The game is unusual in the way any enemies or comet debris not destroyed on the journey make it to earth and do some damage there. Comets do more damage than the enemy ships do. If Earth's damage level reaches 100% the game ends. For every 2,000 points scored, 1% of Earth's damage is repaired, continuing also resets the damage to 0 but the player loses their score and weapon powerups. There are three difficulty settings: Easy, Medium and Hard with each progressively having more, new and tougher enemies.