Debris includes spent rocket stages, pieces of broken-down satellites and random objects, like the tools that drifted off during a spacewalk last year. According to NASA, there are some 18,000 objects in orbit larger than a softball; 300,000 that are larger than a nickel; and millions that are even smaller (among them, the 150,000 sugar-cube-size chunks produced when the Chinese military shot down one of its defunct satellites in 2007). We don't have to worry about those down here, but the flotsam orbits at 18,000 mph — fast enough for a fleck of paint seven thousandths of an inch to gash the space shuttle's window in 1983.