In light of [the sheer size of space], and the fact that the solar system contains 8 planets, 5 dwarf planets, 335 moons and millions of asteroids, minor planets, comets, trojans, centaurs and the like, you really have to wonder why science fiction has obsessed for so long about interstellar and intergalactic empires. Isn't the solar system big enough?
It strikes me now that the answer is fairly obvious: aliens. There aren't any in the solar system, or at least not ones we can wage war on/fuck/misunderstand, so SF authors by necessity need to look beyond it if they want Klingons.
But the fact remains: the Solar System is massive. Consider this for a second: the distance between Jupiter and Saturn is from 600 million kilometres to 2 billion kilometres. Jupiter has, at current count, 66 moons. Saturn has 62. Uranus and Neptune have another 40 between them. There have been 1,200 Transneptunian Objects recorded. There is a lot of stuff out there to get lost in. Indeed, the question I am currently considering is not so much "Why bother with what's outside the Solar System?" as "Couldn't you just set a game among the moons of Jupiter or Saturn?"
The moons of Jupiter vary hugely in size - from massive rocky Ganymede, bigger than Mercury, to tiny Cyllene, only 3km in diameter. They also vary in character: Icy Europa may have a great warm ocean beneath its surface which harbours life, while Io is a world of magma and volcanoes. Probably only 4 - the Galilean moons of Ganymede, Europa, Io and Callisto - are "settle-able" in the SF sense, but think of the potential in the 58 others. Secret bases for space pirates. Impossibly ancient and haunted alien monuments. Mines to be warred over. Space stations right and left. Orbital rigs for siphoning gas from Jupiter itself, built on construction bases bigger than cities, that circle elliptically around Callisto.
The thing about the Moons of Jupiter Planetcrawl is that the distances make sense. These moons are generally about 200,000 km apart at their closest (although of course, at the opposite ends of their ellipses, they are a heck of a lot further), which is traversable in a few hours at the speed of, say, Voyager 1. An inter-lunar craft would probably be a lot slower, because Voyager 1 is a big fat cheat and has the benefit of decades of acceleration, but still - it's the sort of travel that is do-able in the Star Trek "set a course for Ganymede and we'll be there by Friday" sense, and is imaginable without too much handwaving if you want your SF hard. And since your scope is somewhat restricted, you can actually flesh out your locations in a lot of detail - you don't end up with Traveller-esque abstraction in which each solar system has a single interesting planet about which there is a single interesting fact and a single culture. You can go deep: what's going on with Callisto? How does that affect what's going on with Io? What's with the planetary war on Europa and will the Jovian orbiters get involved?
I happen to have recently bought Diaspora. Maybe the planets are aligning.