Time for action – checking the asteroid's position

1. Add the isOnScreen() helper method to the AsteroidManager class:

   Private Function isOnScreen(asteroid As Sprite) As Boolean
       If (asteroid.Destination.Intersects(
               new Rectangle(
                   -screenPadding, 
                   -screenPadding,
                   ScreenWidth + screenPadding, 
                   screenHeight + screenPadding)
                   )
               ) Then
           Return True
       Else
           Return False
       End If
   End Function

What just happened?

The isOnScreen() method checks the passed asteroid to determine if its destination rectangle (where the sprite would be drawn to the screen) intersects a rectangle, generated by expanding the screen by screenPadding pixels in all directions. This padding allows the asteroid to move several pixels off the screen, before the game determines that it has actually left the playfield.