LIKE HER PREVIOUS BOOKS exploring cadavers, coitus, and the afterlife, Mary Roach’s Packing for Mars offers lots of taboo-busting fun, this time on the subject of space travel. The book answers the questions the world’s four-year-olds have about space flight — what do astronauts eat? how do astronauts pee? — and some that they probably haven’t thought of yet: Can you get it on in zero gravity? How exactly do you die if your spacecraft breaks up at thirty-four hundred miles an hour? Does orbit make a man’s penis look bigger? What’s it like to view your own anus through a closed-circuit toilet-cam? (Actually, they may have thought of that last one, but unlike NASA, they don’t have the technology to answer it.)

If all those questions seem a little earthbound, that’s the point. Roach is not out to write a history of the space race, or a philosophical disquisition on off-world exploration. She’s not even really interested in what’s out there. Instead, Roach focuses on the enormous amounts of research and development that go into sustaining human life off Earth. “To a rocket scientist,” she begins, “you are a problem,” and the rest of the book bears her out. Eating, sweating, shedding, peeing, shitting, barfing, farting — the entire budget of the space program appears to be spent dealing with human bodily functions in a place where they have the potential to really screw things up. Game as ever, Roach climbs onto a motion-sickness-inducing chair, watches Japanese astronaut-hopefuls make scads of origami cranes, drinks whiskey with former cosmonauts at eleven a.m., tracks down an alleged zero-gravity porn film, and digs into the archives to clear the name of space-chimp Enos, allegedly nicknamed Enos the Penis for his penchant for public self-love.

All this fun aside, the book circles around an obvious question that has hung in the air at least since the space shuttle Columbia rained down in shards across the nation: is spaceflight worth it? Why go to all the trouble of launching ourselves into an inhospitable place when our home planet suits us so well? The question pervades the book’s wacky anecdotes like a pair of sweaty socks in the corner of the International Space Station, until Roach poses it. Wouldn’t that money and effort, she asks in the last chapter, be better spent on Earth?

Interestingly, her answer is no. The reasons she gives in her summary seem insufficient — money is squandered on more foolish things, like war, and the exploratory urge driving the space program is somehow ennobling. The book makes a better argument as it recounts the myriad ways scientists have grappled with the limitations imposed on our machine dreams by the animal realities of the body. The more we seek to transcend our earthbound carcasses, it seems, the more we come face to face with their demands. We hurled ourselves into space on physics and fossil fuels and wound up staring into a black hole — the human anus.

In the end, even as Roach makes her claim for continuing the space program, Packing for Mars draws our attention to Earth, and how well it answers our annoying, but persistent, needs. Reading about defecation bags, tube food, motion sickness, reprocessed urine, and a space-induced psychosis called “Earth-out-of-view phenomenon,” it’s clear space travel fosters the same appreciation. “Only in space do you understand what incredible happiness it is just to walk,” former cosmonaut Alexander Laveikin tells Roach. “To walk on Earth.”