Do this:
function Update(){
transform.position += transform.TransformDirection(Vector3.right)*moveSpeed;
transform.LookAt(spherePosition);
}
This makes your character move along its x-axis while always pointing at the middle of the sphere.
http://unity3d.com/support/documentation/ScriptReference/Transform.LookAt.html
http://unity3d.com/support/documentation/ScriptReference/Transform.TransformDirection.html