Why are self sustaining space colonies a dream?
Granted, such a notion is at least a few centuries off, but not a pipe dream by any means. IMO, the best space colonies will not be located on a planet, but will be rotating stations located in the outer solar system.
Being in orbit around Saturn, Neptune, or Uranus would be choice locations. They could use tethers to produce electricity from the large magnetic fields of those planets, as well as shielding them from cosmic rays. Unmanned cargo transports could be used to trade locally harvested resources for materials gathered in other parts of the solar system.