>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?

Antigravity is ... science fiction, and will probably remain so. For a start, it would probably violate energy conservation.

Based on available technology - or anything that might seem feasible with current scientific knowledge - we won't get anywhere near the speed of light in the conceivable future. I would guess, to travel to near-by stars, we could use: fusion for energy; propulsion might be with ions or perhaps even photons; and the ship would have to be equiped to travel for many decades, if not centuries. Either a ship for multiple generations, or deep-freeze.

It certainly seems more practical (at least in the short term) to save the planet we are sitting on.