The Drake Equation

Estimate the possible number of communicating civilizations within our galaxy

N* = the number of stars in the Milky Way galaxy: 100 billion 200 billion 300 billion 400 billion
                              (current estimates are 100 to 400 billion)

fp = fraction of stars with planets around them: .1% (1 of 1000) .5% 1% 2% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
                            (current estimates range from 20% to 50%)

   ne = number of planets per star ecologically able to sustain life: .33 .5 .67 1 1.5 2
                           (estimates range from 1 to 2)

fl = fraction of planets where life actually evolves: .1% (1 of 1000) .5% 1% 2% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
                           (current estimates are 100%  ie. where life can evolve it will)

fi = the fraction of fl that evolves intelligent life: .1% (1 of 1000) .5% 1% 2% 5% 10% 20% 30% 40% 50%
                           ( ? )
 

fc = the fraction of fi that communicates: .1% (1 of 1000) .5% 1% 2% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
                            ( ? )

fL = the fraction of the planet's life during which the communicating
                             civilization survives: 1 / 1,000,000,000th (10 years) 1 / 100,000,000th (100 years) 1 / 10,000,000th (1,000 years) 1 / 1,000,000th (10,000 years) 1 / 100,000th (100,000 years) 1 / 10,000th (1,000,000 years) 1 / 1,000th (10,000,000 years) 1% (100 million years) 10% (1 billion years) 20% (2 billion years) 30% (3 billion years) 40% (4 billion years) 50% (5 billion years) 100%

                      eg. expected lifetime of Earth & the solar system is 10 billion years
                          Communication by radio has occurred for less than 100 years
                          (will we overcome our problems and survive indefinitely?)
                         

Nc = the possible number of communicating civilizations in the galaxy =