The game works fine, movement is a bit awkward, and having to run into things with a shield is awkward as a consequence. Decreasing the maximum speed would be nice.
In some instances death is swift, as asteroids can be generated on top of the ship.
Playing the game with a slow moving ship, makes the shield feel rather odd, as collisions have to be very direct rather than just passing over the shield area.
In terms of tactics, directly ramming into an asteroid is the most efficient way of dealing with it and it's children. One good shielded ram, can take out an asteroid and all it's children in one fell swoop. I would suggest making child asteroids spawn further away from the parent, if you want to increase difficulty.
If levels began with asteroids of different sizes that would be a neat touch.
Also, fun fact: Asteroids are made mostly of rock, whereas comets are made more of ice.