This is from a series of space colony ideas NASA commissioned in the 70s. I wish the 70s had lasted forever...
I've been searching for this great article I read about the impracticality of every conceived form of inter-system space travel but can't for the life of me find it. I can only remember a few of the criticized scenarios off-hand. One of the ideas for traveling between planets are these things called generation ships that would be massive ships (possibly created from a hollowed out asteroid) capable of supporting the multiple generations worth of humans needed to survive the passage. Many sci-fi writers have already pointed out that even if the life support systems of these ships didn't break down, within a few (genetically limited) generations it is likely that people would begin to forget where they came from, where they were going and what their purpose was. The cruelest/funniest idea was that should one of these generation ships (or a ship carrying people in induced comas or what have you) finally arrive at its destination it is very likely that humans would have in the meantime invented exponentially faster forms of space travel, passing these older ships in transit to settle these other planets ahead of them so that the original space travelers, instead of stumbling out onto a lifeless planet would be greeted by just another earth outpost station or even city!
So instead of finding this essay I stumbled upon these great renderings of how Mars, Venus and the Moon would look if we terraformed them: