Asteroids in a belt have a large distance between them, but I'd imagine rubble from a planet or moon recently destroyed by the empire would probably be grouped a lot more tightly.
Some grenades can have their pins pulled with teeth, but it's a dumb idea.
Presumably would actually just reform back into a planet since if you blow up a planet the mass is still there, it has just being fractured. If you leave it a couple of years it'll form back into a planet again.
This is probably what happened with the ice moon of Europa.
I'm not an astronomist, so my understanding of the definition of moon versus asteroids is fuzzy at best. That being said, as I understand it, there are several objects that aren't visible to the naked eye that would qualify as moons, but I may be mistaken. As I understand it we have 1 moon that everyone can see, another 12-16 moon like objects that aren't visible, and several hundred asteroids. All of these objects orbit The Earth, so they count as satellites of The Earth.
"We've come out of hyperspace into a meteor shower, some kind of asteroid collision."
Han didn't consider it to be an asteroid field, it wasn't named as such. It was smaller debris.
The asteroid field in Empire Strikes Back isn't given an origin on-screen, it's just there. It's obviously been there for quite a while, though. It's got native megafauna living in it.
I checked Wookieepedia and there's Legends material that establishes a variety of different explanations for the asteroids, but they're all natural and all happened in the ancient past.