gistfile1.cs

// this needs to go into the asteroid's update function.
// It might be prudent to store the gameHeight and gameWidth variables
// in the GameManager script and assign them to variables in this
// script on awake.

// I'm neglecting to add any initialisation here but I'm sure you know what to do 
aWidth = GameManager.instance.gameWidth
aHeight = GameManager.instance.gameHeight

// inside the update function
transform.position = Vector3( Mathf.Repeat(myTransform.position.x, aWidth), Mathf.Repeat(myTransform.position.y, aHeight), 0) ;