Didn't Project Orion determine that it's theoretically feasible to accelerate to not-insignificant fractions of c using NPP? If your journey could average even 0.05c, you can potentially make it within a human lifespan.
- navigation at any non-trivial fraction of *c* (not running into something that would obliterate the vessel)
- slowing down and actually arriving where you wanted to and not overshooting it or stopping .5 LY away
About slowing down, it's pretty predictable. The idea is even to run the ship in constant acceleration.
I don't think you'll want to actually navigate that ship. It's a point and go task. But if needed, you rotate the ship and keep accelerating. But I have no idea on how much you'd be able to fix your route after you discovered it is wrong.
I think the main issues are how do we make a ship where people can live for decades? And how do we launch it from the ground?
