If human beings are ever to colonise other planets – which might become necessary for the survival of the species, given how far we have degraded this one – they will almost certainly have to use generation ships: spaceships that will support not just those who set out on them, but also their descendants. The vast distances between Earth and the nearest habitable planets, combined with the fact that we are unlikely ever to invent a way of travelling that exceeds the speed of light, ensures that many generations will be born, raised and die on board such a ship before it arrives at its destination.
Image: Stanford Torus - Exterior view of a Stanford torus. Bottom center is the non-rotating primary solar mirror, which reflects sunlight onto the angled ring of secondary mirrors around the hub. Painting by Donald E. Davis