They seem to photosynthesize, but they do need whatever is in their atmosphere, either that, or the only thing in space that harms them is rapid decompression. The snacks are just nice to have. Their heads, however must be made of Bouncyus Metaliicus, a mixture of rubber and titanium, allowing them to fall from orbit, fly at terminal velocity, land on their head, and bounce. They could be Green Groots, rapidly regrowing themselves as they burn up in the upper atmosphere. The final possibility is that Kerbals are magic.