Saturn has seven moons large enough to be round: in order of increasing distance from Saturn, they are Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus. Titan is the largest and the only one with a Moon-like gravity: it is the only moon in the Solar System to have a dense atmosphere and is rich in carbon-bearing compounds, suggesting it as a colonization target. Titan has water ice and large methane oceans. Robert Zubrin identified Titan as possessing an abundance of all the elements necessary to support life, making Titan perhaps the most advantageous locale in the outer Solar System for colonization. The small moon Enceladus is also of interest, having a subsurface ocean that is separated from the surface by only tens of meters of ice at the south pole, compared to kilometers of ice separating the ocean from the surface on Europa. Volatile and organic compounds are present there, and the moon's high density for an ice world (1.6 g/cm3) indicates that its core is rich in silicates. Saturn's radiation belt is much weaker than Jupiter's, so radiation is less of an issue here. Dione, Rhea, Titan, and Iapetus all orbit outside the radiation belt, and Titan's thick atmosphere would adequately shield against cosmic radiation. Trans-Neptunian region Freeman Dyson suggested that within a few centuries human civilization will have relocated to the Kuiper belt. Beyond the Solar System Looking beyond the Solar System, there are up to several hundred billion potential stars with possible colonization targets. The main difficulty is the vast distances to other stars: roughly a hundred thousand times farther away than the planets in the Solar System. This means that some combination of very high speed (some more-than-fractional percentage of the speed of light), or travel times lasting centuries or millennia, would be required. These speeds are far beyond what current spacecraft propulsion systems can provide. Space colonization technology could in principle allow human expansion at high, but sub-relativistic speeds, substantially less than the speed of light, c. An interstellar colony ship would be similar to a space habitat, with the addition of major propulsion capabilities and independent energy generation. Hypothetical starship concepts proposed both by scientists and in hard science fiction include: A generation ship would travel much slower than light, with consequent interstellar trip times of many decades or centuries. The crew would go through generations before the journey was complete, so none