Mars does not have a magnetic field, as does the earth. Among many, one of the things which makes a mission to Mars, or anywhere
, difficult, is the necessity to shield the passengers from cosmic radiation. One method is very, very heavy shielding, which at some point has to be lifted out of the mother well (Earth's gravitational field), and which will be a crucial factor in acceleration to cruising speed, and in manoeuvring the vehicle--and god-awful expensive. The other method would be to create an artificial magnetic field around the spacecraft, or within the craft. That would be very, very expensive of energy, and would still involve getting a significantly large amount of material resource out of the mother well. The latter, is however, the most likely scenario, as a great deal of the electronics on board would have to be shielded from cosmic radiation, too, and a compact internal magnetic field would be the most practical method from many points of view.
On "spaceship Earth," we are shielded from the cosmic radiation. Mars has no such shielding. From a practical point of view, the only way to colonize Mars with any significant number of people would be for them to live beneath the surface. One can do that just as well on the Moon, and the resource cost to put large amounts of material and large numbers of people on the Moon would be much less. All the fantasies about terraforming Mars, or any other satellite of the sun, or satellite of one of those satellites, suffers from the same problem--protection from cosmic radiation.
Claims about a future necessity to colonize imposed on us by the sheer numbers of humans are equally silly and ill-considered. When i was a boy, and first old enough to understand the meaning of the idea, i learned that the population of the Earth at that time was approaching two billion people. We were told that the Earth was reaching a saturation point vis-a-vis the exploitation of available resources. We were informed that doom impended without radical action. Now, fifty years later, the population of the Earth is six billion. If the average condition of mankind is no better, it is certainly no worse. Predictions of doom from overpopulation have just not played out as advertised in my lifetime.
But what makes the notion of colonization of other planets/satellites silly and ill-considered is to give realistic consideration to what the material costs would be to make an kind of significant alteration in the planet's population through such a project. Does anyone propose that it would be practical to put two or three billion people in space, with all that they would need for survival, and shielding from cosmic radiation? Anything less than hundreds of millions of people would make no significant dent in the population.
The idea of sending out a few thousand, so as to make a "home away from home" for the race is equally ill-considered. Fewer than two or three thousand (and ten thousand, or tens of thousands would be preferable) would not be sufficient to give goods odds of survival and sufficient genetic diversity. The costs of sending several thousand people into space on a long journey would be astronomical (pun intended). So how practical does anyone propose it would be to ask billions of people to make an enormous sacrifice for many, many years (probably decades) in aid of the future of a few thousand people whom they will never see again, and with whom they will never again communicate?
I haven't even addressed the issue of long-term exposure to "zero-g."
The human race might--only might--be able to colonize other planets/satellites in the cosmos in the very distant future--but don't bet on it. There is too much, especially simple human nature, which would work against it.
The only reasons to go to Mars would have to be human pride, philosophy, "spirituality"--there are no good, pragmatic reasons to do so. If we just want to set up a base in "outer space," the Moon makes much more sense. Neither the Moon, nor Mars, can reasonably be seen as venues for significant colonizing efforts. I do consider such a mission a good idea, but only on the basis of "free research," because such research has always provided unexpected dividends. But it ain't gonna be the Earth's summer house by the lake.
If you think about it--and i have. I suspect that most other people who get that starry-eyed look and dream of cosmic colonization don't.