Sept. 24, 2011— -- The abandoned 6-ton Upper Atmosphere Research Satellite (UARS) satellite entered the earth's atmosphere early this morning but where it crashed remains unknown according to NASA.
In an update posted on NASA's website, the "decommissioned Upper Atmosphere Research Satellite fell back to Earth between 11:23 p.m. EDT Friday, Sept. 23 and 1:09 a.m. EDT Sept. 24."
Officials said it entered the atmosphere somewhere over the Pacific Ocean but the "precise re-entry time and location are not yet known with certainty."
On Friday, officials predicted the satelllite would be passing over Canada, Africa and Australia, and vast areas of the Pacific, Atlantic and Indian oceans.
However, NASA said earlier that the risk to public safety is very remote. So far there have been no reports of injuries.
NASA said some 26 chunks of the old satellite -- which is roughly the size of a bus -- are likely to survive the descent, and fall at hundreds of miles per hour over an area of some 500 square miles. The agency has said it knows of no case in which people have been hurt by space junk.
"We believe that the risk is sufficiently low that no one needs to change their behaviors," NASA's Mark Matney said.
Bill Ailor, principal engineer at the Aerospace Corp., studies incoming space junk for the Air Force. He said pieces of other satellites have come crashing down into villages, farms and random datelines around the planet.
"I actually think a lot of this kind stuff comes down and nobody knows what it is and just thinks it's junk and ignores it," Ailor told ABC News.
Ailor and his colleagues study satellite components in a lab to figure out what will burn up and what will become a potential threat -- just like the pieces of the UARS satellite.
But according to Nicholas Johnson, NASA's chief orbital debris scientist, any one person's chances of getting hit by debris are tiny -- something like 1 in 21 trillion. The chances that of the 7 billion people on Earth, one of them, somewhere, could be hit are more like 1 in 3,200.
Despite those odds, Ailor said that a hazard is a hazard.
"Five hundred pounds of stainless steel represents a hazard -- if you're standing under it," Ailor joked.
Launched in 1991, the UAS satellite is the largest NASA satellite to fall back to Earth uncontrolled since Skylab in 1979.
Skylab was much larger -- about the size of a house -- and debris fell in the Australian Outback and the Pacific.
ABC News' Neal Karlinsky and the Associated Press contributed to this report.