There are three different endings depending on how many times you beat this game non-stop. No such thing as Expert Mode. Only Easy and Normal(Hard). Beat the game and your back at NATOLS, keep going for more endings.
How to Unlock
Beat the Game Once
Beat the Game Thrice
Beat the Game Twice
2nd Best Ending
level four weapon power-up
Though very difficult to acheive it is possible to obtain a level four power up for the zig fighter. First use the zig's tractor beam to pull in a smart bomb, hold this bomb in your tractor beam until another item ship carrying another bomb is on screen, use the bomb you are carrying to destroy that ship, the item it will drop should be the level four power up.
Pause the game, then press C, Up, B, Down, A, Left, Right, B, C, C, Right, Left, Right, A, Down, Start. Here you can change some options, choose any level or view all the endings.