Unable to randomly instantiate prefab from array

New to Unity and programming in general. Looking for a C# solution please.

I am trying to spawn an object that is selected at random from a group of objects (asteroids in this case), however, nothing is spawning in Unity when I run the game. Within the Unity interface I have set the prefabs in the array. There are no errors or warnings. I have similar scripts that instantiate a single object repeatedly with no trouble at all.

I have spent many hours trying to figure this out (including scouring the net for an answer) and I have made no headway. Please help if you can.

The full code I am using for this script: ` using System.Collections; using System.Collections.Generic;

     public class AsteroidSpawnerScript : MonoBehaviour 
     {
         public float numAsteroids;
         public float xMin = 15f;
         public float xMax = 30f;
         public float yMin = 3.5f;
         public float yMax = -4.5f;    
         public GameObject[] asteroidGroup = new GameObject[7];
 
         
         void start()
         {
             InvokeRepeating ("SpawnAsteroid", 1, 5f);
         }
         
         void SpawnAsteroid() 
         {    
             GameObject newParent = GameObject.Find ("L2 Enviro Elements");
             for (int i = 0; i < numAsteroids; i++)
             {
                 Vector3 newPos = new Vector3 (Random.Range (xMin, xMax), Random.Range (yMin, yMax), 0);
                 GameObject asteroidMG = Instantiate (asteroidGroup[Random.Range (0,6)], newPos, Quaternion.identity) as GameObject;
                 asteroidMG.transform.parent = newParent.transform;
             }
         }    
     }

What have I done wrong here? I would be in better shape if Unity would give me an error, but being so new to programming I don't really know what to change.