A few tips
The best part was it was not at all laggy (on this slow comp!) and the graphics were really good, and the filesize was short. But a few flaws.
1:You can score more than one hit off of each alien. You need to make it so that getting more than one per alien is not possible.
2:Make the graphics better and more colorful, 3D, and dynamic but keeping it short and smooth and you should have a top notch game on your hands.