Spawning Enemies off the screen w/ scrolling background

I am working on a 2D sidescroller, the player is on the left and I want to asteroids to spawn off screen on the right. I have script for a scrolling background and a script for random spawning, but I cannot figure out a way to make it so that the asteroids continuously spawn off screen. using UnityEngine; using System.Collections;

Spawning Script

 public class GameController : MonoBehaviour {
 
     public GameObject hazard;
     public Vector3 spawnValues;
     public int hazardCount;
     public float spawnWait;
     public float startWait;
     public float waveWait;
     
     void Start ()
     {
         StartCoroutine (SpawnWaves ());
     }
     
     IEnumerator SpawnWaves ()
     {
         yield return new WaitForSeconds (startWait);
         while (true)
         {
             for (int i = 0; i < hazardCount; i++)
             {
                 Vector3 spawnPosition = new Vector3 (Random.Range (-spawnValues.x, spawnValues.x), spawnValues.y, spawnValues.z);
                 Quaternion spawnRotation = Quaternion.identity;
                 Instantiate (hazard, spawnPosition, spawnRotation);
                 yield return new WaitForSeconds (spawnWait);
             }
             yield return new WaitForSeconds (waveWait);
         }
     }
 }