NASA has repeatedly said there is only a "very remote" risk to the public from the 26 fragments of the Upper Atmosphere Research Satellite (UARS) which were expected to survive the fiery re-entry into the atmosphere.
The satellite fell back to earth today but the precise re-entry time and location "are not yet known with certainty", NASA said.
"The satellite was passing eastward over Canada and Africa as well as vast portions of the Pacific, Atlantic and Indian oceans during that period," the space agency said in its latest update.
On its Twitter feed, NASA said that if debris fell on land Canada was the most likely area.
The two dozen parts of the UARS that may have survived re-entry could weigh anything from one to 158 kilograms, the space agency said, and the debris field is expected to span 800 kilometres.
The tumbling motion of the satellite has made it difficult to narrow down the location. And given that the world is 70 per cent water, an ocean landing was considered likely.
"In the entire 50-plus-year history of the space program, no person has ever been injured by a piece of re-entering space debris," said Mark Matney, an orbital debris scientist at NASA.
"Keep in mind we have bits of debris re-entering the atmosphere every single day."
On Friday, a NASA spokeswoman said the US Department of Defence and the space agency were busy tracking the debris and keeping all federal disaster agencies informed.
The Federal Aviation Administration issued a notice on Thursday to pilots and flight crews of the potential hazard and urged them to report any falling space debris and take note of its position and time.
Also on Friday, Italy's civil protection agency warned that the probability of a crash in its northern territory had risen from 0.6 to 1.5 per cent, and urged residents to stay indoors, on lower floors, preferably near load-bearing walls.
Orbital debris experts say space junk of this size from broken-down satellites and spent rockets tends to fall back to earth about once a year, although this is the biggest NASA satellite to fall in three decades.
NASA's 85-tonne Skylab crashed into Western Australia in 1979.
The surviving chunks of the tour-bus-sized UARS, which was launched in 1991 and was decommissioned in 2005, will likely include titanium fuel tanks, beryllium housing and stainless-steel batteries and wheel rims.
"No consideration ever was given to shooting it down," a NASA spokeswoman said.
The craft contains no fuel and so is not expected to explode on impact, and NASA also said on Twitter that talk of "flaming space debris" was a "myth".
"Pieces of UARS landing on earth will not be very hot. Heating stops 36km up, cools after that," NASA said, adding that UARS contains nothing radioactive but its metal fragments could be sharp.
The US space agency has warned anyone who comes across what they believe may be UARS debris not to touch it but to contact authorities for assistance.
Space law professor Frans von der Dunk from the University of Nebraska-Lincoln told AFP that the US would likely have to pay damages to any country where the debris falls.
"The damage to be compensated is essentially without limit," von der Dunk said, referring to the 1972 Liability Convention to which the US is one of 80 state signatories.
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