Despite the fact that future technology could eventually open up all of these worlds for human habitation, only a few of them may attract "the masses" after the first person sets foot upon their dusty soil due to the "evil R word"--radiation.
Contrary to the various rumors, taking heavy doses of radiation does not turn one into the Hulk, one of the members of the Fantastic Four or Spider Man via a radioactive spider bite.
Radiation, whether cosmic or solar has the potential of seriously roasting you alive, if not turning one into a vegetable.
Even though humans can tolerate "various degrees" of radiation, our bodies seem to be quite content with the level of background radiation our species receives on planet Earth, which is about 0.35 REM's (aka Roentgen Equivalent Man) a year.
Higher doses of radiation can prove to be fatal towards future colonies, and some researchers do not recommend levels above 50 REM within a year or 25 REM during a 30 day period as it can lead towards some serious side affects (as highlighted in the chart below).
While radiation can be countered by using water, lead and aluminum, parents may be hesitant to breed upon foreign planets and moons (let alone raise kids upon them) if it will result in their children acquiring serious birth defects.
In order to determine which worlds are "family friendly," one only has to look at how much radiation a world receives to determine whether or not it is suitable for large populations or should be left alone for industrial space companies.
Starting out with Mars, one often dreams about metropolises dotting the surface of that crimson sphere. While Mars may hold much promise for future colonies, its annual dose of 15-20 REM may give some settlers second thoughts.
While future Martians may be able to combat the threat of radiation by building cities within its lumpy magnetic field, the red planet as a whole may not spawn dense cities until a globe sized artificial magnetic field can be constructed.
Moving outward to the Jovian system future space settlers may find more fortune living on Jupiter's moon Callisto. Orbiting just outside of its angry parents radiation belt, Callisto receives approximately 0.01 REM a day (or about 3.65 REM a year).
Coupled with its prime location in the outer solar system, Callisto may outpace its Martian rivals population wise, and may be second only to Earth as far as future inhabitants go.
Unfortunately Jupiter's other lunar daughters do not fare as well as Callisto, with all three of these worlds (Ganymede, Europe, Io) bathed in Jupiter's harsh radiation belt, putting them at a disadvantage compared to their much colder, "uglier" sister.
Traveling further outward towards Saturn, one may find it strange that humans may call the smog world of Titan home sweet home. While its surface may be hidden from the human eye, its atmosphere may be thick enough to protect residents from both solar rays as well as Saturn's radiation belts.
Even though there are other worlds such as Luna (aka Earth's moon), Ceres, and even Ganymede that may eventually be civilized by our ever growing race, these worlds may not conquered right away due to the "invisible killer" lurking in the shadows.
While it would not be surprising to see scientists and industrial corporations setting up shop on these hostile worlds, the bulk of humanity may choose to remain on these radiation safe worlds until over population forces them to conquer these overlooked spheres roaming silently among the stars.
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