how do you assign a prefab to a script.

i dragged it from the project window, the prefab shouldn't be in the Hierarchy window, at least that's what my school course said for this assignment.

public class ShipBehaviour : $$anonymous$$onoBehaviour {

 private float speed = 0.03F;

 public Text scoreText;

 public Text messageText;

 private int score = 0;

private bool gameOver;

 Teleporter porter;

 
 public GameObject laserPrefab;

 
 public GameObject asteroidPrefab;

   
 void Start()
 {
     
     porter = new Teleporter(Camera.main);

     // reset score text
     score = 0;
     scoreText.text = "0";

     
     createAsteroid();
     createAsteroid();
     createAsteroid();

     
     InvokeRepeating("createAsteroid", 1.0F, 3.0F);

 }

 
 void Update()
 {
     
     if (!gameOver)
     {
         
         transform.Translate(speed, 0, 0);

         
         float rotation = Input.GetAxis("Horizontal") * 5.0F;

         
         transform.Rotate(0, 0, -rotation);

         
         porter.Wrap(gameObject);

         
         if (Input.Get$$anonymous$$eyDown("space"))
         {
             
             fireShot();
         }
     }
 }

 void OnTriggerEnter2D(Collider2D otherObject)
 {
     
     string otherTag = otherObject.gameObject.tag;

     
     if (otherTag.Equals("asteroid"))
     {
         
         gameOver = true;
         messageText.text = "Game Over";
     }
 }

 
 public void IncreaseScore()
 {
     score++;
     scoreText.text = score.ToString();
 }

 
 void createAsteroid()
 {
     
     if (!gameOver)
     {
         
         float worldHeight = Camera.main.orthographicSize * 2.0F;
         float worldWidth = worldHeight * Camera.main.aspect;

         
         Vector2 position;
         position.x = Random.value * worldWidth - worldWidth / 2.0F;
         position.y = worldHeight / 2.0F;  

         
         Instantiate(asteroidPrefab, position, Quaternion.identity);
     }
 }

 
 void fireShot()
 {
     
     GameObject shot = (GameObject)Instantiate(laserPrefab, transform.position, transform.rotation);

     
     StartCoroutine(scheduleDestroy(1.0F, shot));
 }

 
 IEnumerator scheduleDestroy(float delay, GameObject target)
 {
     yield return new WaitForSeconds(delay);

     Destroy(target);
 }

}