According to the European Space Agency (ESA), 80 percent of satellites and other man-made objects delivered into space stay at an altitude of 100 miles to 1,300 miles.
This zone is called low Earth orbit, or LEO. The International Space Station is in LEO - as are all space shuttle missions. In this ESA-created image, it's clear just how crowded LEO has become. NASA has long feared this crowding will result in the Kessler Syndrome - a circumstance in which orbiting objects smash into one another and create even more debris, which might eventually impact and destroy vital satellites.