need a hand with a plot element

>Would you give me a hand with something I need in a story? First,
>what would you call rather small communities, designed to hold several
>generations of a family or several families, which are a cross between
>space colonies and space ships insofar as they are intended to venture
>into deep space over a long period of time?

I've heard them called Generation Ships or Arks, but I couldn't tell you
where those terms came from.

>And second, what would be a plausible economic mission of such a
>ship/dwelling? In other words, what do we need in deep space that we
>cannot get from objects in the near vicinity of earth?

Economics typically doesn't focus on arranging to purchase things that can't
be delivered for many generations. It's just not possible to know with much
certainty what your descendants might need.

The closest I can think of is a scientific mission with possible economic
consequences. It's already been demonstrated that we can discern some
things about the atmosphere of exoplanets, via light absorption spectra when
the exoplanet passes between its star and us. When the interferometry
telescope missions get lauched within the next 10 years we'll be able to
visualize nearby planets for the first time, without the blinding glare of
their stars. If we find that an exoplanet's atmosphere suggests that it has
life (and the combination of oxygen, water vapor and carbon dioxide in
sufficient amounts has been suggested as the marker for that), then the
question quickly becomes, what is living there? Is it intelligent life?
Plant life? Suppose that earth's biodiversity (the number of species) is
getting smaller due to man's impact on earth, and the scientific community
concludes that we may need to urgently explore and take plant samples from a
nearby planet which has the atmospheric evidence of life. A generation ship
might be a way to gather samples and return them. Hence the urgent
scientific mission. If the families who return with the plants get a
royalty or patent or other economic reward, there's your economic hook.
Maybe they're part of a privately-funded effort competing for a bounty
offered by a government or organization.

Feel free to use it if it will do,
Brad W.