Diaspora, within the next millenium.

We are too close -- in generations of techonology -- to constant boost capability and reliable enclosed remote environments.

Add in population pressure as an incentive....



Actually, the development of a reliable fusion engine to power a constant boost ship would make the entirety of the solar system available for use in short order. It would be my candidate for a "Manhattan" style project.

With constant boost ships, trips between stars become arduous and extremely costly (as opposed to flights of pure fancy).