What planet would you colonize first? Why and how?
Mars is the obvious choice but there are a few problems we would need to solve first. It needs a magnetic field, and some oceans.
Having read science fiction for decades and considered this question at some length, I keep returning to Winston Duarte’s published thesis “Logistics-Based Strategy in Interplanetary Conflict” within the Expanse universe. There is something very germane that even he seemed to miss which does come up later in the series; transfer points.
The spacecraft systems required to reach orbit are essentially non-overlapping with the systems required to travel between planets, and then land and take off again. Elon Musk is taking the same approach as The Martian did; one rocket to get to orbit, and another rocket to shuttle back and forth between Earth and Mars. What’s interesting is that even he does not consider the moon as the obvious transfer point where a permanent settlement could be built and then serve as a natural place for the rockets to meet.
Building and controlling transfer-points bears not only enormous economic and political significance but also dramatically empowers growth and expansion. In a later Expanse book, transfer points are constructed deliberately around the terrestrial planets in order to control traffic, but it just seems so obvious today that these are the most critical and overlooked missing piece to the interplanetary puzzle. There is another critical feature of large transfer points as we will see…
The Earth already has a large transfer point, Luna. But Mars has two tiny baby satellites which are essentially useless because they do not have enough mass to create geological activity and a corresponding planetary magnetic field to protect against harmful radiation. Based on the theory that the reason Mars is geologically and magnetically dead is its lack of a large satellite, adding a large satellite would solve both problems at once by making Mars geologically and magnetically active, and also creating an obvious transfer-point for interplanetary travel.
I have conducted a survey of the available moons in the solar system and I think Titan would be the most logical one to move over to Mars. It is somewhat larger than Luna with respect to Mars’ relative mass compared to the Earth. I think this would accelerate the process of activating the planet’s geology and magnetic field. We would still need to create liquid oceans, and so Ceres seems like the most logical choice. Scientists estimate that its mass is about ten percent water. So if we direct Ceres to crash into Mars, we would add about 10^17 kg of water to Mars’ surface, or about a quarter of the amount of ice in Antarctica; not a lot but definitely something to work with.
I think these changes would be a good first step in terraforming and eventually colonizing Mars.