Creating Meteor with Physics (where the Particle System Works)

Hi, So you will have to explain the answer to me very basically as i am just a beginner. I created a meteor with working physics. But i can't get the particle system to work very well. So what I want to happen is create the cone shaped particle system at the point of the meteor in the direction that it is moving. I also want it so that when the meteor turns, the particles dont turn with it, just the newly created ones. You will need to break it down very slowly for me to understand. Here is what i already did:

     //Particle system is on the Sphere, which has the rigidbody
     //Im sure there is a problem with Parent objects and etc stuff
     //There is also error saying "Object reference not set to an instance of an object"
     Rigidbody rb;
     // Use this for initialization
     void Start () {
         Rigidbody rb = GetComponentInParent<Rigidbody>();
     }
     
     // Update is called once per frame
     void Update () {
         transform.position = rb.velocity.normalized * 1.25f; 
     }
 }

Beside that Thanks!