Click the mouse cursor on any alien object to fire the earth defense rocket. If the earth rocket hits the alien object you earn ten points. If you destroy all alien objects in a level you go to next level. If the alien object hits the earth the game ends. Click the mouse cursor on any alien object to fire the earth defense rocket. If the earth rocket hits the alien object you earn ten points. If you destroy all alien objects in a level you go to next level. If the alien object hits the earth the game ends.