The NROL-21 USA-193 satellite was launched for the U.S. Department of Defense in December 2006 and failed within hours. The solar arrays never deployed, and in January 2007 U.S. officials reported that they were unable to communicate with the spy satellite.
The satellite, estimated to weigh 7,000 pounds, is expected to continue disintegrating upon re-entry into the Earth's atmosphere in February or early March. GlobalSecurity.org, a military information Web site, said the odds of debris landing on solid ground, instead of in the ocean, are about one in four. That's because about 75% of the Earth's surface is covered water.
"Although the safety hazard of the impacting debris was small, there was some concern that secrets of the spacecraft could be compromised if the debris were recovered by a hostile intelligence agency," GlobalSecurity explained in a statement on its Web site.
U.S. Strategic Command, or STRATCOM, acknowledges that re-entry of space debris is an inexact science because of limitations on tracking equipment, which is located primarily in the Northern Hemisphere, and environmental factors, like variations in gravitational fields, solar radiation pressure, and atmospheric drag.
"Objects re-entering may skip off the Earth's atmosphere, much as a stone skipped across a pond, causing it to impact much further away than originally forecast," STRATCOM explains in a military fact sheet.
The command said that most objects disintegrate from heat created upon re-entry into the atmosphere.
"The chances of someone being struck by a re-entering object are slight," the fact sheet states. "Only about 25% of the Earth's landmass is actually inhabited. If the object is forecast to make landfall in North America or Hawaii, the JSpOC [Joint Space Operations Center] will notify the Federal Emergency Management Agency and/or Public Safety Canada."
Amateur observers and experts have said it appears this satellite is likely to land in North America.
More than 20,176 man-made objects that the Department of Defense tracked have re-entered the Earth's atmosphere. Government sources estimate that more than 15,000 man-made objects currently orbit the Earth.