Re: Mission energy requirements....
As long as you are in the interplanetary space, that is you have escaped the practical extent of the gravitational field of Earth, even a solar sail will do the trick. Or you can use radioactive carbon as mentioned in here earlier, with a half life of about 5000 years.
To escape Earth's field you need a bit more oomph to get out in a reasonable amount of time. And that is where nuclear power might help.
Biting the hand that feeds IT
