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WASHINGTON (AP) -- The U.S. military is developing contingency plans to deal with the possibility that a large spy satellite expected to fall to Earth in late February or early March could hit North America.

17,000 manmade objects have re-entered the Earth's atmosphere in the past 50 years.

Air Force Gen. Gene Renuart, who heads of U.S. Northern Command, told The Associated Press on Tuesday that the size of the satellite suggests that some number of pieces will not burn up as the orbiting vehicle re-enters the Earth's atmosphere and will hit the ground.

"We're aware that this satellite is out there," Renuart said. "We're aware it is a fairly substantial size. And we know there is at least some percentage that it could land on ground as opposed to in the water."

A U.S. official confirmed that the spy satellite is designated by the military as US 193. It was launched in December 2006 but almost immediately lost power and cannot be controlled. It carried a sophisticated and secret imaging sensor but the satellite's central computer failed shortly after launch. The official spoke on condition of anonymity because the information is classified as secret.

Renuart added that, "As it looks like it might re-enter into the North American area," then the U.S. military, with the Homeland Security Department and the Federal Emergency Management Agency, will either have to deal with the impact or assist Canadian or Mexican authorities.

Military agencies, he said, are doing an analysis to determine which pieces most likely would survive re-entry. But he cautioned that officials won't have much detail on where or when it will crash until it begins to move through the atmosphere and break up.

Renuart added that there does not as yet appear to be much concern about sensitive technologies on the satellite falling into enemy hands.

"I'm not aware that we have a security issue," he said. "It's really just a big thing falling on the ground that we want to make sure we're prepared for."

The satellite includes some small engines that contain a toxic chemical called hydrazine -- which is rocket fuel. But Renuart said they are not large booster engines with substantial amounts of fuel.

Video images of the satellite captured by John Locker, a British amateur satellite watcher, show it to be about 13 feet to 16.5 feet across. He believes it weighs a maximum of 10,000 pounds. Locker calculated its size with data on its altitude and location provided by other amateur satellite watchers, using the International Space Station as a yardstick.

Satellite watchers -- a worldwide network of hobbyists who track satellites for fun -- have been plotting the satellite's degradation for a year. They estimate it is now at an altitude of about 173 miles, and Locker believes it is dropping about 1,640 feet a day.

Where it lands will be difficult to predict until the satellite falls to about 59 miles above the Earth and enters the atmosphere. It will then begin to burn up, with flares visible from the ground, said Ted Molczan, a Canadian satellite tracker. From that point on, he said, it will take about 30 minutes to fall.

In the past 50 years of monitoring space, 17,000 manmade objects have re-entered the Earth's atmosphere.