duplicating an object and its code

[undecillion]

Hi everyone, i was making a space shooting game, where you control
a horizontally moving ship and fire with the space bar. I want to get asteroids to come from the top of the screen and go down, and if they go past the bottom of the screen, you lose a life.

so far, I have one asteroid that does precisely what i want it to do, the code for the asteroid is as follows (entered in the main timeline) :

for (var i = 0; i<bulletCount; i++) {
        var b = bulletArray[i];
        if (b._y>0) {
            b._y -= bulletSpeed;
        } else {
            removeMovieClip(b);
            bulletArray.splice(i, 1);
        }
        if (!hit) {
            if (b.hitTest(ob)) {
                hit = true;
                ob.play();
                b.play();
                _root.score +=500;
                b._y -= bulletSpeed;
                bulletArray.splice(i, 1);
                removeMovieClip(b);
            }
        }
    }
}

var nrEnemies = 3;
for (i = 1; i < nrEnemies; i++)
{
    _root.ob.duplicateMovieClip("asteroid" + i, _root.getNextHighestDepth());
}

var hit = false;
var obSpeed = 5;
var earthSpeed = 1;
function createObject(o) {
    ob = this.attachMovie(o, o, 100);
    ob._x = random(460) + 20;
}
function resetObject() {
    ob._x = random(460)+20;
    ob._y = -40;
    ob.gotoAndStop(1);
    ob.restart = false;
    hit = false
    }
function loseLife() {
    earthMc._y = 360;
    }
function lostlife() {
    earthMc._y += earthSpeed;
}

function moveObject() {
    if (ob._y<400) {
        ob._y += obSpeed;
        _root.score +=1
        lostlife();
    } else {
        ob.play();
        resetObject();
        _root.lives -=1;
        loseLife();
        
    }
    if (ob.restart) {
        resetObject();
        }
    }
createObject("asteroid", "asteroid", (0));

my problem is that i only have one asteroid and the game is far too easy.
Is there a way to change the code or make a new one so that there will be more than one asteroid on the screen with the same properties?

any and all help is greatly appreciated.