NASA has admitted that it is "concerned" about a dead satellite the size of a school bus that is scheduled to fall back to Earth but insisted that it isn't a danger to anyone on the planet.

The 20-year-old Upper Atmosphere Research Satellite is likely to fall sometime between late September and October, but NASA doesn't know the exact date or where it will land. The space agency predicts that pieces of the satellite could land anywhere in the world from Alaska to the tip of South America.

Scientists have assured the public that there is just a one in 3,200 chance that any part of the satellite will hit anyone and that most of the satellite will burn up as it enters the Earth's atmosphere.

Daily changes in the Earth's atmosphere make it harder for scientists to pinpoint where UARS will fall, said Maj. Michael Duncan, deputy chief of space situational awareness at the U.S. Space Command.

"We do know with 99.9 percent accuracy that it'll re-enter the atmosphere somewhere between 57 degrees north and 57 degrees south, which means it will be anywhere from northern Canada to southern South America and that is truly the best estimation we can give you at this point of time," Duncan told reporters Friday. "It takes until it gets much closer to the actual re-entry time before we can give any sort of prediction better than that."

Launched in 1991, UARS has been gradually falling out of orbit after it completed its mission of studying the chemicals in Earth's atmosphere in 2005.

NASA has analyzed the UARS design using an orbital debris program to determine if any parts of the satellite would survive re-entry.

"We believe there will be 26 different components that will hit the surface of the Earth somewhere with a total mass of a little over 500 kilograms [1,100 lbs]," NASA's orbital debris program chief scientist Nick Johnson said. "The largest piece that we think is going to come back is part of the structure of UARS and it is going to have a mass of just in excess of 150 kilograms, so a little more than 300 pounds."

Johnson also pointed out there has been no incident of anyone in the world being injured by falling debris in the 54-year history of the space age.

The odds of any one individual among the Earth's 7 billion inhabitants being struck by the satellite are about one in 21 trillion.