The US space agency has stressed that the risk is "extremely small" of any of the 26 chunks expected to survive the fiery re-entry into Earth's atmosphere hitting one of the planet's seven billion people - a one in 3,200 chance.
"Re-entry is possible sometime during the afternoon or early evening of Sept. 23, Eastern Daylight Time," NASA said on its website Thursday night.
"It is still too early to predict the time and location of re-entry with any more certainty, but predictions will become more refined in the next 24 hours."
The Upper Atmosphere Research Satellite (UARS) was not expected to fall over North America, NASA added.
The influence of solar flares and the tumbling motion of the satellite make narrowing down the landing a particularly difficult task, experts said as the Internet lit up with rumors of where and when it would fall.
The US Department of Defense and NASA were busy tracking the debris and keeping all federal disaster agencies informed, a NASA spokeswoman said.
The Federal Aviation Administration issued a notice Thursday to pilots and flight crews of the potential hazard and urged them to "report any observed falling space debris to the appropriate (air traffic control) facility and include position, altitude, time and direction of debris observed," CNN said.
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, though this is the biggest NASA satellite to fall in three decades.
NASA's Skylab crashed into western Australia in 1979.
The surviving chunks of the tour-bus sized UARS, which launched in 1991, will include titanium fuel tanks, beryllium housing and stainless steel batteries and wheel rims.
The parts may weigh as little as two pounds (one kilogram) or as much as 350 pounds (158 kilograms), NASA said.
Orbital debris scientists say the pieces will fall somewhere between 57 north latitude and 57 south latitude, which covers most of the populated world.
The debris field is expected to span 500 miles (800 kilometers).
NASA has also said that in 50 years of space exploration no one has ever been confirmed injured by falling space junk.
The craft contains no fuel and so is not expected to explode on impact.
"No consideration ever was given to shooting it down," NASA spokeswoman Beth Dickey said.
NASA 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's College of Law told AFP that the United States will 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 United States is one of 80 state signatories."Damage here concerns 'loss of life, personal injury or other impairment of health; or loss of or damage to property of States or of persons, natural or juridical, or property of international intergovernmental organizations,'" he said, reading from the agreement.
However, the issue could get thornier if the debris causes damage in a country that is not part of the convention.
"The number of countries so far theoretically at risk is rather large, so there may be an issue if damage would be caused to a state not being party to the Liability Convention," he said. (AFP)
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