Same era. Missile Command. Yes, I remember. Maybe there is a black hole that asteroids enter if you they are on the path. As the count builds up and gets critical, you have to enter it which brings you to a new gameplay mode that shows you the asteroids raining down on the planet that you defend much closer to home (missile command). So the story line is, intercept the asteroids in deep space, but if they get too close, defend the planet as the last resort. Clear the asteroids and re-enter the portal to go back into deep space to defend from further away.