A 6-ton decommissioned NASA satellite the size of a bus is expected to fall to Earth on Friday, according to the U.S. space agency.

The Upper Atmospheric Research Satellite, which was launched into orbit in 1991 and shut down in 2005 after completing its mission, is being monitored by the Joint Space Operations Center (JSOC) of U.S. Strategic Command at Vandenberg Air Force Base in California. The center detects, identifies and tracks all man-made objects in Earth orbit and space junks.

Donald J. Kessler, a retired senior scientist for orbital debris research at NASA, said the satellite is likely to break up and burn in the atmosphere, so the chance of anyone on the ground getting hit by the debris is remote, or 1 in 3,200.

Although not all pieces of the satellite will burn in the atmosphere, the L.A. Times quoted Kessler as saying that it's almost impossible to predict where a piece of space debris that does not burn might fall.

NASA experts suggest at least 26 durable parts of the UAR will remain intact on re-entry and may crash in a wide area; up to 400 to 500 miles. Experts are betting that Ohio will be the crash site.

The Upper Atmospheric Research Satellite studied atmospheric changes and pollutant effects. It also measured chemical compounds found in the ozone layer, wind and temperature in the stratosphere, and the energy input from the sun in order to help define the role of Earth's upper atmosphere in climate and climate variability.

The Joint Space Operations Center originally predicted that the UAR would crash on Saturday. Early monitoring surmised that the satellite's fall would be between late September and early October before narrowing it to the last week of September.