Missle Distry after it reaches distance

I have the following codes making my missles launch from my aircraft and it all works. My problem lies in the fact that when the bullets fire and if it dont hit anything it never disapears. I would like it to self destroy after it reaches a decent amout of distance from the ship.

This is the missle launcher:

var missile : GameObject;

function Update () {
	if (Input.GetMouseButtonDown (0))
	{
		var position : Vector3 = new Vector3(0.004, -.001, 0) * 10.0;
		position = transform.TransformPoint (position);
		var thisMissile : GameObject = Instantiate (missile, position, transform.rotation) as GameObject;
		Physics.IgnoreCollision(thisMissile.collider, collider);
	}
	
	
}

This is for theTrajectory:

var explosion : GameObject;

function FixedUpdate () {
	rigidbody.AddForce (transform.TransformDirection (Vector3.forward) * 200.0);
}

function OnCollisionEnter(collision : Collision) {
	var contact : ContactPoint = collision.contacts[0];
	var thisExplosion : GameObject = Instantiate (explosion, contact.point + (contact.normal * 5.0) , Quaternion.identity);
	
	if (collision.gameObject.tag == "enemy")
	{
		Destroy (collision.gameObject);
	}
	
	Destroy (thisExplosion, 2.0);
	Destroy (gameObject);
}

The easiest way is to give the missile a life-span as soon as it is Instantiated

function Start()
{
    Destroy( gameObject, 4.0 );
}

then 4 seconds after it appears , Destroy (or give it long enough to fly far enough into the distance).

the method you’re asking for is to check the distance. For this you can use Vector3.Distance : Unity - Scripting API: Vector3.Distance

function Start()
{
    InvokeRepeating( "CheckDistance", 0.5, 0.2 );
}

function CheckDistance()
{
    if ( Vector3.Distance( player.transform.position, transform.position ) > 20.0 )
    {
        Destroy( gameObject );
    }
}