Shooting length

 var projectile : Transform;
 
 var bulletSpeed : float = 100;
 
 function Update () 
 {
     // Put this in your update function
     if (Input.GetMouseButtonDown(0)) {
 
     // Instantiate the projectile at the position and rotation of this transform
     var clone : Transform;
     clone = Instantiate(projectile, GameObject.Find("spawnPoint").transform.position, transform.rotation);
 
     // Add force to the cloned object in the object's forward direction
     clone.rigidbody.AddForce(clone.transform.forward * 1000);
     }
     Destroy(clone.gameObject, 5);
     }

Okay, so what this code does is generates a clone of my fireball (projectile) when the user has clicked on the LMB. However when it instantiates, it goes forward by 10m and then falls to the ground. How do I make it so that when it's created it goes in a straight line (like a raycast). I've set it to destroy after 5 seconds so it should disappear in the air.

So the real question is, how do I make it stay in the air?

Look forward to your answers.