>It has occurred to me that if mankind is going to expand to other planets sometime in
>the future (The sooner the better) then a new engine type must be invented.
>Chemical rockets will not do it and solar sails only work in solar wind.
>Ion engines are not powerful enough either. If anti-gravity becomes possible
>it will need gravity to work againts to produce motion and the further away
>from a gravitaional source, the weaker it would get. So, do you have any ideas
>on a solution for this future NASA problem?

Nuclear.