Custom Asteroids I created for ship to ship battles in SW Saga RPG / miniatures.
I used chunks of high density blue foam 1.5 to 2 inch thickness. I carved or broke them apart to a shape I liked. Then I took small needle nose pliers and pulled chucks off of them to make a pocked look. Then you use black spray paint and spray the crap out of them. Normally I don't use spray paint on my terrain unless I coat it good with a brush on paint so you don't melt the foam. But in this case its spray the heck out them and melt away. The propellant causes the foam to melt and develop more little bubbles to give it a tiny pocked mark appearance on the majority of the little asteroid. I then used some grey spray paint to create the main color. I'm sure one could then use a dry-brush method to add even more detail. I just need to make one or two with a little hidden pirate base on the surface or a smuggler hide-out.