I imagine Sagan has probably already been mentioned but if he hasn't this omission can not abide.
(http://en.wikipedia.org/wiki/File:Carl_Sagan_Planetary_Society.JPG)
Sagan (1934 - 1996) was one of the nations leading minds in the area of space exploration and leading proponents of the Tenants of Dude
That is he never advocated harm, the royal "pissing on" of anyone else's belongings and fought peacefully for the Voyager program and other exploration endeavors that extended our knowledge of this solar system fantastically.
My favorite talk of his was his "Pale Blue Dot Speech"
(Look it up, really ties the room together)
Little known fact: The big pauses in his were replaced with silence to replace the bong rips he took.
(That may or may not be true, but I have no doubt he partook of the holy herb.)