NASA says it has no reports of a person being injured or property being significantly damaged by reentering debris. But there was an incident in 1997.

"There actually was a lady in Oklahoma who was hit by a piece of very light debris from a reentering satellite, but it didn't hurt her. It was a piece of insulation. She was out jogging, and it hit her," recalls Matney. "That same reentry dropped two tanks over Texas."

Matney says debris reentry is a common occurrence, averaging about one piece per day, but that the pieces usually are small. But he says this will be the first time in 30 years that a U.S. space agency satellite of this size will have crashed back to Earth.

Most of UARS is expected to burn up in the atmosphere.

Computer Modeling

Even though officials at NASA and the Department of Defense cannot yet provide a precise landing footprint, Matney says the science of figuring out what will land is exacting.

"We actually take time to get the original specifications, to get the different parts of the spacecraft, the material types, their shape, their mass," explains Matney. "And we actually have computer programs that model the dynamics as it begins to heat up and break up and look at the temperatures those pieces reach and whether they reach the melting point of the metal."

For instance, he says that because of aluminum's relatively low melting temperature, aluminum objects usually disintegrate before they reach the surface of the Earth.