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The Doclopedia #2,664
People Of The Solar System…: Jovians
Few people visit any other Jovian moon besides Callisto, home to the joint US/French research and mining colony. This is because the other three Galilean moons are home to a dangerous and probably insane religious cult.
The Family of Jupiter first came to these moons in 1888 and established a colony on Europa. Later, more cultists arrived and settled on Io and Ganymede. Nobody knows how many people make up the Jovian part of the cult, but the estimate is around 7,000. They have declared themselves to be owners of those moons and will destroy any ship that lands on one. What they do out there is unknown, but travelers should not go anywhere near them.