WASHINGTON: The largest piece of U.S. space junk to fall in three decades is expected to crash land to Earth within the next 12 hours, NASA said.

The U.S. Space Agency has struggled to predict where and when the defunct satellite would crash.

NASA stressed there was only an "extremely small" risk that the 26 fragments expected to survive re-entry would hit any of the planet's seven billion inhabitants.

Italy has taken the unusual step of warning residents to stay indoors late Friday to avoid a 1.5% risk of the six-tonne satellite hitting the northern part of the country.

NASA still guessing

But the trajectory still has NASA guessing. Its latest data showed the Upper Atmosphere Research Satellite (UARS) was changing its path so that a U.S. landing could no longer be ruled out. On Thursday, the U.S. space agency had said North America was out of range.

"The satellite's orientation or configuration apparently has changed, and that is now slowing its descent," NASA said in a recent update posted on its website.

"There is a low probability any debris that survives re-entry will land in the United States, but the possibility cannot be discounted because of this changing rate of descent," it added.

"It is still too early to predict the time and location of re-entry with any certainty, but predictions will become more refined in the next 12 to 18 hours," or between 0230 GMT and 0830 GMT Saturday.

A similar time frame was issued by the Center for Orbital and Reentry Debris Studies at the California-based Aerospace Corporation, which said the debris would come in at 0316 GMT Saturday, plus or minus five hours.

Ocean landing likely

Parts that survive the fiery re-entry into Earth's atmosphere may weigh as little as one kilogram or as much as 158 kg, NASA said, and the debris field is expected to span 800 km.

Orbital debris is expected to fall somewhere between 57 north latitude and 57 south latitude - a range that covers most of the populated world.

The tumbling motion of the satellite makes it difficult to narrow down the location. And given that the world is 70 % water, an ocean landing was considered likely.

"The chances that you (yes, I mean YOU) will be hit by a piece of the #UARS satellite today are one in several trillion. Very unlikely," NASA said in a message on Twitter.

Constant tracking

The U.S. Department of Defense and NASA were busy tracking the debris and keeping all federal disaster agencies informed, a NASA spokeswoman said.

The U.S. Federal Aviation Administration issued a notice 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.

Italy's civil protection agency warned that the probability of a crash in its northern territory had risen from 0.6 to 1.5 percent, 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, though this is the biggest NASA satellite to fall in three decades.

NASA's Skylab crashed into western Australia in 1979.

No past injuries from space junk

The surviving chunks of the tour-bus sized UARS, which launched in 1991, will likely include titanium fuel tanks, beryllium housing and stainless steel batteries and wheel rims.

NASA has also said that in 50 years of space exploration, no one has ever been confirmed injured by falling space junk.

"No consideration ever was given to shooting it down," NASA spokeswoman Beth Dickey 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."

Liability for damage

The U.S. 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 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 U.S. is one of 80 state signatories.