So, a slightly different progress:
- Climate change, freshwater exhaustion and population rise are severe stresses 50 years from now. The situation is serious but (just barely) manageable.
- About 2070, an alien species that has migrated permanently away from planetary surfaces establishes a fuel processing center around Saturn. Part of their strategy is to increase custom by contracting to locals for food supplies (still cheaper to grow on a planet) and to trade for technology that will get those customers into space and thus buying fuel and equipment.
- The drive for space is to establish humanity on other worlds so all our eggs are not in one basket. Colonists would be people with a wide range of objectives from political and religious idealists to those planning to become the privileged aristocracies of new kingdoms. The bulk of the people eager to migrate, however, would be landless and displaced peasants of the developing world and displaced industrial workers of the rich world.
- First colonies by 2100 (if not earlier).
- Now add a few hundred years; but not too many.