NASA UARS Satellite Probably Crashed in Pacific, Agency Says

VIDEO: Exact location not yet known for decommissioned NASA
WATCH Satellite Crashes Into Earth

NASA's abandoned 6-ton UARS Satellite probably crashed in the northern Pacific Ocean, the space agency said today -- though it conceded it did not know precisely where any remains may have come down. It said it did not believe anyone on the ground was injured.

"We don't know where the re-entry point exactly was. We don't exactly know where the debris field is," said Nicholas Johnson, NASA's chief scientist for orbital debris. He said there were "no credible" reports that anyone saw pieces of the satellite burning up in the atmosphere.

In a statement, NASA said, "Data indicates the satellite likely broke apart and landed in the Pacific Ocean far off the U.S. coast."

"This was not an easy re-entry to predict because of the natural forces acting on the satellite as its orbit decayed," said Johnson.

"Yes, you are safe to return to your regularly scheduled weekend activities," said a light-hearted post from NASA on Twitter.

On its final orbit, UARS -- short for Upper Atmosphere Research Satellite -- would have traveled on a northeasterly course that would have taken it close to Seattle and Calgary, Alberta, if it had gotten that far. But it was steadily descending into the upper layers of the atmosphere, where air resistance slowed it.

Once it reached a height of about 60 miles, the resistance would have been too much for the bus-sized satellite to continue at orbital speeds.

The satellite, launched from the space shuttle Discovery in 1991 to study the upper atmosphere, was originally placed in an orbit 350 miles high. In 2005, before it ran out of fuel, its orbit was lowered so that it would eventually burn up.

NASA's website said 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."

NASA said most of the satellite would have burned up on re-entry, but some 26 parts were dense or sturdy enough to survive the descent, and fall at hundreds of miles per hour in an oval area of some 500 square miles.

The agency has said it knows of no case in which people have been hurt by space junk.

Johnson said he knows of one woman, Lottie Williams of Tulsa, Okla., who was tapped on the shoulder in 1997 by what turned out to be a piece of insulation from an old Delta rocket. (ABC News interviewed her; she said she was disappointed that the piece of blackened debris, little larger than a soda can, was of earthly rather than celestial origin.)

Bill Ailor, principal engineer at the Aerospace Corporation in El Segundo, Calif.., who studies incoming space junk for the Air Force, said pieces of other satellites have come crashing down into villages, farms and random locations 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 Johnson, 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 have been hit by part of UARS were roughly 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, Gina Sunseri and Leezel Tanglao contributed to this story. Additional information from The Associated Press.