Newbie... Instantiating a prefab?

Here is my code in my GameMaster script to create an asteroid in my game every second:

var NextAsteroidTimer = 0.0;

var aster : transform;

function Update () {

NextAsteroidTimer += Time.deltaTime;
if (NextAsteroidTimer > 1)
{
    NextAsteroidTimer = 0.0;
    var asteroid = Instantiate(aster, Vector3(0,0,0), Quaternion.identity);
}

}

But I am not understanding the 'var aster : transform' along with 'aster' being part of the Instantiate statement. Somehow 'aster' has to refer to my prefab which is in my project view as 'prefab_asteroid'. What are the additional step/steps I need to do? I am not getting clarity on what I am reading in the Unity documentation and searching this board and the community board.

Thanks for any help.

Capitalise your 'transform' variable type (updated code below). Then, find the GameObject that this script is attached to, and there will be a field that says "aster" and the value will be "None (Transform)". Drag your prefab_asteroid object onto that field and the link will be complete. Then your game should spawn an asteroid every second.

var aster : Transform;
var NextAsteroidTimer : float = 0.0;

function Update ()
{
   NextAsteroidTimer += Time.deltaTime;
   if (NextAsteroidTimer > 1)
   {
      NextAsteroidTimer = 0.0;
      var asteroid = Instantiate(aster, Vector3(0,0,0), Quaternion.identity);
   }
}