To prevent the aliens from returning fire, allow all of the aliens to appear on the screen before you fire a shot a them, then shoot all of them apart from the first two (yellow/blue colour) on the left hand side of the screen. Put the kettle on and make a cup of coffee while the two remaining ships are left to fly around for fifteen minutes. When you are certain that they have stopped firing at you then shoot them. All other aliens will now refrain from firing back at you for the rest of the game.