A couple of gas clouds or an asteroid field is vital in a game to provide not just cover, but to mask of areas of the board, allow for tactical movement and to out position your oponent. With no debris you are looking at two fleets lined up opposite each other blasting away as thaey both advance, fly past, turn around, and do it again, by which time the game is over.
3 medium areas of terrain, say 6x4 inches or 8x6 inches should be enough, depending on the size of your table, to provide you with enough dead zones and reduced lines of sight to provide an interesting game with more of a tactical challenge