But space junk falling on housetops is not the biggest worry. Scientists are concerned about the “Kessler” syndrome. It is named for the American scientist who first thought of the idea in nineteen seventy-eight. Imagine what happens when an empty rocket strikes another while orbiting the earth. Two big things become many smaller things. They then hit other things. The pieces get smaller and smaller until they form a cloud of junk that blocks the path of future space vehicles.
Source: tatoeba (12229988)