In addition to about 550 operational spacecraft in Earth orbit there are an estimated 9500 other pieces of junk from previous space missions which are large enough to be tracked from the ground. A collision with any of these objects, which are about 10 cm and larger in size, would destroy a spacecraft. The number density of this debris peaks at a value of about 10-17 m-3. Estimate the probability that a spacecraft of cross sectional area 30 m2 will be hit in its operational lifetime of 5 years if it is located in the region of peak density. (Scientific American, August 1998).