Life Aboard A Colony
I don't think there's anything improbable about a colony ship heading out for a multi-generation trip to another star. In the event that we can build such self-contained colonies in the future, it will come after we've fully matured the technology of inhabiting ones orbiting here within the solar system. Thousands, or millions of them. People living inside will already be accustomed to spending their whole lives inside one colony, just as people still grow up, age and die without ever leaving their county or shire. Sustainability will be the way of life for everyone. It won't change their lives much if the colony is orbiting Earth, Mars, among the asteroids or coasting through interstellar space.
However, no-one is going to be traveling inside a big lump of raw rock. Imagine the energy required to move such inert mass, and consider that it's probably just a big pile of loose rubble. A mountain of unprocessed asteroid rock is just a waste. Instead, spacecraft colonies will protect themselves inside a shell of already-processed and refined resources, and lots and lots of water ice. Every bit of that will be useful to maintaining life and propulsion.
Biting the hand that feeds IT
