One of the definitive treatments of the generation ship concept in science fiction - if faster than light travel is impossible, create a starship large enough to sustain a human population for as long as the trip will take.
Orphans is a collection of two connected novellas, Universe and Common Sense, written by Robert A. Heinlein and published in 1941 in the magazine Astounding. The collected edition was first published in 1963.
Orphans/Universe pioneered the common twist of generation ship stories where the ship's population somehow forgets its mission, and even the existence of a universe beyond the walls of the ship. In this case, generations since a mutiny that claimed the lives of the crew, the descendants of the loyalists eke out a primitive existence tending the remaining agricultural sections, while defending themselves from the 'Muties', mutated descendants of the mutineers of old, now exiled to the irradiated inner decks. They are ruled by their 'Scientists' , a caste of priests who maintain the ship by rote and interpret the few remaining physics texts with no comprehension of their purpose (Newton's Law of Gravitation as love advice, for instance), while exalting the name of the ship's creator as their god.
The protagonist of the story, Hugh Hoyland, is kidnapped by a group of Muties shortly after being accepted into the Scientist caste. Their leader, a two-headed mutant named Joe-Jim, takes the boy deep into the mutants' territory... to the bridge of the ship, and reveals the universe to him. Joe-Jim shares his/their knowledge with Hugh, who eventually decides to try to unite the ship in order to complete its original mission.
A similar work that escaped into the public eye is the infamous Starlost TV series, disowned by Harlan Ellison.