After thinking about this topic for a while, I decided that we humans need to concentrate on developing space habitats at least as much as dreaming of ways to colonize a planet. Don’t get me wrong, I am all for colonizing large natural bodies (minor planets and larger), as have numerous advantages. However, even assuming a “quick terraformability” scenario for the planets (especially Mars), they would still suffer from multiple problems based on two primary shortcomings: gravity and radiation:
*Deep Gravity Well (even if not quite to the extent as Earth’s) - increased transportation expense for goods and services. Space piers, space fountains, and space elevators can help reduce the expense, but the fact remains that escaping a space habitat’s gravity well is much easier than escaping even Ceres’ well.
*Gravity Substantially Weaker than Earth: Advantageous for construction of surface structures, but quite perilous for humans (assuming people want the option to visit Earth in the flesh). This is especially true of the Moon and Ceres. I do admit that this point becomes less relevant if (a) we develop drugs to strengthen human bones, muscles, and organs when visiting Earth, (b) gene-enginnering so that space colonists can adapt very quickly to their present gravity-environment, (c) gene-engineer local Martian (or other body) foods so that the foods contain vitamins and minerals that rapidly replenish the bones, muscles, and organs (this is my favorite long-term solution, if it’s achievable)
*Radiation Hazzards - these can be easily overcome by (a) filling a dome with ozone AND creating a coating that makes the dome largely opaque to UV. An electrically-generated magnetic field can also screen out much of the magnetism in each local settlement (we can presume they’d have multiple backup generators should a mag-field generator fail). Even so, time and again, nature proves more reliable than even the most advanced human technology, and probably even foreseeable human technology as well.
For the above reasons, I now believe space habitats / orbital habitats are the optimum solution, at least for the next several centuries. They have several advantages over colonizing a planet.
* Negligible Gravity Well: means less energy (and expense) when transporting goods and people to and from the habitat.
* Easily Adjustible Rotation Speed: This translates into adjustable centrifugal pseudo-gravity, meaning that inhabitants won’t have to worry about low-gee-induced bone, muscle, and organ deterioration. This reduces the need for any future drugs/treatments to maintain the human body to earth-gee tolerance standards.
*Can Predetermine Their Orbits (including their eccentricity): Useful for the following purposes.
-- Solar energy available is already known, meaning you can set the orbit so that on-board equipment can function at it’s optimum solar energy level (i.e. Venus’s solar energy is about 200% that of earth’s, Mars is about 44%, so there’s a wide range of choices) . This ties into the next point.
---Easily adjustable magnetic field shield strength - the closer you are to the sun, the more concentrated the solar energy is. This translates into more power for the habitat’s magnetic field. Combined with thick habitat shells, this can easily keep cosmic radiation to tolerant levels.
-- Highly Taylorable Natural Environment - Climate Control is much easier. Indeed, it might be possible to develop climate-oriented habitats for differing human tastes, perhaps even including those not found on earth to a significant extent. Just on temperature alone, you have your choice of the following for any one habitat
---Tropical - Always Warm or Hot
---Subtropical - Hot Summers, Cool to Chilly Winters
---Temperate Continental - Warm to Hot Summers, Chilly to Cold Winters
---Temperate Oceanic - Mild to Warm Summers, Cool to Chilly Winters
---Subarctic - Warm Summers, Cold to Frigid Winters
---Arctic - Cool to Chilly Summers, Long Frigid Winters
Moreover, each type of climate listed can have its humid, sub-humid, semi-arid, and desert variants (some of which might not naturally exist on earth!). Engineering moisture reliably is likely to be complicated, at least if we wish to maintain a natural looking habitat. In fact, orbital habitats can be an excellent, if expensive way, to preserve endangered ecosystems and cultures (for example, making a polar arctic habitat with all its trimmings for polar bears and their main food source, seals. Also in this scenario, it’s theoretically possible for such places to house indigenous cultures - although there’s the question about the authenticity of such a native culture in an orbital vs. the original one on Earth, but we can presume that at least some members of these cultures will jump at the opportunity to recreate their culture in the habitat.
I freely admit that preserving/recreating entire ecosystems is well beyond our capabilities on at least two counts: (a) such orbital would probably need an interior space of at least 50,000 square km or so, possibly more, (b) we still have little knowledge about how any particular ecology operates even regarding macroscopic life - don’t even ask about the microbes at the bottom of the food chain! Even so, I think we can partially overcome (b) by using microbes already present in the original Earth environment.
Last but certainly not least -- Assuming we’re still around for the next few billion years, we can adjust the orbit or our habitats to the new “Earth Solar Constant Line” (the point from the sun where solar energy = 1,368 Watts per sq meter per second). Because it’s much easier to move an orbital than it is to move a planet, we have more assurance of our survival.