Each empire can manage a fleet of up to six ship classes at a time. Once this limit is reached, creating a new ship class requires decommissioning an existing class — and therefore scrapping all surviving ships of that class. This is important as ship classes cannot be updated as new technologies are discovered.
Colony ships are ship classes designed with a colony landing module. Increasingly hostile planet biomes require further evolutions of the landing modules, and therefore designing new colony ship classes. Terran, Jungle, Ocean, Arid, Steppe, Desert, and Minimal planets can be landed on with starting technology, but Barren, Tundra, Dead, Inferno, Toxic, and Radiated require increasing levels in Planetology.
Ship travels depends on two critical devices, their drives and their fuel cells. Improved fuel cells give a greater range (eventually unlimited), while improved drives give a greater speed (eventually nine times the speed of the starting drive).