A 20-year-old satellite from NASA is expected to crash back to earth on Sept. 23, but the problem is that the agency still doe not know where it will fall.

The Upper Atmosphere Research Satellite, which was used to measure the ozone layer, spans 35 by 15 foot (three by 10 meters) and weighs 13,000 pounds (5,900 kilograms). The satellite, which carries 10 scientific instruments for measuring wind, temperature and ozone chemistry, was officially decommissioned in 2005.

"As of Sept. 16, 2011, the orbit of UARS was 140 mi by 155 mi (225 km by 250 km). Re-entry is expected Sept. 23, plus or minus a day. The re-entry of UARS is advancing because of a sharp increase in solar activity since the beginning of this week," NASA said in a brief update on its Web site.

"Most of #UARS will burn up in the atmosphere. The risk to you is extremely small.," NASA said in a tweet. NASA though acknowledges that although the spacecraft will break into pieces during re-entry, not all of it will burn up in the atmosphere.

NASA noted that since the beginning of the Space Age in the late-1950s, there have been no confirmed reports of an injury resulting from re-entering space objects. Nor is there a record of significant property damage resulting from a satellite re-entry.

The decommissioned satellite could land anywhere between 57 degrees north latitude and 57 degrees south latitude, a vast swath of populated territory. Predictions will only be more reliable as the landing approaches.

"It is impossible to pinpoint just where in that zone the debris will land, but NASA estimates the debris footprint will be about 500 miles (800 kilometers) long," the space agency said.

The UARS satellite was sent into orbit in 1991 by the space shuttle Discovery.

To those looking for fragments from the satellite for additional cash or for a collector's item, NASA has this message:

"If you find something you think may be a piece of UARS, do not touch it. Contact a local law enforcement official for assistance."