Change 50 in line 35 to 100.
  
  
            double distance = Math.hypot(x1, y1);
Actor missile = new Missile(); getWorld().addObject(missile, getX(), getY()); missile.setRotation(getRotation());
import greenfoot.*;
public class Background01 extends World
{
    private int scrollSpeed = 2;
    private int picHeight;
    private GreenfootImage bgImage;
    private int scrollPosition;
    GreenfootSound bgMusic;
    
    public Background01()
    {    
        super(900, 600, 1);
        bgImage = new GreenfootImage(getBackground());
        picHeight = bgImage.getHeight();
        Greenfoot.start();
        bgMusic = new GreenfootSound("Star Wars.mp3"); 
        bgMusic.play();
    }
    
    public void act()
    {        
        scrollPosition -= scrollSpeed;
        while(scrollSpeed > 0 && scrollPosition < -picHeight) scrollPosition += picHeight;
        while(scrollSpeed < 0 && scrollPosition > 0) scrollPosition -= picHeight;
        paint(scrollPosition);       
        if (!bgMusic.isPlaying()) Greenfoot.setWorld(new Background02());
    }
    
    private void paint(int position)
    {
        GreenfootImage bg = getBackground();
        bg.drawImage(bgImage, 0, position);
        bg.drawImage(bgImage, 0, position + picHeight);
    }
}