WASHINGTON — A decommissioned NASA research satellite is expected to re-enter the Earth’s atmosphere between late September and early October, NASA and U.S. military officials said Sept. 9.

The satellite, known as the Upper Atmosphere Research Satellite (UARS), is a school bus-size craft NASA launched aboard the space shuttle in 1991 and kept in service until 2005 when it decommissioned the nearly 6-ton satellite in preparation for an eventual atmospheric re-entry.

With no means of controlling the spacecraft, NASA and U.S. military officials cannot say exactly when or where the satellite will re-enter. Officials expect the satellite to break into 26 pieces of various sizes when it comes down somewhere between Canada and South America.

While NASA and U.S. military officials are constantly tracking the dead satellite, they said they will not have a better idea of where the pieces might land until about two hours before re-entry.

“We simply will not know where it’s going to come down until it comes down,” said Air Force Maj. Michael W. Duncan, deputy chief of U.S. Strategic Command’s space situational awareness division. “There are so many factors that will affect it. The atmosphere changes on a daily basis, so it’s impossible to say how this will affect re-entry.”

Duncan said Strategic Command will be sending updates to NASA and other agencies, including the Federal Emergency Management Agency, as UARS gets closer.

There is a 1-in-3,200 chance that a person somewhere on Earth could be struck by falling debris from UARS, but the odds of UARS re-entering around a populated area are remote, NASA officials said.

UARS is the largest NASA satellite to make an uncontrolled re-entry in years, agency officials said. UARS is about 10.7 meters long and 4.5 meters wide, and weighs 5.7 metric tons.

While such large satellites only re-enter about once a year, other satellites and debris fall back to Earth on a weekly basis. But most of those objects burn up in the atmosphere before reaching the ground, said Nick Johnson, chief scientist of NASA’s orbital debris program at the Johnson Space Center in Houston.

“Throughout the entire 54 years of the Space Age there has been no report of anyone being injured or impacted by any re-entering debris,” Johnson said.

Launched from the Space Shuttle Discovery in September 1991, the $750 million UARS mission spent 14 years studying the interaction between the Earth’s atmosphere and the sun.

In 2005, NASA intentionally used the last of the satellite’s propellant to lower its orbit to hasten its eventual re-entry, Johnson said. Otherwise UARS would not have re-entered the atmosphere until around 2025, he said.