Think of the possibilities.
Generations from now, when technology is advanced enough (and humanity hopefully united enough through continuing globalization) to make such an endeavour possible, we will be sending a ship into space.
Travelling to these faraway, habitable planets will take that ship several human LIFETIMES, even taking into account the progress in propulsion technology that may have been made until then.
When a journey takes this long, it will most likely be impossible to pre-equip the ship with everything it needs to finish it. This means that it will have to be
completely self-sustaining in all fields that are necessary to 1) feed its population, and 2) continue its travel to the final destination. It will have to house a fairly large population, which will have to be comprised of very healthy and extremely disciplined specimen that follow a very concrete "breeding program" of sorts to avoid incest on the ship, while of course at the same time restricting the passengers to a reasonable number- just enough to avoid incest, as said, and at the same time just the right number to keep the ship to a size that doesn't make it a near impossible feat to keep it moving. It must never stop approaching the planet,
it has to keep moving at all times, because the energy that is required to get it going again once it has come to a halt will be enormous.
In a place like this, that has to be efficient above all other things, there sadly would be no room for elderly people. Once a person has reached an age where he/she can't carry his/her own weight and can't make much of a contribution any more, that person will have to go. (It has to be said.) Therefor, the ship will also require a euthanasia program. That shouldn't pose much of an ethical problem though, it will be natural and everybody will understand it's necessary.
The ship would also require a huge warehouse space to carry with it everything that is necessary to enable its passengers to build a functioning colony on the planet once it has reached it. It would still have to be restricted to a reasonable size though. Highly developed, practical housing technology will be indispensable.
It won't happen in our lifetimes, I don't think so. Which is a shame, because witnessing something like this become reality would be exciting.
EX-CI-TING.
PS.:
An excellent education system would also be necessary. Contact to earth will probably become rare at some point, but knowledge would still have to be passed on and preserved somehow. It would be disastrous if the ship reached the planet, but then the people on it wouldn't know what to do with solid ground beneath their feet because all they knew how to do was what had been necessary while they were on the ship.
