Luna's return and Twilight's ascension to rulership of Equestria helped usher in an age of scientific and magical innovation. She and her slow-aging and long-lived friends lived through tumultous times of discovery and progress, and now Equestria is beginning its first true forays into deep space.
But someone has to be out there, representing Twilight and speaking on her behalf. She can't go herself--FTL travel is still primitive and time consuming. She needs someone out there trekking the stars, keeping up with the growing network of colonies. Someone with diplomatic chops, a vivid imagination, impeccable manners, and a lot of charisma. Rarity is eager to help her wife out in a once in a lifetime opportunity to see the worlds, literally.