atmosphere Research Satellite (UARS) is the largest American space agency satellite to return uncontrolled into the atmosphere in about 30 years.
The fall to Earth was monitored by the Joint Space Operations Center (JSPOC) at Vandenberg Air Force Base in California. Its best estimate for the timing of the re-entry would have seen UARS come in at a point well out into the North Pacific.
However, if UARS re-entered many minutes after 04:16, it is possible debris could have reached the American landmass.
There were some unconfirmed reports of glowing wreckage moving across the sky in western Canada, but Nasa said it had yet to receive credible evidence that this was so, less still that any debris items had been found.
"I've got no reports that I've seen that talk about people who think they might have recovered debris," Nick Johnson, Nasa's chief scientist for orbital debris at the Johnson Space Center in Houston, told reporters during a media teleconference.
"Obviously, we're going to continue to keep our eyes and ears open, and if we receive any reports like that we'll try to go verify."
Most of the 20-year-old satellite should simply have burnt up on re-entry into the Earth's atmosphere, but modelling work indicated perhaps 500kg could have survived to the surface.
Calculations estimated this material would have been scattered over an 800km path. Nonetheless, with more than 70% of the Earth's surface covered by water, many experts had offered the view in recent weeks that an ocean grave was going to be the most probable outcome for UARS.
"Because we don't know where the re-entry point actually was, we don't know where the debris field might be. If the re-entry point was at the time that JSPOC has its best guess of 04:16 GMT then all that debris wound up in the Pacific Ocean," Nick Johnson reiterated.
UARS 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.
Astrophotographer Thierry Legault's video of the falling UARSIn the past few days, Nasa had warned members of the public not to touch any pieces of the spacecraft that might survive the fall to land, urging them to contact local law enforcement authorities instead.
"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 Mark Matney, an orbital debris scientist from Nasa's Johnson Space Center.
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 is found to have damage property or injured 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 orbited the Earth between 57 degrees North and SouthNasa calculated some 26 components might survive the fall to EarthThe largest was a moveable instrument platform weighing almost 160kgIn total, about half a tonne could have made it all the way to the surfaceThe risk of any one of 7bn people being hit was 1 in 3,200, Nasa saysJonathan.Amos-INTERNET@bbc.co.uk
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