I just started with javascript and am working on a little game to wrap my head around it and came across an error I can't seem to fix.
Code:
 if (AsteriodXpos[i] < -50 || AsteriodYpos[i] < -100 || AsteriodYpos[i] > WinHeight + 100) {

                    AsteriodXpos[i] = WinWidth;

                    AsteriodYpos[i] = Math.round(Math.random() * WinHeight);

                    AsteriodXSpeed[i] = Math.random() * 5 + 7;

                    AsteriodYSpeed[i] = Math.random() * 7 - 5;
                    AstriodsAvoided += 1;
                    if (AstriodsAvoided % 15 == 0) {
                        document.write('<div style="position:absolute;top:0px;left:0px">');

                        document.write('<div style="position:relative">');
                        document.write('<div id="ast" style="position:absolute;font-size:2px"><img src="asteroid.png" height=50 width=50></div>');
                        document.write('</div>');
                        document.write('</div>');

                        AsteriodYpos[AsteriodYpos.length] = Math.round(Math.random() * WinHeight);
                        AsteriodXpos[AsteriodXpos.length] = Math.round(Math.random() * WinWidth) + WinWidth;
                        AsteriodXSpeed[AsteriodXSpeed.length] = Math.random() * 10 + 5;
                        AsteriodYSpeed[AsteriodYSpeed.length] = Math.random() * 7 - 5;
                        asteriods += 1;
                    }
                }
                ast[i].style.pixelLeft = AsteriodXpos[i];

                ast[i].style.pixelTop = AsteriodYpos[i]+hscrll;
When it seems to not like these last two lines as it says "Error: Unable to get value of the property 'style': object is null or undefined". Essentially it is a game where you are flying through space avoiding astroids. after you avoid so many it increases the number of astroids which is where the problem is happening. In case someone would like to see this in context of what im doing the whole code is
Code:
<html>
<head>
</head>
<body bgcolor="#000000" onload="fly()" onkeypress="displayunicode(event)" style="overflow: hidden">
    >
    <script language="JavaScript">



<!-- Begin
        x = 0;

        var startTime = new Date();
        // create object
        imageObj = new Image();

        // set image list
        images = new Array();
        images[0] = "ship1.png"
        images[1] = "ship2.png"
        images[2] = "ship3.png"
        images[3] = "ship4.png";
        images[4] = "ship5.png"
        images[5] = "ship6.png"

        // start preloading
        for (i = 0; i <= 5; i++) {
            imageObj.src = images[i];
        }
        AstriodsAvoided = 1;
        wasCollision = 0;
        SmallStars = 20;

        LargeStars = 50;
        asteriods = 5;
        SmallYpos = new Array();

        SmallXpos = new Array();

        LargeYpos = new Array();

        LargeXpos = new Array();

        Smallspeed = new Array();

        Largespeed = new Array();

        AsteriodXpos = new Array();
        AsteriodYpos = new Array();
        AsteriodXSpeed = new Array();
        AsteriodYSpeed = new Array();

        document.write('<div style="position:absolute;top:0px;left:0px">');

        document.write('<div style="position:relative">');


        for (i = 0; i < SmallStars; i++) {

            document.write('<div id="si" style="position:absolute;top:0;left:0;width:1px;height:1px;background:#fffff0;font-size:1px"></div>');

        }

        document.write('</div>');

        document.write('</div>');


        document.write('<div style="position:absolute;top:0px;left:0px">');

        document.write('<div style="position:relative">');


        for (i = 0; i < LargeStars; i++) {

            document.write('<div id="li" style="position:absolute;top:0;left:0;width:3px;height:2px;background:#ffffff;font-size:2px"></div>');

        }

        document.write('</div>');

        document.write('</div>');


        document.write('<div style="position:absolute;top:0px;left:0px">');

        document.write('<div style="position:relative">');

        for (i = 0; i < asteriods; i++) {

            document.write('<div id="ast" style="position:absolute;top:0;left:0;font-size:2px"><img src="asteroid.png" height=50 width=50></div>');

        }

        document.write('</div>');

        document.write('</div>');


        imgWidth = 150;
        imgHeight = 150;
        document.write('<div style="position:absolute;top:0px;left:0px">');

        document.write('<div style="position:relative">');

        document.write('<div id="shipLayer" style="position:absolute;top:0;font-size:2px"><img id="ship" src="ship.png" height=150 width=150></div>');

        document.write('</div>');

        document.write('</div>');


        WinHeight = window.document.body.clientHeight;

        WinWidth = window.document.body.clientWidth;


        ShipYpos = Math.round(Math.random() * WinHeight);
        ShipXpos = Math.round(Math.random() * WinWidth) / 2;

        ShipXSpeed = 0;
        ShipYSpeed = 0;

        shipLayer.style.pixelLeft = ShipXpos;

        shipLayer.style.pixelTop = ShipYpos;

        for (i = 0; i < SmallStars; i++) {

            SmallYpos[i] = Math.round(Math.random() * WinHeight);

            SmallXpos[i] = Math.round(Math.random() * WinWidth);

            Smallspeed[i] = Math.random() * 5 + 1;

        }


        for (i = 0; i < LargeStars; i++) {

            LargeYpos[i] = Math.round(Math.random() * WinHeight);

            LargeXpos[i] = Math.round(Math.random() * WinWidth);

            Largespeed[i] = Math.random() * 10 + 5;

        }


        for (i = 0; i < asteriods; i++) {

            AsteriodYpos[i] = Math.round(Math.random() * WinHeight);

            AsteriodXpos[i] = Math.round(Math.random() * WinWidth) + WinWidth;

            AsteriodXSpeed[i] = Math.random() * 10 + 5;

            AsteriodYSpeed[i] = Math.random() * 7 - 5;
        }





        function fly() {
            document.getElementById("ship").src = images[x];
            x += 1;
            if (x == 6) { x = 0; }
            var WinHeight = window.document.body.clientHeight;

            var WinWidth = window.document.body.clientWidth;

            var hscrll = document.body.scrollTop;

            var wscrll = document.body.scrollLeft;


            for (i = 0; i < LargeStars; i++) {

                LargeXpos[i] -= Largespeed[i];

                if (LargeXpos[i] < -10) {

                    LargeXpos[i] = WinWidth;

                    LargeYpos[i] = Math.round(Math.random() * WinHeight);

                    Largespeed[i] = Math.random() * 10 + 5;
                }


                li[i].style.pixelLeft = LargeXpos[i];

                li[i].style.pixelTop = LargeYpos[i] + hscrll;

            }

            for (i = 0; i < SmallStars; i++) {

                SmallXpos[i] -= Smallspeed[i];


                if (SmallXpos[i] < -10) {

                    SmallXpos[i] = WinWidth;

                    SmallYpos[i] = Math.round(Math.random() * WinHeight);

                    Smallspeed[i] = Math.random() * 5 + 1;

                }


                si[i].style.pixelLeft = SmallXpos[i];

                si[i].style.pixelTop = SmallYpos[i] + hscrll;

            }


            for (i = 0; i < asteriods; i++) {

                AsteriodXpos[i] -= AsteriodXSpeed[i];

                AsteriodYpos[i] -= AsteriodYSpeed[i];
                //for (j=0; j<30; j++){
                //	xDistance = AsteriodXpos[i]+25-AsteriodXpos[j]+25;
                //	yDistance = AsteriodYpos[i]-25-AsteriodYpos[j]-25;
                //	distance =xDistance*xDistance+yDistance*yDistance;
                //	if (distance<3000 && i!=j){



                //	}
                if (AsteriodXpos[i] < -50 || AsteriodYpos[i] < -100 || AsteriodYpos[i] > WinHeight + 100) {

                    AsteriodXpos[i] = WinWidth;

                    AsteriodYpos[i] = Math.round(Math.random() * WinHeight);

                    AsteriodXSpeed[i] = Math.random() * 5 + 7;

                    AsteriodYSpeed[i] = Math.random() * 7 - 5;
                    AstriodsAvoided += 1;
                    if (AstriodsAvoided % 15 == 0) {
                        document.write('<div style="position:absolute;top:0px;left:0px">');

                        document.write('<div style="position:relative">');
                        document.write('<div id="ast" style="position:absolute;font-size:2px"><img src="asteroid.png" height=50 width=50></div>');
                        document.write('</div>');
                        document.write('</div>');

                        AsteriodYpos[AsteriodYpos.length] = Math.round(Math.random() * WinHeight);
                        AsteriodXpos[AsteriodXpos.length] = Math.round(Math.random() * WinWidth) + WinWidth;
                        AsteriodXSpeed[AsteriodXSpeed.length] = Math.random() * 10 + 5;
                        AsteriodYSpeed[AsteriodYSpeed.length] = Math.random() * 7 - 5;
                        asteriods += 1;
                    }
                }
                ast[i].style.pixelLeft = AsteriodXpos[i];

                ast[i].style.pixelTop = AsteriodYpos[i]+hscrll;

               
            }




            if (ShipXpos - ShipXSpeed < -150) {
                ShipXpos = WinWidth;
            }
            else {
                if (ShipXpos - ShipXSpeed > WinWidth) {
                    ShipXpos = -150;
                }
            }

            if (ShipYpos - ShipYSpeed < -150) {
                ShipYpos = WinHeight;
            }
            else {
                if (ShipYpos - ShipYSpeed > WinHeight) {
                    ShipYpos = -150;
                }
            }
            ShipXpos -= ShipXSpeed;
            ShipYpos -= ShipYSpeed;
            shipLayer.style.pixelLeft = ShipXpos;

            shipLayer.style.pixelTop = ShipYpos + hscrll;


            DidCollide()
            sleep(10);
            setTimeout('fly()', 10);


        }



        //  End -->

        function displayunicode(e) {
            var unicode = e.keyCode ? e.keyCode : e.charCode;
            var keyPressed;
            keyPressed = (String.fromCharCode(unicode));
            moveShip(keyPressed)
        }

        function moveShip(keyPressed) {
            switch (keyPressed) {
                case "w" || "W":
                    if (ShipYSpeed != 25) {
                        ShipYSpeed += 5;
                    }
                    break;
                case "a" || "A":
                    if (ShipXSpeed != 25) {
                        ShipXSpeed += 5;
                    }
                    break; ww
                case "s" || "S":
                    if (ShipYSpeed != -25) {
                        ShipYSpeed += -5;
                    }
                    break;
                case "d" || "D":
                    if (ShipXSpeed != -25) {
                        ShipXSpeed += -5;
                    }
                    break;
            }
        }
        function DidCollide() {
            for (i = 0; i < asteriods; i++) {

                xDistance = AsteriodXpos[i] + 25 - ShipXpos - 75;
                yDistance = AsteriodYpos[i] + 25 - ShipYpos - 75;
                distance = xDistance * xDistance + yDistance * yDistance;
                if (distance < 4225) {

                    wasCollision = 1;
                    document.write('<img src="Explosion.png">');
                    var endTime = new Date();
                    var totalTime = new Date();
                    totalTime.setTime(endTime.getTime() - startTime.getTime());
                    test = "this is a test";
                    document.write('You managed to last ' + totalTime.getSeconds() + '.' + totalTime.getMilliseconds() + ' seconds.  You avoided a total of ' + AstriodsAvoided + 'astroids');

                }
            }
        }
        function sleep(milliseconds) {
            var start = new Date().getTime();
            for (var i = 0; i < 1e7; i++) {
                if ((new Date().getTime() - start) > milliseconds) {
                    break;
                }
            }
        }
    </script>
    <!-- Script Size:  3.79 KB -->
</body>
</html>
Any help someone could provide would be wonderful, and if you can explain to me whats going wrong and why its not working that would be even better!