The satellite would create a fraction of the debris created by the explosion of the space shuttle Columbia in 2003, he said.
Satellites have natural decay periods, and it's possible this one failed as long as a year ago and is just now getting ready to reenter the atmosphere, he added. (Learn more about Earth's atmosphere.)
Jeffrey Richelson, a senior fellow with the National Security Archive, said the spacecraft likely is a photo reconnaissance satellite.
Such "eyes in the sky" are used to gather visual information from space about the activities of adversarial governments and terror groups, including construction at suspected nuclear sites or militant training camps.
The satellites also can be used to survey damage from hurricanes, fires, and other natural disasters.
The largest uncontrolled reentry by a NASA spacecraft was Skylab, the 78-ton abandoned space station that fell from orbit in 1979. Its debris dropped harmlessly into the Indian Ocean and across a remote section of western Australia.
In 2000 NASA engineers successfully directed a safe de-orbit of the 17-ton Compton Gamma Ray Observatory, using rockets aboard the satellite to bring it down in a remote part of the Pacific Ocean.
In 2002 officials believe debris from a 7,000-pound (3,175-kilogram) science satellite smacked into Earth's atmosphere and rained down over the Persian Gulf, a few thousand miles from where they first predicted it would plummet.
Copyright 2008 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.
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