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

Air Force Gen. Gene Renuart, who heads the U.S. Northern Command, said on Tuesday that the size of the satellite suggests that some 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, which lost power and no longer can be controlled, was launched in December 2006 and could weigh as much as 4.5 tons. It carried a sophisticated and secret imaging sensor but the satellite's central computer failed shortly after launch.

The official, who spoke on condition of anonymity because the information is classified as secret, said the satellite, designated by the military as US 193, never reached its final orbit and the Pentagon declared it a total loss in early 2007.

Because it looks like it will land in North America, the U.S. military, Homeland Security Department and Federal Emergency Management Agency will either have to deal with the impact or assist Canadian or Mexican authorities in dealing with it, Renuart said.

Military agencies are doing an analysis to determine which pieces are most likely to survive re-entry, he said. But officials can't give a good estimate of where or when it will crash until it begins to move through the atmosphere and break up, he said.

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

Initial estimates were that the satellite would take years to degrade and re-enter the atmosphere.

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 278 kilometres. British amateur satellite watcher John Locker, who has taken video images of the satellite, believes it is dropping about 500 metres a day.

It will be difficult to predict where the satellite will land until it falls to about 95 kilometres 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, it will take about 30 minutes to fall, he said.

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