Well, no, those beliefs are beliefs that things exist without any evidence. Beliving that humans can colonize another planet is a belief that something can be a

Well, no, those beliefs are beliefs that things exist without any evidence. Beliving that humans can colonize another planet is a belief that something can be accomplished that we currently have no idea how to do. 200 years ago, there were no UFO beliefs and no "easter bunny" as we know it today, but there were sasquatch legends. The claims are as unsubstantiated today as they were then. 200 years ago some people believed the humanity could one day visit the moon. At the time they had no idea how, but 200 years later we have not only gone to the moon but become bored by it. But I also don't see why you don't think we could establish a martian colony even with the technology we have today. I suppose I don't know what you mean by "sustainable," and if you mean truly terraforming the whole planet to make it a second earth, you're right that that is still the realm of science fiction. But you don't need that. All you need is a large climate-controlled enclosure with hydroponic farming equipment. The martian atmosphere contains enough oxygen (0.14%) to keep this enclosure supplied, and water exists on mars, certainly in solid form. Such an enclosure would be sufficient for a person to live an entire lifetime without assistance from earth, which is a reasonable criteria for sustainability. The project would be economically possible, but probably politically impractical.