I'm really no expert. I guess the iron core of the earth is too big to be feasibly replaced by asteroids "crashing" on mars. I'd further guess that it would be

I'm really no expert. I guess the iron core of the earth is too big to be feasibly replaced by asteroids "crashing" on mars. I'd further guess that it would be easier to do the same with some sort of electromagnetic field. To enclose the whole planet would also be a giganteous undertaking. Much easier would be local field generators on the surface, maybe powered by a fusion generator. At best this would be centuries in the future, at worst this doesn't make sense at all because it would require less energy/resources to construct huge greenhouses/domes that achieve the same thing and more. The more immediate obstacles to colonizing Mars seem to be getting there in the first place, getting enough stuff there on a permanent/continuing basis for a few dozen people to live there for a few years, including radiation shielding. It would be a really long time until there is a financially self-sustaining "colony", let alone one that can produce all of its own foods and much of its own technology and other resources. I figure it will be centuries until people even think about anything terraforming-related seriously.