/**** 
* Classes
****/
// Missile class
// Alien class
var Alien = Container.expand(function () {
	var self = Container.call(this);
	var alienGraphics = self.attachAsset('alien', {
		anchorX: 0.5,
		anchorY: 0.5
	});
	self.speed = 2;
	self.move = function () {
		var dy = earth.y - self.y;
		var dx = earth.x - self.x;
		var angle = Math.atan2(dy, dx);
		self.x += self.speed * Math.cos(angle);
		self.y += self.speed * Math.sin(angle);
	};
});
// Shield class
var Shield = Container.expand(function () {
	var self = Container.call(this);
	self.attachAsset('shield', {
		anchorX: 0.5,
		anchorY: 0.5
	});
	self.orbitRadius = 200;
	self.orbitSpeed = 0.02;
	self.orbitAngle = 0;
	self.update = function () {
		self.orbitAngle += self.orbitSpeed;
		self.x = earth.x + self.orbitRadius * Math.cos(self.orbitAngle);
		self.y = earth.y + self.orbitRadius * Math.sin(self.orbitAngle);
	};
});

/**** 
* Initialize Game
****/
var game = new LK.Game({
	backgroundColor: 0x000000 // Init game with black background
});

/**** 
* Game Code
****/
// Game variables
// Initialize assets used in the game.
var aliens = [];
var earth;
var scoreTxt;
var spawnAlienInterval = 120; // Frames until a new alien spawns
var nextAlienSpawn = spawnAlienInterval;
// Initialize Earth
function initEarth() {
	earth = game.addChild(LK.getAsset('earth', {
		anchorX: 0.5,
		anchorY: 0.5,
		x: game.width / 2,
		y: game.height - 1000
	}));
	window.shield = game.addChild(new Shield());
	window.shield.x = earth.x;
	window.shield.y = earth.y - window.shield.orbitRadius;
}
// Initialize score display
function initScore() {
	scoreTxt = new Text2('0', {
		size: 150,
		fill: "#ffffff"
	});
	scoreTxt.anchor.set(0.5, 0);
	LK.gui.top.addChild(scoreTxt);
}
// Spawn an alien
function spawnAlien() {
	var alien = new Alien();
	alien.x = Math.random() * (game.width - alien.width) + alien.width / 2;
	alien.y = -alien.height / 2;
	aliens.push(alien);
	game.addChild(alien);
}
// Check for collisions
function checkCollisions() {
	for (var i = 0; i < aliens.length; i++) {
		if (window.shield.intersects(aliens[i])) {
			aliens[i].destroy();
			aliens.splice(i, 1);
		}
	}
}
// Game over check
function checkGameOver() {
	for (var i = 0; i < aliens.length; i++) {
		if (aliens[i].intersects(earth)) {
			LK.effects.flashScreen(0xff0000, 1000);
			LK.showGameOver();
			break;
		}
	}
}
// Touch event to move shield
game.on('down', function (obj) {
	window.shield.orbitSpeed *= -1;
});
// Game tick event
LK.on('tick', function () {
	// Move aliens
	for (var j = aliens.length - 1; j >= 0; j--) {
		aliens[j].move();
		if (aliens[j].y > game.height + aliens[j].height) {
			aliens[j].destroy();
			aliens.splice(j, 1);
		}
	}
	// Spawn aliens
	nextAlienSpawn--;
	if (nextAlienSpawn <= 0) {
		spawnAlien();
		nextAlienSpawn = spawnAlienInterval;
	}
	// Update shield position
	window.shield.update();
	// Check for collisions and game over
	checkCollisions();
	checkGameOver();
});
// Initialize game elements
initEarth();
initScore();

===================================================================
--- original.js
+++ change.js
@@ -81,9 +81,16 @@
 	aliens.push(alien);
 	game.addChild(alien);
 }
 // Check for collisions
-function checkCollisions() {}
+function checkCollisions() {
+	for (var i = 0; i < aliens.length; i++) {
+		if (window.shield.intersects(aliens[i])) {
+			aliens[i].destroy();
+			aliens.splice(i, 1);
+		}
+	}
+}
 // Game over check
 function checkGameOver() {
 	for (var i = 0; i < aliens.length; i++) {
 		if (aliens[i].intersects(earth)) {