How to destroy an instantiated object

I'm trying to make the asteroid destroy itself after the player gets too far

{

 public Transform transform;
 public Rigidbody2D rb;
 public Vector2 randomVector;
 float rotationForce;
 GameObject Player;
 private Transform playerTransform;
 GameObject Asteroid_PreFab;
 GameObject LogicMaster;
 private logic_Master logicMaster;

 void Awake()
 {
     Player = GameObject.Find("Player");
     Asteroid_PreFab = GameObject.Find("Asteroid_PreFab");
     LogicMaster = GameObject.Find("LogicMaster");

     playerTransform = Player.GetComponent<Transform>();
     logicMaster = LogicMaster.GetComponent <logic_Master> ();

 }
 
 // Use this for initialization
 void Start () {

     Vector2 randomVector = new Vector2(Random.Range(-3,3), Random.Range(-3,3));

     Debug.Log(randomVector + "" + randomVector.magnitude);

     rotationForce = Random.Range(-1, 1);

     float scaleFactor = Random.Range(0.5f, 2f);
     Vector2 scaleVector = new Vector2(scaleFactor, scaleFactor);
     transform.localScale = scaleVector;

     rb.velocity = rb.velocity + randomVector;

     rb.AddTorque(rotationForce, ForceMode2D.Impulse);
     
 }
 
 // Update is called once per frame
 void Update()
 {
     Vector2 playerDistance = new Vector2(transform.position.x - playerTransform.position.x, transform.position.y - playerTransform.position.y);

     if (playerDistance.magnitude > 30 )
     {
         Debug.Log("true");
         Destroy(Asteroid_PreFab);
     }
 }

} At the very end when the magnitude exceeds 30, its outputting true as per debug.log, but is not destroying the asteroid.