Asteroids Hyper 64

During the game press start, 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, and relentless mode:
When you are 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.