Scientists expect NASA satellite's dive within days

— -- What goes up, must come down. So NASA officials predict 6.2 tons of defunct satellite will make a fiery re-entry to the Earth's atmosphere sometime from Thursday to Saturday.

Debris from the space agency's defunct Upper Atmosphere Research Satellite (UARS) poses a 1-in-3,200 risk of hitting someone, according to a space agency analysis.

Depending on the exact altitude at which the satellite's final plunge starts, most likely on Friday, the debris could scatter anywhere from Siberia to South America.

If the satellite enters over a populated part of the world, "people should see quite a show," resembling a shooting star, even in the daytime, says NASA orbital debris expert Nicholas Johnson of the Johnson Space Center in Houston.

"We'll just not know precisely where it comes down, until it comes down," Johnson says.

The risk of debris striking anyone, he says, "is very low. In 45 years of spacecraft falling back to Earth, no one has been hurt."

The satellite will re-enter along a 500-mile path. When it comes down, the 17,000-mph re-entry should scatter 26 "potentially hazardous" pieces of debris, weighing 1,200 pounds total. Similar-sized spacecraft re-enter the atmosphere about once a year, Johnson says.

"NASA has to let the public know about it, but the chances of this harming anyone are just extraordinarily small," says Ray Williamson of the Secure World Foundation, a space law foundation in Superior, Colo. "Meteorites are a lot more commonplace, and we don't hide under our desks all day because of them."

Launched in 1991 on a space shuttle mission, the satellite measured upper air chemistry, including the depleted ozone layer, and gauged the effects of the sun's solar cycle on the atmosphere.

In 2005, space agency managers halted the spacecraft's operations and used up its fuel to lower the spacecraft to its present re-entry orbit to keep it from cluttering space and producing orbital debris hazardous to other spacecraft.

UARS suffered a small strike from a small piece of debris earlier in the year that shed four small pieces from the spacecraft, Johnson says.

UARS travels on a 140-mile-high orbit that carries it from 57 degrees north latitude to 57 degrees south latitude.

The orbit continuously shifts westward as the Earth turns.

Simulations suggest a re-entry within that range "with 99% accuracy," says U.S. Air Force Maj. Michael Duncan of California's Vandenberg Air Force Base, which is monitoring the satellite.

Much of the satellite's orbit passes over the Pacific or uninhabited Siberia, Duncan says. Air Force monitors will give updates two hours apart as the expected re-entry time approaches.

Historically, U.S. and Russian space programs have controlled satellite re-entries by firing rockets to precisely aim them to Earth.

In 1979, for example, NASA used maneuvering rockets to delay the re-entry of the 77-ton Skylab to ensure it would re-enter over the Atlantic and Indian Oceans. Some parts landed in the Australian Outback.

NASA will face a big re-entry decision when the 460-ton International Space Station reaches the end of its service life after 2020.

Discussions are underway to extend the station's service to 2028.

UARS lacks any fuel to take it lower than its present orbit, necessitating the uncontrolled plunge.

According to NASA's analysis, sturdier parts from the satellite, such as solid steel gyroscope rings or fuel tanks, will survive the fall.

"NASA faces a bit of a bind here where alerting the public to the re-entry probably makes more news than the hazard here really deserves," Williamson says. "But then they have to worry about the public reaction if they didn't say anything."