Move rotating object

I have a ball which rotates around the point 0,0,0 in the Z-axis. When the space button is pressed, the ball has to go inside the large circle. Now my code looks like this. When you press space, the ball does not behave as they should. I want to know how to make a balloon down exactly down

that's how the ball should behave

 void Update () {
         if (Input.GetKeyDown (KeyCode.Space)) {
             transform.position = new Vector3 (transform.position.x - 1, transform.position.y - 1, 0);
         } else {
             transform.RotateAround(new Vector3(0,0,0), new Vector3(0,0,1), 2);
         }
 }