Callisto

Callisto is a Galilean moon of Jupiter whose residential orbitals continue to supply aspirants to the Imperial Fists. Aspirants are candidates for recruitment into that force.

Callisto lay within the industrial swarm that Perturabo overran during the Solar War. Its habitats formed part of that concentration of manufacturing infrastructure.

The residential orbitals belong to Terra’s shipbuilding belt. Callisto is therefore not a separate fortress-world, and its role as a recruiting source for the Imperial Fists continues within that industrial setting.