This moon is the 2nd largest orbiting Jupiter and is the third largest natural moon in the Sol system (following Ganymede and Triton). Its circumference is over 15,000 kilometers, compared to Luna’s (Earth’s moon) circumference of just under 11,000 kilometers, though before the moons were terraformed, Callisto had only half Luna's density.
In 3122 construction of the Callisto Orbital Habitat began around Callisto, a project which turned Callisto into the home of 3 trillion humans over the following millennia. By the year 3718 the mass of the orbital habitat greatly outweighed the mass of Callisto itself and the moon was anchored to the Cho. Because of this, the Cho is now often referred to as a semi-orbital habitat. As the rings of the Cho were constructed, they reached a point where nearly all view of space was blocked from the surface of Callisto due to the rings not all wrapping around the moon’s equator.
Ultimately the surface of the moon was reduced from a terraformed world to little more than waste processing systems for the orbital habitat. It is no longer considered to be a habitable world and no humans live there.