Cheat Mode
Pause during a game, then hold L and press C-Left (x2), C-Right (x2), B, A, C-Up (x2), C-Down (x2), Start.
Original Asteroids, Credits, & Relentless Mode
From 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.
Original Asteroids
Shoot the green object in Zone 1, Level 15 to play the original Asteroids game.