Cheat mode
Pause the game, then hold L and press C-Left(2), C-Right(2), B, A, C-Up(2), C-Down(2), Start.

Original Asteroids, credits, and 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.