Serious propulsion

Well, you have lots of options in that area - solar sails, ion drives,
even direct LOX/LH propulsion will work. Mass slingers have some very
hard problems to overcome - and in the end would almost certainly at
best have the same thrust and Isp as ion drives. The areas you
mention still receive a lot of energy from the sun, so ion drives (or
solar sails if you can afford the risk) are probably the best bet. The
main problem with ion drives for this is that the best propellant,
Xenon, is rather rare - there may not be enough available in the
forseeable future for this mission.

-David