dehammer,
You forget its still got to be 'boosted` onto
the orbit for the trip.
Actually, you could use water, (also usefull
as reaction mass for decelleration with some
drive systems, or drinking, or needed when you
finally arrive), but you've still got to push a heluva lot more mass on a manned trip and that either costs energy or slows you down a lot.
We're probably stuck with robot probes for the
near term.

Pragmatist