How to make a Prefab move in the X and Z directions while also orbiting in a circular pattern?

I have C# experience but it's my first project in Unity. I'm making a 3D top-down Galaga-style space shooter. When I spawn in an "alien" it rotates in a circular pattern in the X and Z directions (I'm using Width and Height to make the orbit path the shape I want it to be), but it doesn't move around the screen in the X and Z directions while also orbiting like I want it to. It just spawns center screen and orbits the center position.

I used AddForce in the Start method to try to make this happen but no luck. Here's a portion of the code with which I can get the object to orbit, but not move around at the same time:

 private void Start()
 {
     Rigidbody rigidBody = GetComponent<Rigidbody>();
     rigidBody.AddForce(StartForce, ForceMode.Impulse);
 }

 void Update()
 {
     TimeCounter += (Time.deltaTime * Speed);
     XCoord = Mathf.Cos(TimeCounter) * Width;
     ZCoord = Mathf.Sin(TimeCounter) * Height;

     transform.position = new Vector3(XCoord, YCoord, ZCoord);
 }