459
xkcd #2908: Moon Armor Index
(imgs.xkcd.com)
Alt text:
Astronomers are a little unsure of the applicability of this index, but NASA's Planetary Protection Officer is all in favor.
I got to see one about 7 years ago. Made a whole vacation of it, and was not disappointed. In the darkness, all the birds stopped singing. And to top it off, at the motel I stayed there was a cleaning lady yelling at me to get back into my room because this was a sign from her god saying this was the end of days.