Just writing this more for myself than anything...
Thought of a "health bar" which would be the count of the global population.
On easy mode missing an asteroid will kill say half a billion people (there's 7 billion people, so effictively 14 lives), on hard a billion (7 misses aloud before game over and total annihlation)
scoring i guess would be multiplied with higher difficulties.
finishing the stage/song would yield a bonus based on remaining population of the world.
was giving thought to what the 8 markers could be. I thought that they could either be a nuclear missle launch area (would need a missle launch animation coming from the earths surface) or have it as some form of satelite microwave grid which superheats the water inside the asteroids causing them to explode. the explanation of the latter would be that the earth only has enough spare power to activate one microwave grid at once for a split second which would add continuity to the reason for the zones. the former would simply be down to deployment time, and accuracy. too close and the fallout and objects wouldn't burn up... to far and the nuke would miss or only partially destroy the asteroid hence the reasons for perfect timing.
just some more clarification on controls. 8 direction joystick. point the joystick in desired direction which highlights the zone (none highlighted when joystick not pointed) then press fire to activate nuke/microwave array (indicator briefly turns orange for visual confirmation).