In Alien Invasion, the player controls a rocket ship that appears at the bottom
center of the screen. The player can move the ship right and left using the arrow
keys and shoot bullets using the spacebar. When the game begins, a fleet of aliens
fills the sky and moves across and down the screen. The player shoots and destroys
the aliens. If the player shoots all the aliens, a new fleet appears that moves
faster than the previous fleet. If any alien hits the player's ship or reaches
the bottom of the screen, the player loses a ship. If the player loses three
ships, the game ends.