Also known as the "Asteroid Belt Zone 1", while the Vic Viper was flying through outer space, it landed in an asteroid belt, and has to navigate past it while various enemies try to shoot it down. Eventually, you run into Tetran, and after defeating it, the game goes back to being horizontal for the third stage.
In Life Force, the background has been changed to make it appear as organic, and the enemies are changed into organic beings instead of mechanical ones as well. Instead of asteroids crashing down, chunks of pink growth will fall. And finally, the boss of the stage, Tetran, has of course, been converted into an organic being, known as Zylon.