Callisto

Callisto is one of the moons of Jupiter and a human colony. Hosting the Jovian system's second-largest population, Callisto is also home to a considerable UNSC military presence, with a Naval base and a key shipyard.

History
The only Galileian moon to orbit outside Jupiter's radiation belt, Callisto was the target of considerable interest during the Golden Age of Space Colonization, and became the first Jovian moon to host a permanent colony in 2060. From the beginning, Callisto played a key role in supplying missions on the other moons. As the Jovian system developed, Callisto became an economic and commercial hub as well as a waystation for missions further out-system. Though Callisto's population has experienced a decline following the Domus Diaspora, it remains the most second-most populous Jovian moon, after Ganymede. However, the recent centuries have seen its purpose pivot more towards military industries and naval facilities, and many of its habitat-cities are effectively military towns. Callisto's orbit also houses key elements of Sol's early-warning slipspace monitoring network. One of the remote scanning stations, Pythagoras, was the first to detect a sensor ping from the Prophet of Regret's fleet near Io during the initial Covenant assault on Earth.

Topography
One of Jupiter's four largest moons, the Galileian moons, Callisto is an icy world relatively similar to Europa and Ganymede. While in some ways less remarkable than its neighbors, Callisto's minimal levels of radiation compared to the other moons, coupled with the moon's geological stability and lack of dramatic surface formations, make its surface conditions more conducive for human settlement. This also allowed colonies to be built on the surface (rather than ice warrens) from the outset.

Surface features on Callisto are named after Norse mythology. These lend names to many of the moon's population centers and facilities.