Satellites are tracked by United States Space Surveillance Network (SSN), which has been tracking every object in orbit over 10 cm (3.937 inches) in diameter since it was founded in 1957. There are approximately 3,000 satellites operating in Earth orbit, according to the US National Aeronautics and Space Administration (NASA), out of roughly 8,000 man-made objects in total. In its entire history, the SSN has tracked more than 24,500 space objects orbiting Earth. The majority of these have fallen into unstable orbits and incinerated during reentry. The SSN also keeps track which piece of space junk belongs to which country.