Terminator

Your planet is being attacked by an alien force which are more or less invincible. Instead of sending thousands to their deaths to try to destroy it, the solution is to send a fighter pilot to the aliens planets and destroy them to force the enemy into submission. You have been selected for this task and you must destroy a certain amount of alien ships as you fly over the surface of each planet. When you have killed enough then the planet turns a different colour and you fly over the surface again. Once enough aliens have been killed again then the planet is destroyed.