In live, when an asteroid first opened, it began a timer, whether or not anything at all was removed from it. If the timer expired before the asteroid was emptied (it was fairly long, not "empty this thing in 30 seconds or BOOM!"), the asteroid poofed as if it had been emptied. I don't recall if this released a pop-rock mob when it happened, because I always cleaned a field out completely when I prospected if I wasn't driven away by a something I couldn't handle at the time.
This would work equally well if not better for hulks (oops, you disturbed the wreckage, setting off a chain reaction that will eventually lead to its complete destruction).
Cheers.