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);

An easy way to make this happen is to use Viewport coordinates. Viewport coordinates go from 0.0 in the lower left, to 1.0 in the upper right. So you can use a ‘x’ value of line 1.1 for off screen creation. So you would do something like this:

Vector3 pos = Vector3(1.1, Random.value, someValue);
pos = Camera.main.ViewportToWorldPoint(pos);

‘someValue’ is the distance in front of the camera to spawn the object. Note I’m a bit confused by your code now since it spawns at random on the ‘x’ axis but has a fixed ‘y’ and ‘z’ position for spawning.