Additional asteroids

What used to be a free sample of lava rocks from the home improvement store soon will be an asteroid field for my starship games.  They just need priming and drybrushing.  Here's a comparison shot with an Avarian cruiser, so you can see how they'll look in a game. 

Speaking of playing, what kind of rules do you have for space rocks in your battles?  Do they block line of sight?  Can they hit a ship (or vice versa)?  Or are they just there for decoration?