RPC54 - The Expanse
The Belt — short for the asteroid belts between Mars and Jupiter — is a region of the solar system that has only been seeing colonization in the last 100 years. In modern parlance it is a reference to the roughly 1,000 stations scattered through the solar system that are owned by Inner Planet governments and corporations. The biggest of these stations can often house millions if not tens of millions of people. Earth estimates put the population of the Belt at around 30 million people while Mars estimates put that estimate higher at 50 million. Many Belter activist groups put the number even higher than that.
After the E-Drive was invented the speed these new drives allowed for was used to create hauling ships to travel to Ganymede and setup ice mining operations by Martian corporations. Mars had long been a slave to Earth when it came to certain natural resources, water chief among them, and with this new transportation technology combined with Martian advancement in ship building, Mars was able to become much less dependent on Earth.
While the E-Drive decreased travel times, it did not make them instantaneous. Small space stations were constructed at convenient places for refueling, refinement, and relaxation from the long hauls to the icy moons. Eventually some miners would take residence in these stations and they would grow and prosper as small colonies in their own right, needing resources bought from Earth and Mars and dependent on them for a large amount of goods.