Thomas Jefferson

"I would find it easier to believe that two Yankee professors would lie than  that rocks should fall from the sky"
(1807)
 

John Milton

 "Witness this new-made world, another Heav'n
 From Heaven Gate not farr, founded in view
 On the clear Hyaline, the Glassie Sea;
 Of amplitude almost immense, with Starr's
 Numerous, and every Starr perhaps a world
 Of destined habitation." (Paradise Lost)

Hermann Oberth

 "And what would the purpose of all this?
For those who have never known the relentless urge to explore and discover there is no answer.  For those who have
felt this urge, the answer is self evident. For the latter there is no solution but to investigate every possible means of
gathering knowledge of the universe.
This is the goal:  To make available for life every place where life is possible.  To make inhabitable all worlds as yet
uninhabited, and all life purposeful." (1957)

Thomas Paine

 "We hold it in our power to begin the world anew-" (1776
 

Carl Sagan

 "All civilizations become either spacefaring or extinct."

"Our time will be remembered, because this was when we first set sail for other worlds."  (1987)

Konstantin Tsiolkovsky

 "The Earth is the cradle of mankind, but one cannot stay in the cradle forever."
 (1895)