The Earth's orbit is a giant wasteland filled with space junk -- about 20,000 pieces of debris that are four inches or larger. This space junk doesn't always stay in space; sometimes it falls back to Earth.
One large piece of junk is the six-tonne Upper Atmosphere Research Satellite (UARS). The UARS was deployed as part of a Space Shuttle mission to measure atmospheric levels of ozone and other chemicals. The satellite is 4.5m in diameter and equipped with 10 scientific instruments. It was in service from 1991 to 2005 and has floated aimlessly ever since.
In the upcoming month, the satellite will begin its descent back to earth. Though most of the structure will burn up during re-entry, some pieces may break through and fall to the earth. Right now, NASA has no idea where these pieces will land. They could hit in the ocean or in a major metropolitan area.
Before you pull a chicken little, NASA points out that the risk of injury from falling space junk is small. In over 60 years of space flight, there are no reports of injury or significant property damage from falling space junk. [NASA via Discovery News]