From edwardtufte.com –
Nick Johnson, who worked at a military contractor in tracking possible ICBMs and distinguishing a missile attacks from space junk, prepared the 2 drawings used in my Envisioning Information. He told me in 1988 that he decided to make the space-junk object symbol just a little bit bigger than a random speck that might show up in making xerox copies!
Here is a current, non-visual summary of space junk as reported by space.com: there are 4,000,000 pounds now in orbit, and the estimated probability of a penetration of the Space Station in a 10-year period is .45.
— Edward Tufte, January 10, 2002