User blog comment:Shnopp.shniederheimer/Tips and Tricks - Treehouse of Horror XXV Event/@comment-24.242.250.193-20141010113136/@comment-24499545-20141010121840

Depends on which one you're talking about. If it's the one with Kodos in it, which you shoot down in your own town, then that's it. It's only once. If it's the UFO you shoot in your neighbor's town, you need Ammo which I describe how to get above.