Scientists have proposed a massive 36-mile-wide "generation ship" capable of carrying 1,000 humans on a 250-year journey to another star system. Inside the rotating structure, artificial gravity, farms, water recycling, and entire ecosystems would allow people to live their entire lives in space. Children born aboard the ship would grow up knowing only one world: The starship.
By the time the mission ends, the people who arrive would be the great-great-grandchildren of the original crew. Humanity would become a travelling civilization.