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Probability Moon is a 2000 science fiction novel by the American writer Nancy Kress. The novel concerns a xenological expedition to the planet World, where aliens live who have developed a strange form of telepathy or collective unconscious, "shared reality", which causes piercing "head-pain" whenever "Worlders" attempt to hold strongly differing opinions. Simultaneously, an artificial satellite is found in orbit of the planet which has uncharted powers, and may be the key to winning a war against a xenocidal alien race, the "Fallers".