Star Battle is a clone of Galaxian written in 1981 by HAL Laboratories and published by Commodore.
This is a straight-up clone of Galaxian; you control a little ship that can move left and right across the bottom of the screen, and shoot upwards. A phalanx of alien ships moves slowly down from the top, in the particular pattern of such games; the formation moves right until it hits the screen edge, drops a bit, then moves left until it hits the screen edge, drops a bit, and so on. Whenever the formation loses ships, it increases in speed slightly. Countering this, if you can remove an entire column off the side of the phalanx, it must move further to reach the edge of the screen.
Occasionally, enemy ships peel out of formation and dive-bomb you. As the levels increase, this becomes more frequent. The main difference from Galaxian is that you can't capture enemy ships.
If the formation reaches the bottom of the screen, you lose. The enemy ships also fire shots downwards, and if you are hit by a shot you lose a life.
Places to Buy
Look for Star Battle on your favorite VIC-20 ROM site.