Ah, that's easy. The best sites to test designs for sustainable colonies on Mars are in the deserts; the best sites to test designs for sustainable colonies on

Ah, that's easy. The best sites to test designs for sustainable colonies on Mars are in the deserts; the best sites to test designs for sustainable colonies on Europa are in the oceans; the best sites to test designs for sustainable colonies on asteroids are at the tops of mountains. Once you have sustainable colonies in the deserts, the oceans and the mountaintops, you have much less urgency to move them out to space...