You'll use a few scripts for your asteroids, so to keep organized it's a good idea to make an "Asteroids" folder. Now, you will need 3 new C# scripts: CreateAsteroids, MoveAsteroid, and DestroyAsteroid.

Attach the "CreateAsteroids" script to "CreateAsteroids" and add this code:

public GameObject asteroid;

void Update()
{
 Vector3 createPosition = Vector3.zero;
 Instantiate(asteroid, createPosition, asteroid.transform.rotation)
}

To understand what's happening here you need to know what instantiate means. Instantiating something is like building something from plans or instructions. If you're baking a cake, the cake is the instance and the recipe is the Instantiate() function. In the game world, your cake is instead a GameObject!

Drag the Asteroid prefab form the "Prefabs" folder and drop it into the "asteroid" box for your "CreateAsteroids" script in the Inspector for your "Asteroids" object. Try running your game.

WOAH! That was a lot of asteroids being created!The Update() function happens really fast so a lot of asteroids get created. You can control how fast the asteroids are created with the InvokeRepeating() function. Add this to your previous code:

 public float creationTime = 1f;

 // Use this for initialization
 void Start()
 {
 // 0f is when to start invoking repeat
 InvokeRepeating("createAsteroid", 0f, creationTime);
 }

Now change Update() to createAsteroid() and put the "asteroid" prefab into the the script's "asteroid" slot in the "Asteroids" object Inspector.

Try running the code now, it should create asteroids much slower.

If you create too many objects, your computer wont be able to keep track of them all. So when you create an asteroid you need to make sure it is destroyed. Lets use the Destory() function in the "DestoryAsteroids" script:

Attach the "DestroyAsteroids" to the Asteroid prefab. Add Destroy(gameObject, 10f); to the Start() function.

gameObject is the object the script is attached to. 10f destroys the asteroid after ten seconds.

Lets make your asteroids move! You'll use the "MoveAsteroids" script. Add this above Start ():

public Rigidbody rb;
public float asteroidSpeed;

and this into Start(): rb.velocity = -(transform.up) * asteroidSpeed

You changed the Rigidbody's velocity (the speed) property. -(transform.up) is the direction to move.

The final step is attaching the asteroid prefab's Rigidbody to the "MoveAsteroid" script and set the value of asteroidSpeed to 2.

Lets make it more fun by creating asteroids at different. You can make a function that returns a random position to do this! Add this function to the "CreateAsteroids" script:

 Vector3 getRandomPosition()
 {
     float xPos = Random.Range(.05f, .95f);
     Vector3 randomPosition = Camera.main.ViewportToWorldPoint(new Vector3(xPos, 1.1f, 15f));
     return randomPosition;
 }

Putting Vector3 instead of void in front of a function declaration means that the function will return a Vector3 object.

Random.Range(.05f, .95f) returns a random number between the two numbers given in the parameters (a parameter is anything in the parenthesis following a function).

The camera's viewport dimensions are 1 by 1 (the bottom left being (0,0) and the top right being (1,1)).

You then create a Vector3 to return called randomPosition. and set the randomPosition to (our randomly generated x position, a y position that creates the asteroids above the screen, and the z position that is level with your player object).

Finally, change Vector3 createPosition; to Vector3 createPosition = getRandomPosition();.

Try the game out!