|This is NASA's old research satellite that is expected to come crashing down through the atmosphere on Saturday, Australian time. (AP Photo/NASA) AP|
FRAGMENTS from an old NASA satellite are hurtling towards Earth, while the exact site of the crash-landing remains a mystery.
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.
The Upper Atmosphere Research Satellite (UARS), which weighs more than five tonnes, is expected to re-enter the Earth's atmosphere at 1058 AEST tomorrow.
The US-based Centre for Orbital and Re-entry Debris Studies estimates re-entry could occur up to seven hours before or after this time.
The satellite's flight path includes several passes over Australia.
The editor of the Australian Space News website Jonathan Nally said the satellite poses a negligible threat to life and property on Earth.
"Most of the satellite will burn up on re-entry, with perhaps as many as 26 stronger or harder small pieces surviving to reach the surface," Nally said in a statement.
"But with the majority of the Earth comprising oceans or uninhabited (or very sparsely populated) remote regions, the chances are overwhelming that any pieces ofUARS that survive re-entry will fall harmlessly and never be seen again."
NASA said on its website the satellite would re-enter "sometime during the afternoon or early evening of September 23 (US time)".
"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 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 rumours 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 on 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 one kilogram or as much as 160kg, 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 800km.
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 organisations,'" 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.