Since I mentioned my reservations about Musk's plans, it's only fair that I substantiate them - heavy, high-energy stellar radiation particles. The interplanetary space is flooded with them, and their effect on the human electrochemical systems would be quite severe. Even a few months of exposure would leave any potential Martian colonists debilitated. As currently envisioned, such an attempt would be suicidal.
Overall, I consider the idea of colonizing and then terraforming other planets a folly - manipulating the atmosphere and climate would inevitably change the landscape of the planet in question, very likely endangering the established human infrastructure and thus causing economic damage that would economically cripple, if not destroy, the fledgling colonies. Instead, we should start the terraforming process before there is a significant permanent human presence there.
It's common knowledge that life on Earth can exist because our planet's magnetosphere protects us from the solar wind and stellar radiation. Thus, it's obvious that such protection, whether for space transportation or for exo-planetary habitation, is absolutely required for any sustainable human expansion into the Solar System. However, although we do know how to create powerful magnetic fields, such as those used by the CERN collider, we don't have the technology to make such generators mobile, or to build them in situ on the Moon or Mars.
Since the two most suitable (and still hellish) nearby worlds, Mars and Venus, don't have it, why do we? All three planets in question have hot iron cores, yet the Martian and Venusian no longer rotate, and thus produce no current. We do have a good idea how planetary cores produce magnetic fields, but not how this process is kicked off. In my opinion, the answer is obvious and is visible in our skies most of the time. In short, we have to recreate the setup nature has granted us here on Earth - we have to give each a stable, rocky moon, proportionate to the parent planet's mass, with a hot iron core. The interaction between the planetary and moon cores would provide the necessary first step to establishing a stable local magnetosphere. What we don't know is how long it would take - years? Centuries? Millennia?
I understand the magnitude of such an endeavor, and we are obviously nowhere near the necessary level of technology. If it ever comes to pass, this would be to largest, most audacious project the humanity would have had attempted up to that time. And a dangerous one, since migrating and parking such massive objects would inevitably impact gravitational interactions of the inner planets and the asteroid belt objects, and thus their orbital dynamics. In addition, there inevitably will be an impact on the host planet as well - quakes, volcanic eruptions, other tectonic activity. But conceptually, it is technologically feasible and such risks can be managed.
Where could we source such objects? For Venus, the obvious candidate is Mercury, although wrangling it from the Sun's grasp would be no easy fit. Otherwise, there is quite a field of candidates in the gas giant systems, possibly even in the asteroid belt or as far away as the Kuiper belt.
In the meantime, we should focus on the Moon. It's proximity would allow us to establish a lifeline even with the current level of technology, it is far closer and has all the elements needed for establishing a sustained human presence, including water, vast underground caverns, etc. Colonizing the Moon will push the innovation necessary for the creation of portable magnetic field generators of appropriate power to protect shipping and a base, then a city, etc. It will also provide a vast amount of wealth to fund such exploration, and with time, invaluable experience. As a species, we will learn how to colonize other planetary bodies there. Mars can wait.
