Astroids are typically made up of debris. A few more ashes would hardly be distinguishable from the rest of the rubble.
And no. You can use ash to signify a plural when using it in a mass noun form. takn from Oxford Dictionary of English:
Ash -n 1.[mass noun]( also ashes ) The powdery residue left after the burning of a substance.
You are welcome for todays grammer lesson.
