Rigidbody does not rotate along its trajectory [SOLVED]

I would like to shoot a missile, and I did this:

function Fire () {
       var clone : Rigidbody;
       clone = Instantiate(projectile, transform.position, transform.rotation);
       clone.velocity = transform.TransformDirection (new Vector2(speed, 0));

It works properly, but how can I do to rotate “clone” along its trajectory?


See if this helps:

function Fire () {
        var clone = Instantiate(projectile, transform.position,   transform.rotation) as Rigidbody;
        clone.rotation = Quaternion.LookRotation(clone.velocity);

Check here for a similar script I wrote and tested that works:

    var speed = 5.0f;
    var rotateSpeed:float ;
    var sphere:GameObject;
    var child:Transform;
    var ballPrefab:Rigidbody;
    var ballSpeed:float = 200;
    var spawnPoint:Transform; 
    function Update () {
            var ball = Instantiate(ballPrefab, spawnPoint.position,   spawnPoint.rotation) as Rigidbody;
            ball.rotation = Quaternion.LookRotation(ball.velocity);

Update: I solved it!
The scripts are all correct, but my sprite rotated on wrong axis, I created an Empty gameObject as a Father and then I have corrected the rotation, now it works!
Thanks at all for the support!