If you're running it using miniatures and a battlemat, do the wraparound map trick - everything that goes off one side comes on the other. Use the excuse of it having its own center of gravity to explain why missiles and spells don't go flying off into the void, so can come looping back around to literally get the PCs in the arse. Set up several small maps using this trick, and then have them draw close during the fight, so people can jump from one to the other. Possibly limit lines of sight so to represent the curvature of the asteroids.
That's on the outside of the asteroids of course. Inside, if you want complication, make it so it's constantly rotating, and at any given moment (or at start of the turn), everything can suddenly move 90 degrees or so, make the floor and roof the walls, and the walls the roof and floor. Anyone native to inside the asteroid would have everything fastened down, and have some sort of climb speed to negate the worst of it.
Have a zero g fight. This easier than it sounds - everyone simply has a flight speed (with hover) and everything that has a physical or force damage type automatically has a push effect. Watch PCs and enemies go flying around like mad.