Pause game play. Then, hold L and press C-Left(2), C-Right(2), B, A, C-Up(2), C-Down(2), Start.
Shoot the new, strange green object in Zone 1, Level 15 to unlock an selection to play the original game on the main menu. Note: Save the game configuration so it does not have to be unlocked again.
Hold L and press C-Left, C-Right, C-Up, C-Down, A, C-Up, C-Down, C-Left, C-Right, Start at the main menu.
Hold R and press C-Left, C-Up, C-Right, C-Down, B, C-Left, C-Down, C-Right, C-Up, Start at the main menu.
Successfully complete the game under normal mode to unlock relentless mode. In this mode, the asteroids will keep appearing. Note: Save the game configuration so it does not have to be unlocked again.