So: Asteroids placed in specific positions via FRED cannot be destroyed or collided with. Asteroids generated by the asteroid field editor cannot be used in SEXPs - e.g. in an 'is_destroyed_delay' SEXP.
Is there a simple way that I'm not aware of to make a lone asteroid destroyable. Alternately, is there a way to count the number of asteroids destroyed if they're generated by the asteroid field editor?
EDIT: Shortly after writing this, I decided to just take the "no collide" flag out of the table, but an answer to one of the above questions would be best interesting.