Last Updated 7:25 a.m. ET
NASA's Upper Atmosphere Research Satellite, out of gas and out of control after two decades in space, plunged back into the atmosphere early Saturday, heating up, breaking apart and presumably showering chunks of debris along a 500-mile-long downrange impact zone.
But NASA officials could not immediately confirm where or exactly when the satellite came down, saying only that re-entry occurred during a two-hour period, reports CBS News space analyst Bill Harwood.
"NASA's 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," the agency said in a statement released more than three hours - two complete orbits - after the predicted impact time.
Details were still sketchy, but the U.S. Air Force's Joint Space Operations Center and NASA said the six-ton, bus-sized satellite first penetrated Earth's atmosphere somewhere over the Pacific Ocean. That doesn't necessarily mean it all fell into the sea. NASA's calculations had predicted that the 35-foot former climate research satellite would fall over a 500-mile swath.
NASA said it didn't know the precise time or location yet.CBS News: Bill Harwood's Space Place blog
NASA: UARS updates
Some 26 pieces of the satellite - representing 1,200 pounds of heavy metal - were expected to rain down somewhere. The biggest surviving chunk should have been no more than 300 pounds, scientists said.
The Upper Atmosphere Research Satellite is the biggest NASA spacecraft to crash back to Earth, uncontrolled, since the Skylab space station and the more than Pegasus 2 satellite, both in 1979.
Russia's 135-ton Mir space station slammed through the atmosphere in 2001, but it was a controlled dive into the Pacific.
Before UARS fell, no one had ever been hit by falling space junk and NASA expected that not to change. NASA put the chances that somebody somewhere on Earth would get hurt at 1-in-3,200. But any one person's odds of being struck were estimated at 1-in-22 trillion, given there are 7 billion people on the planet.