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, saying only that re-entry occurred during a two-hour period.
"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.
"The satellite was passing eastward over Canada and Africa as well as vast portions of the Pacific, Atlantic and Indian oceans during that period," NASA said. "The precise re-entry time and location are not yet known with certainty."
The Joint Space Operations Center at Vandenberg Air Force Base, Calif., reported that UARS entered the atmosphere over the Pacific Ocean, according to NASA spokesman Bob Jacobs, but the "precise time and locale aren't yet known."
Given the trajectory of the bus-size, 6.3-ton satellite, experts said it was unlikely any falling remnants would result in injuries or significant property damage. But there was no immediate confirmation as to how much debris might have reached the ground or where the debris "footprint" might be located.
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.
"As of 10:30 p.m. EDT on Sept. 23, 2011, the orbit of UARS was 85 miles by 90 miles (135 km by 140 km)," NASA said in a 10:50 p.m. statement. "Re-entry is expected between 11:45 p.m. Friday, Sept. 23, and 12:45 a.m., Sept. 24, Eastern Daylight Time (3:45 a.m. to 4:45 a.m. GMT). During that time period, the satellite will be passing over Canada and Africa, as well as vast areas of the Pacific, Atlantic and Indian oceans. The risk to public safety is very remote."
The final update from U.S. Strategic Command, which operates a global radar network used to monitor more than 20,000 objects in low-Earth orbit, predicted entry around 12:16 a.m. EDT Saturday above the Pacific Ocean just west of Canada. But the prediction was uncertain by plus or minus two hours and at orbital velocities of 5 miles per second, just 10-minutes of uncertainty translates into 3,000 miles of uncertainty in position.
Nick Johnson, chief scientist with NASA's Orbital Debris Program at the Johnson Space Center in Houston, 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 in a 500-mile-long down-range footprint.
"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.