[adastra]

Mars is a non-starter? I think you've received some bad information.

Average radiation on the Mars surface is 10-20 rem/year. With a thicker atmosphere it will be less. And for people spending 12-15 hours a day indoors in shielded dwellings it will be a lot less. It's not a showstopper at all.

[mixmastamyk]

The low gravity may not be enough to hold a thick atmosphere, no?

[jswhitten]

Not for more than 100 million years or so. But then, Earth will also become uninhabitable within the next few hundred million years.