Destroy all the invading aliens before they're able to land on Earth(the bottom of the game). Be warned: The aliens are smart; and as they're defeated, they get progressively smarter, faster, and more upset with your attempts to prevent their invasions. Your shields may take up to 4 directhits (from you or the aliens) before being destroyed. Coming in contact with an alien will kill both you and it. If an alien contacts a shield, they will both be destroyed. If an alien lands, your mission is a failure. Enjoy!