The player's ship appears in the middle of the screen, with 4 large asteroids drifting around. Each large asteroid (20 points) breaks into 2 medium-sized ones (50 points) when shot, which in turn breaks into 2 small (100 points) asteroids. The medium and small asteroids can travel at widely varying speeds. Periodically one of two types of flying saucers ("UFOs") will fly onto the screen: the big one (worth 200 points) shoots in random directions, while the small one (1000 points) attempts to aim at the player. They tend to appear more often when few asteroids remained on the screen and/or the player hasn't shot an asteroid recently.
The screen wraps around, allowing the player's ship, as well as asteroids and shots but not saucers, to fly off the one edge of the screen and reappear on the opposite side. Once a level has been cleared of all asteroids and UFOs, a new set of large asteroids would appear, increasing by 2 each round up to a maximum of 12.
The maximum score possible is 99,990 points, after which it turns back over to zero. A player who desires to get onto the top score list must be careful to shoot just enough asteroids/UFOs to reach this score without going over (including committing suicide with the last ship left to reach the final total!).
On some early versions of the game it was possible to hide the ship
in the score area indefinitely without being hit by asteroids.