In conducting research into the physiology of astronauts in space, Mary Roach found out that one man on a Space Shuttle flight wore a sound monitor on his belly for the duration of his voyage. It is Ms. Roach’s style to be less interested in the belly-noise findings than in the freaky-deaky part of the story. “Don’t feel bad for him,” she writes in “Packing for Mars” about that awkwardly wired astronaut. “Feel bad for the Air Force security guy assigned to listen to two weeks of bowel sounds to be sure no conversations including classified information had been inadvertently recorded.” Ms. Roach has already written zealously nosy books about corpses (“Stiff”), copulation (“Bonk”) and charlatans (“Spook”). Each time, what has interested her most is the fringe material: exotic footnotes, smart one-liners, bizarre quasi-scientific phenomena. Yet her fluffily lightweight style is at its most substantial — and most hilarious — in the zero-gravity realm that “Packing for Mars” explores. Here’s why: The topic of astronauts’ bodily functions provides as good an excuse to ask rude questions as you’ll find on this planet or any other.