Artist's concept of the Upper Atmospheric Research Satellite. Credit: NASA

NASA's decommissioned 6.3-ton 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 Pacific Ocean impact zone.

Maybe.

U.S. Strategic Command radar tracking indicated re-entry would occur around 12:16 a.m. EDT (GMT-4) Saturday as the satellite was descending across the Pacific Ocean on a southwest-to-northeast trajectory approaching Canada's west coast. If re-entry occurred on or before the predicted time, any wreckage that survived atmospheric heating almost certainly fell into the Pacific Ocean.

"Because we don't know where the re-entry point actually was, we don't know where the debris field might be," said Nicholas Johnson, chief orbital debris scientist at NASA's Johnson Space Center.

"If the re-entry point was at the (predicted time) of 04:16 GMT, then all that debris wound up in the Pacific Ocean. If the re-entry point occurred earlier than that, practically the entire pass before 04:16 ... is over water. So the only way debris could have probably reached land would be if the re-entry occurred after 04:16."

Johnson said amateur satellite watchers in the U.S. northwest and the Canadian southwest were "looking to observe UARS as it came over. Every one of those attempts came up negative. That would suggest that the re-entry did, in fact, occur before it reached the North American coast, which, again, would mean most of this debris fell into the Pacific."

But it's not yet certain and it's equally possible a delayed re-entry resulted in debris falling somewhere in northern Canada or elsewhere along the trajectory.

"We may never know," Johnson told reporters in an afternoon teleconference.

The centerpiece of a $750 million mission, the Upper Atmosphere Research satellite was launched from the shuttle Discovery at 12:23 a.m. EDT (GMT-4) on Sept. 15, 1991. The solar-powered satellite studied a wide variety of atmospheric phenomena, including the depletion of Earth's ozone layer 15 to 30 miles up.

The long-lived satellite was decommissioned in 2005 and one side of its orbit was lowered using the last of its fuel to hasten re-entry and minimize the chances of orbital collisions that could produce even more orbital debris. No more fuel was available for maneuvering and the satellite's re-entry was "uncontrolled."

As with all satellites in low-Earth orbit, UARS was a victim of atmospheric drag, the slow but steady reduction in velocity, and thus altitude, caused by flying through the tenuous extreme upper atmosphere at some five miles per second.

UARS' final trajectory as it neared the discernible atmosphere proved difficult to predict. The descent slowed somewhat Friday, presumably because the spacecraft's orientation changed. As the day wore on, the predicted impact time slipped from Friday afternoon to early Saturday.

Johnson said falling satellites typically begin breaking up at an altitude of around 50 miles. In the case of UARS, computer analysis indicated about 26 pieces of debris would survive to reach the surface, spread out along a 500-mile-long down-range footprint. Johnson said the heel of the footprint, the area where the lightest debris might fall, is typically 300 miles or so beyond the breakup point.

But so far, "we've got no reports of anyone seeing anything that we believe are credible," Johnson said.

Johnson told reporters last week he expected most of the satellite to burn up as it slammed into the dense lower atmosphere at more than 17,000 mph. But computer software used to analyze possible re-entry outcomes predicted 26 pieces of debris would survive to impact the surface, the largest weighing some 330 pounds. Impact velocities were expected to range from 30 mph to 240 mph.

"We looked at those 26 pieces and how big they are and we've looked at the fact they can hit anywhere in the world between 57 north and 57 south and we looked at what the population density of the world is," he said. "Numerically, it comes out to a chance of 1-in-3,200 that one person anywhere in the world might be struck by a piece of debris. Those are obviously very, very low odds that anybody's going to be impacted by this debris."

For comparison, some 42.5 tons of wreckage from the shuttle Columbia hit the ground in a footprint stretching from central Texas to Louisiana when the orbiter broke apart during re-entry in 2003. No one on the ground was injured and no significant property damage was reported.