NASA's decommissioned Upper Atmosphere Research Satellite, out of gas and out of control, is not descending toward re-entry as rapidly as expected, officials say, likely delaying the satellite's kamikaze plunge to Earth by a few hours to late Friday or early Saturday. Experts expect more than two dozen chunks of debris to survive re-entry and hit the ground in a 500-mile-long footprint somewhere along the satellite's orbital track.

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

But given the bus-size 6.3-ton's satellite's trajectory and the vast areas of ocean and sparsely populated areas UARS passes over, experts say it is unlikely any falling debris will result in injuries or significant property damage. Additional radar tracking is required to pinpoint when -- and where -- the satellite will make its final descent.

"As of 10:30 a.m. EDT on Sept. 23, 2011, the orbit of UARS was 100 miles by 105 miles (160 km by 170 km)," NASA said in a brief update. "Re-entry is expected late Friday, Sept. 23, or early Saturday, Sept. 24, Eastern Daylight Time. Solar activity is no longer the major factor in the satellite's rate of descent. The satellite’s orientation or configuration apparently has changed, and that is now slowing its descent.

"There is a low probability any debris that survives re-entry will land in the United States, but the possibility cannot be discounted because of this changing rate of descent. It is still too early to predict the time and location of re-entry with any certainty, but predictions will become more refined in the next 12 to 18 hours."

A subsequent 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 the satellite would re-enter sometime around 11:34 p.m. EDT Friday as the spacecraft flies over the southern Indian Ocean. But the prediction was uncertain by several hours and at orbital velocities of 5 miles per second, just 10-minutes of uncertainty translates into 3,000 miles of uncertainty in position.

The centerpiece of a $750 million mission, the Upper Atmosphere Research satellite was launched from the shuttle Discovery in September 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 is available for maneuvering and the satellite's re-entry will be "uncontrolled."

Nick Johnson, chief scientist with NASA's Orbital Debris Program at the Johnson Space Center in Houston, told reporters last week he expects most of the satellite to burn up as it slams into the dense lower atmosphere at more than 17,000 mph. But computer software used to analyze possible re-entry outcomes predicts 26 pieces of debris will 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.