Simply shoot the green asteroid in Zone 1, Level 15. You will unlock the original Asteroids mini-game.

Codes

Cheat Mode - During the game, press pause. Then hold L and press C-Left, C-Left, C-Right, C-Right, B, A, C-Up, C-Up, C-Down, C-Down, Start.

Original Asteroids, Credits, Relentless Mode - At the main menu, hold L and press C-Left, C-Right, C-Up, C-Down, A, C-Up, C-Down, C-Left, C-Right, Start.