vemvare said:
Terraforming is of course extremely speculative. A "terraforming future" isn't a future with a couple of thousand NASA/ISA scientist-astronauts flying around the solar system, it is a future where millions of people live their lives in massive space stations, where thousands of people leave and return to Earth each day, and where asteroids are moved around, smashed to pieces and mined! As such, it is likely to be distant future, which of course means that huge advances will be made in science, so much of what we're speculating about here will be laughably wrong if read by the generation that actually start the terraforming process.
I agree, the timescales here are not centuries, but millenia. That's why we have to start the process soon, so that capacity keeps up with the projected growth of our civilization.
Because of this, I believe that Venus is a poor candidate for terraforming, as it will eventually be subject to the increasing luminosity of the sun as it finishes the main sequence. Earth, too, will be in danger, so it would be wise to select a planet that has a good chance of escaping the red giant phase if we really do intend to survive and prosper indefinitely.
Mars is quite appropriate for several reasons.
1) Reasonably similar gravity to Earth's (0.376g)
2) Conveniently reduced escape velocity for transfer of materials to and from orbit
3) High oxygen content (in iron oxide)
4) Bearable temperature & pressure ranges
5) Proximity (~7 minute delay in communications, regular resupply from Earth feasible)
6) Familiarity - we've already started exploring it and have the technology today for landing.
The easiest way to go about the terraformation would be to send over a probe full of our favorite lichen, moss, and (genetically modified?) microbes that would start clambering over the surface, devouring iron oxide and releasing heat & H20.
Perhaps we can one day slingshot magnetic materials around it at ridiculous speeds to jump-start the core and provide a magnetosphere to boot... if not, we'll find a way to generate the shielding we need.
Eachus said some very interesting things about starting in Martian valleys. I think that process could start within a year if we applied ourselves. Mars One estimates the cost of sending 4 crewmembers to Mars permanently to be $6 billion, about as much as the U.S. spends on defense every 2-3 days in 2012 money - so the resources are there if we can find common ground for cooperation.
Don't forget, the asteroid belt is right next door to Mars. Once we get a foothold on and around the red planet, it will be that much easier to access those resources and relieve scarcity even back home.
The moons of the gas giants will be great resources for a network of space stations throughout the solar system, but terraformation will be much less feasible owing to the gravitational stripping of their atmospheres caused by the planets they orbit.
With the limitations we experience here, I can't help looking past Earth and imagining an interstellar future for humankind. Our homeworld will always be sacred, but future lies among the stars.