I'm a bit late to this party (welcome, Lindsey!), but Josh's notes on asteroids got me thinking.
1. Can asteroids perturbed by outside forces hit each other? Should they? What effects should that produce that would be fun/appropriate for this game?
2. If asteroids can move, then even if they're affected by Magical Space Drag

eventually they will disperse from their original volume.
Is some kind of shepherding feature needed at the edges of a field-containing-asteroids that gently corrals them back into the field's bounding volume?
3. Simulation of gravity from solid objects (or fields) at asteroid size on up. Want. Don't expect to get. Still want.
