How many alien civilizations are capable of contacting us? In 1961, radio astronomer Frank Drake presented an equation designed to estimate that number on the basis of seven assumptions. For now, the Drake Equation tells you more about your own views on life in the universe than about scientific fact. But the scientific community continues to use it as a yardstick for the SETI search. Plug in your estimates for the following seven factors and come up with your own answer. Click here.