During the years of 1973 to 1978, The British Planetary Society undertook a study called Project Daedalus.
The purpose of this study was to find out if it was possible to send a robotic emissary of Humanity to Barnard's Star, 5.9
light years away from Earth in a Human lifetime.
It was determined that Project Daedalus could reach Barnard's Star in fifty Earth years.
Click on the link below to learn more.
http://en.wikipedia.org/wiki/Project_Daedalus