space shooter asteroids are invisible

When I play the game, the asteroids are invisible, even though the asteroid prefabs are getting added and removed from my hierarchy.

This is my Mover.cs script:

public class Mover : MonoBehaviour {

	private float acceleration = 1f;
	public float speed;
	public float maxSpeed = 10f;

	private Rigidbody rb;

	// Use this for initialization

	void Start () {
		rb = GetComponent<Rigidbody> ();
		rb.velocity = transform.forward * speed; // transform.forward represents foward movement along the z-axis
		Debug.Log ("asteroid is moving");

	// Update is called once per frame
	void Update () {
		// Add acceleration to speed
		speed += Time.deltaTime * acceleration;
		// Set object velocity
		rb.velocity = transform.forward * speed;
		// Check to make sure speed is under top speed
		if (speed > maxSpeed) {
			speed = maxSpeed;

Is there anything else that would be helpful for me to provide?

Why do you think this script has anything to do with visibility? Are there differences between Edit and Play mode? How are the prefabs instanciated? The information you provide is not sufficient to even understand what you are asking.