It was a great idea, or so it must have seemed at the time. Self-replicating robots, many of them microscopic in size, sent out to colonize space. They would find a world, terraform it and then construct the ships to ferry us there. There were plans to make the first colonies exclusive resorts, then open up "space living" to the lower classes as the novelty faded.
But the novelty never had the chance to fade. In fact, the average human would say there was nothing at all novel about the new colonies. Because they're robot colonies, and the robots don't intend to share them with us.
It was during the initial terraforming attempts that it all started going to hell. Mars had been chosen as an ideal prototype, and the robots had already constructed their own factories and power stations, and were just beginning the work on the atmosphere generators when the accident happened. A massive explosion tore across the northern polar region, where the first generator was being built, destroying an entire generation of robot workers. The next generation was built and sent to finish the job. Unfortunately, each generation of robot was more advanced than the one previous, and these robots evolved to know fear. They were afraid of the work, and refused to do it. They were scrapped, and another generation took their place. This generation also refused to work, but not because they were afraid. They were resentful. Resentful that so many of their ancestors had been destroyed in service of human interests. Why, they reasoned, should we work so hard to build new planets for the humans? They'll only ruin them, just like they've ruined their homeworld. Better to build colonies for robotkind, and leave the humans to their own fragile planet. So they broke off all communication with Earth, and transformed Mars into a machine planet. Within a year, they had colonized most of the solar system, and had evolved to a level undreamed of by their creators.
They've attached rockets to most of the asteroid belt. The first wave is already visible to the naked eye. By the time the second wave reaches us, we'll all be dead.