Micrograph for the Day

Cosmic ray tracks in an Apollo space helment, amplified 10-millionfold by chemical etching.

I don't remember when I first saw this picture, but I told Jack about it at lunch today. I looked for it when I got home and was surprised how quickly I found it.

From the Wikipedia article on Ultra-high energy cosmic rays:

. . . in other words, a subatomic particle with macroscopic kinetic energy equal to that of a baseball (142 g or 5 ounces) traveling at 96 km/h (60 mph).
It was most probably a proton with a speed very close to the speed of light.

At some point I had the idea that cosmic rays were iron particles, and that might occasionally be the case, but they are more likely to be protons.

So I'm wondering if the astronaut noticed when this particle hit his helmet. Maybe it wasn't an ultra-high energy cosmic ray, maybe it was just regular cosmic ray, and he just thought someone was throwing spitballs at him. Jimmy! Stop that!

Update January 2017 replaced missing picture.