Nice idea, but I'd agree with dummie999 - it goes from slow to mayhem very abruptly. Also, maybe when the game restarts after you lose a life the screen should clear. Currently when the area of play gets full and you lose a life it restarts with the same chaos and imminent death. Can go from 3 lives to game over inside a second of game play.
Different levels? Could have different speeds of smaller asteroids as the game progresses to later levels. Smaller ones would get round the problem of the screen just being filled with a few massive blobs.
Could introduce some screen-clearing smart bombs, wildcard asteroids (that will annihilate any colour).
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