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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.
In the novel The Robots of Dawn, Asimov reveals why the majority of Settlers came from the short-live 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.)