Movement problem?

I have two spheres.One large one small. The small one is on the large one. Now I am moving the small sphere on the large force. But I can't move it the way I want it too.

The Gravity on the sphere:

 rigidbody.velocity = (-transform.position + planet.transform.position) * gravity ;

The Movement Script:

 if(Input.GetKey(KeyCode.A))
         {
             rigidbody.AddForce(Vector3.right * speed);
             
         }
         if(Input.GetKey(KeyCode.S))
         {
             rigidbody.AddForce(Vector3.down * speed);
             
         }
         if(Input.GetKey(KeyCode.D))
         {
             rigidbody.AddForce(Vector3.left * speed);
             
         }
         if(Input.GetKey(KeyCode.W))
         {
             rigidbody.AddForce(Vector3.forward * speed);
             
         }

The placement of forces may be wrong.But in order to make the small rotate around the large one my keys change some way. Like first was "S" , then "A" and then "D".