NullReferenceException: Object reference not set to an instance of an object script_bullet.OnTriggerEnter (UnityEngine.Collider other) (at Assets/script_bullet.js:35)

I am getting an Null Reference Exception when trying to call CreateAsteroid() (which is inside my script_asteroid file) from within script_bullet. I am not exactly sure how I should be calling the asteroid. Can somebody show me what I am doing wrong? Thanks in advance.

 //Bullet Script
 #pragma strict
 
 //Inspector Variables
 
 var bulletSpeed         : float = 40.0;                //Bullet Speed
 var bulletBounds        : float = 15;                //Bullet Boundry 
 var asteroid             : script_asteroid;
 asteroid                 = GetComponent(script_asteroid);
 var explosion            : Transform;
 
 
 function Start () {
 
 }
 
 function Update () {
     var moveBullet        : float = bulletSpeed * Time.deltaTime;
     
     //Moves Player based on stored input data
     transform.Translate(moveBullet, 0,0);
     
     if(transform.position.x >= bulletBounds)
         Destroy(gameObject);
 }
 
 function OnTriggerEnter (other : Collider) 
 {
     //Print what the bullet is colliding with
     print("bullet collison: " + other.gameObject.tag);
     
     //Check for collision with asteroid
     if(other.gameObject.tag == "Asteroid")
     {
         asteroid.CreateAsteroid();
         
         //Create the explosion on impact
         Instantiate(explosion,transform.position,transform.rotation);
         Destroy(other.gameObject);
         Destroy(gameObject);
     }
     
 }
 //Player Script
 #pragma strict
 
 //Inspector Variables
 
 var asteroidSpeedHMin         : float = 1.0;                //Asteroid Horizontal Speed Min
 var asteroidSpeedHMax         : float = 6.0;                //Asteroid Horizontal Speed Max
 var asteroidSpeedV             : float = 10.0;                //Asteroid Vertical Speed
 var asteroidBoundsH            : float = 15;                //Limits for Asteroid movement Horizontal
 var asteroidBoundsV            : float = 10;                //Limits for Asteroid movement Vertical
 var asteroid                    : Transform;
 var asteroidHealth            : int    = 3;
 
 //Private Variables
 private var asteroidPosition;
 private var asteroidRotation;
 private var transH;
 private var transV;
 
 function Start () {
     asteroidPosition = Vector3(transform.position.x,Random.Range(-8,8),0);
     asteroidRotation = transform.rotation;
     transH             = -Random.Range(asteroidSpeedHMin,asteroidSpeedHMax)*Time.deltaTime;
     transV             = Random.Range(-asteroidSpeedV,asteroidSpeedV)*Time.deltaTime;
 }
 
 function Update () {
 
 
     //Moves Asteroid based on Random numbers
     transform.Translate(transH,transV,0);
     
     if(transform.position.x >= asteroidBoundsH || transform.position.x <= -asteroidBoundsH || transform.position.y >= asteroidBoundsV || transform.position.y <= -asteroidBoundsV) {
         CreateAsteroid();
         Destroy(gameObject);
     }
 }
 
 function CreateAsteroid() {
     Instantiate (asteroid, asteroidPosition, asteroidRotation);
 }