Callisto, a moon of Jupiter, has been a subject in both science fiction and scientific speculation for potential future human colonization. Callisto is the second largest moon in the Jovian system and the third largest in the solar system. Because of its low radiation levels, geologic stability, and large abundance of water ice, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.
In 2003 NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer solar system. The target chosen to consider in detail was Callisto.
It was proposed that it could be possible to build a surface base on Callisto that would produce fuel for further exploration of the Solar System. It could facilitate remote exploration of Europa, or be an ideal location for a Jovian system waystation servicing spacecraft heading farther into the outer Solar System, using a gravity assist from a close flyby of Jupiter after departing Callisto.
In a December 2003 report, NASA expressed belief that an attempt for a manned mission to Callisto may be possible in the 2040s.
Callisto has a large amount of valuable elements on or near its surface that might be useful for sustaining human life. Compounds detected spectroscopically include water ice, carbon dioxide, silicates, and organic compounds. The abundance of water ice on Callisto is a benefit to any considerations for colonization, as water can be provided for colonists' drinking needs or could be broken down to provide breathable oxygen.
Possible subsurface ocean
In addition to the ice water on it's surface Callisto is thought to have a liquid water ocean underneath it's icy exterior. If it exists access to this liquid water ocean would be a major difficulty, but could prove to be highly beneficial. Space colonists could create (or, possibly, find) a pocket between the icy surface and the liquid interior in which to establish a base. This location would be protected by the ice overhead, and would be at a more reasonable temperature than the surface, as indicated by the presence of liquid water.
Compared to the other Jovian moons Callisto receives much less radiation from Jupiter's radiation belt. On other moons such as Europa radiation levels would be lethal to Humans within days without proper shielding. Callisto escapes this problem entirely as it is almost completely outside Jupiter's radiation belt. Surface settlements therefore would need much less radiation protection and could prove to be easier and safer to build then on other moons in Jupiter orbit.
While far out from the inner solar system, Callisto lies within close proximity to several resource rich locations such as the other moons of Jupiter, Jupiter itself, and the Trojan asteroids. Callisto could be used as a base to launch further exploration or colonization to these worlds. It is also possible that ships bound for locations further out in the Solar System (such as Saturn or Uranus) could stop in Callisto to refuel or resupply.