NASA announced today the presence of enormous salt-water lakes only several kilometers below the surface ice of Jupiter's moon Europa. From the standpoint of colonization (and also incidentally exobiology), this is vastly preferable to the ocean of salt-water which also exists on Europa, but at some 100 kilometers below the surface ice. This puts Europa back in the running as a plausible candidate for outmigration, and possibly even in the top tier.
With this data, the three best candidates for a permanent and self-sustaining colony are, arguably, Ceres, Europa and Enceladus.
Saturn's moon Enceladus is in some ways the most appealing, but by far the least accessible. At 1.5 BILLION kilometers, it is more than twice the distance to Jupiter/Europa, and six times the distance to Ceres. And it is the smallest of the three candidates. But essentially limitless supplies of water and energy are relatively accessible to anyone living on the surface, and from the standpoint of self-sustainability that's huge.
Europa now meets most of the same criteria, is six times larger than Enceladus (and just a bit smaller than our own moon), and is much closer. The biggest drawback with Europa is still the very high amounts of ionizing radiation from Jupiter that would be experienced on the surface. Burrowing under the ice would provide shielding, but you have to get down there first. It is also possible that we could use another of the Galilean moons such as Callisto as a base-camp while drilling down to to the Europan lakes.
Dwarf planet Ceres is practically in our own backyard, and about twice the size of Enceladus. It too is covered with water ice over a salt-water ocean, but we don't know yet how thick that ice-mantle is. When the Dawn spacecraft arrives there in 2015 we'll know a lot more. Ceres receives ample sunlight for solar power, so even if there were no geothermal energy such as on Europa, or whatever-the-hell is generating 16 gigawatts of energy on Enceladus, it could probably support a substantial colony even if the surface ice had to be melted for water. At this time, I'm inclined to think that Ceres may be our best shot at getting a permanent and self-sustaining colony established quickly, but we'll know a lot more once the Dawn spacecraft starts sending back data.
Many people have speculated that Ceres would be an important stepping-stone for colonization of the outer planets, but she may prove to be a critical destination in her own right.
I think that Europa, Enceladus and Ceres offer three very different possibilities for exploration. Europa is still and again probably the best candidate for extraterrestrial life in our solar system. Enceladus is intriguing because of the Tiger Stripes; I believe, and I think you do too, that there is a strong possibility that the source of heat we're seeing is artificial. That's worthy of exploration, at least by robot. Ceres is maybe the least interesting of the three, but the closest and the best candidate for establishing a human colony in the foreseeable future. And it probably goes without saying, your Stanford torus idea would work equally well on Ceres, and with small modification on Europa.
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