In a subset of science fiction novels and stories, the journey to the stars is longer than a human lifetime. The space vehicles are huge, as they must be to house many cosmonauts during thousands of years. The speeds of these spacecraft are not very high, on the order of a few million kilometers per day. At that rate it takes thousands of years to cross the interstellar chasms.
The cosmonauts who reach the end of the journey are not the same ones who started: hundreds or even thousands of generations have passed their entire lives aboard a spaceship and don’t know the Sun, or life in the open air. The vehicle where they travel is a small planet: a closed entity, in perfect ecological balance. Nothing is wasted. Wastes from normal activity are recycled and reused. The cycles of water, oxygen, carbon, nitrogen and other essential elements of life must be perfectly controlled. They only eat hydroponic vegetables.
