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Settlers are fictional human colonists that replaced the earlier Spacer emigrants who were in dominance in the vague period between Isaac Asimov's Robot series near-future short stories (of the type collected in I, Robot) and novels.
Emigrants from Earth establish colonies on fifty worlds, the first being Aurora, the last Solaria, and the Hall of the Worlds located on Melpomenia, the nineteenth. However, sociological forces possibly related to their sparse populations and dependence on robot labor lead to the collapse of most of these worlds; their dominance is replaced by new, upstart colonies known as Settler worlds in the Milky Way galaxy. Comporellon was the first colonized world of the Settlers. Unlike their Spacer predecessors, the Settlers detested robots, and so by the time of the Empire series, robotics is almost an unknown science.
The Settlers were born from a movement founded by noted Earth hero and police officer Elijah Baley, who had some years earlier, become the first Earth born human in centuries to travel beyond Earth (to the Spacer colony world Solaria and later colony Aurora to investigate murders) and to walk under open skies as opposed to remaining in the underground cities all Earthmen then lived in. The experience of doing so had a profound affect on Baley, who upon his return from Solaria, he began gathering young risk takers, willing to reject the stasis of life within the cities and struggle against the now culture wide agoraphobia now experienced by all Earth born humans. Over time the movement began to grow and progressed from simply practicing to overcome the agoraphobia and began studying manual agriculture, determined to learn how to survive without robotic aid.
The governments of Earth took a dim view of Baley's movement, but his status as a hero to the public and his political connections with the Spacers kept the early Settlers unmolested and they gradually grew to become a major force in Earth's politics and led by Baley's son Ben Baley they managed to achieve their goal, settling what would be named 'Baleyworld'. From there the Settler's expanded with a far greater speed than the Spacers ever demonstrated and soon surpassed their long living cousins in number of planets held, if not in technology, which prompted concern and threats from the conservative side of the Spacer government.
Unlike the Spacers who took a haughty view of Earth, the Settlers revered the Homeworld, turning to Earth to arbitrate any disputes between them and believing their connection to Earth's past made them superior to the advanced but now stagnant Spacers.
In the novel The Robots of Dawn, Asimov reveals why the majority of Settlers came from the short-lived Earth population, as opposed to Spacer worlds, which would have used humaniform robots in the process.
By the time of the sequel, Robots And Empire, the number of the Settler worlds exceeded a hundred ("about half" of the "hundreds of... planets... recorded and studied").
(In Asimov's novel Nemesis, the main colony is a member of the collection of orbital colonies known as the Fifty Settlements. It should not be confused with the later Settlers that are the matter of this article, as it is possible that the Fifty Settlements were the basis for the fifty Spacer worlds in the Robot stories.)