Rotate Around and Look At Problem

I'm using the following code to get prototype for moving an object around a spherical object and keeping the object perpendicular to the sphere.

Code (csharp):

1.  
2. var speed = 100.0;
3. var planet : Transform;
4.  
5. function Update () {
6.     var x = Input.GetAxis("Horizontal") * Time.deltaTime * speed *-1;
7.     transform.RotateAround(Vector3(0, -512, 0), Vector3.forward, x);
8.     transform.LookAt(planet);
9. }
10.  

The result is sort of working. I see the following behavior when the object orbiting crosses the halfway point of the sphere. It rotates 180 on the z axis.