When developing a space fantasy campaign setting, the GM can easily use Earth's own history—namely, the Age of Sail—as a model. With that in mind, four stages in the expansion of exploration and exploitation of the galaxy are suggested below.
Stage 1: More Theory Than Practice
At this point, most people in a campaign setting don't believe that space travel is even possible. There are a few visionaries, however, who are willing to try it and discover the truth. More importantly, at least a few of them receive the patronage of a wealthy sovereign or some other backer, providing the means for testing the theory. When those intrepid explorers set out on their voyages, many dismiss any hope of ever seeing them again. Indeed, some do not return, and their disappearances become part of local legend. Others manage to complete their voyages, however, and bring with them the first evidence of wealth and culture from other worlds. Their success is cause for sensation and celebration, and others begin to consider undertaking expeditions of their own.
The voyages of Christopher Columbus and Ferdinand Magellan are good examples of this stage.
Stage 2: Investigations and Investments
Once the first explorers have proven that aetherial travel is possible, governments and individuals with the wealth to do so begin plotting expeditions in more systematic ways. This includes more frequent voyages, undertaken with religious, political and/or mercantile objectives, likely accompanied by military support. Indeed, different powers are likely to compete against each other in the hope of claiming territory and resources for themselves. Of course, this is also when pirates begin their activity, since any kind of law enforcement is still remote and scattered.
The expeditions of the Spanish conquistadors, such as Cortez, Pizarro and Coronado, are good examples of this stage.
Stage 3: Companies and Colonies
As the level of interplanetary travel increases, the Old World countries gradually make their presence felt in bigger and bigger ways. This includes the creation of settlements and colonies on other worlds, as well as the creation of organizations dedicated to promoting exploration and trade. Specific routes become well established, although hazards still remain. Piracy is one such lingering danger, but can be met with a forceful reprisal. There is the possibility of insurrection in once-loyal colonies. Note, too, that cultures and societies on the Old World become increasingly blended, since travel between nations is much easier. What is more, visitors from other worlds are seen in larger cities.
In Earth history, the Golden Age of Piracy, along with the Revolutionary period that followed it, are good examples of this stage.
Stage 4: The Way of the Worlds
By this stage, travel between planets becomes commonplace. Visitors to and from other worlds are frequent sights in port. Additionally, post-colonial political situations occur; for example, an old country might have to learn to deal with one of its former settlements as a rival and an equal.
The developments during the late Eighteenth and early Nineteenth Centuries on Earth are exemplary of this stage.