How to kill an object when I shoot bullets

Hello. I'm new here. I want someone to help me solve something. I have made a game like asteroids hitting and my problem is that when I shoot the asteroid with my spaceship is not destroyed. How can I make it work? I have prefabs (asteroid and bullets also). The code I have for bullets is the following:

 #pragma strict
 
 var speed : float = 20;
 var destroyobj: float = 70;    
 
 function Update () 
 {
     transform.Translate(0,speed,0, Space.World);
     
     Destroy (gameObject,destroyobj* Time.deltaTime);
 }

and this is the code for the asteroid:

 #pragma strict
 
 var speed : float = 50;
 
 function Update () 
 {
     transform.Translate(Vector3.down * speed, Space.World);
 
     if (transform.position.y <= -4528)
     {
         resetEnemy();
     }
 }
 
 function resetEnemy ()
 {
     transform.position.y = 1400;
     transform.position.x = Random.Range (-3100, 3117);
 }

I have try different code from google but I can't understand why this does not work.