As a result of 50 years of spaceflight, the useful orbits around Earth are littered with derelict satellites, burnt-out rocket stages, discarded trash and other debris. In September 2012, the U.S. Space Surveillance Network tracked about 23,000 orbiting objects larger than 2-4 inches (5-10 centimeters). By extrapolation it is estimated that there could be a total of 750,000 orbiting objects larger than 0.4 inch (1 cm).