need a hand with a plot element

At 11:07 PM 5/17/2006, you wrote:
>something to consider is what is the condition of earth at the time
>the ship is built. if its getting more and more crowded and is having
>trouble sustaining life, the trip [to the start] might be the only way for
>ppl to
>have a better life with more room.

Why not just live in orbital colonies? A lot easier than going to the
nearest star. Total living area should be roughly 1000x the surface area
of the Earth if most asteroidal and significan lunar materials are
used. Much more if you are willing to completely use up a few moons.