If we assume that faster than light travel is not possible, then one alternative are ships that support multiple generations of people while traveling long distances. This is also assuming that cryogenics are not feasible or desirable, and that reproduction in space is possible. This will be a (hopefully) realistic design of a spaceship that can stay in space indefinitely using current technology.

Other assumptions:

1. The minimum viable population is 160 (https://www.newscientist.com/article/dn1936-magic-number-for-space-pioneers-calculated/#.VBiC_XtDLwo)
2. It will have to be a modular design that is hauled to space on multiple trips.
3. The main ship will only be used in space, so size will be less of an issue. A second, smaller ship will need to be carried for exploring planets, asteroid mining, etc.

Questions:

1. Is power finite? When leaving the solar system is there enough sunlight to power solar panels and push a solar sail? A nuclear reactor may be needed to provide power when solar panels are not sufficient.