Happily, Roach does not dwell on Lisa Nowak, the astronaut who drove from Texas to Florida, allegedly in diapers, to confront her ex-boyfriend’s new girlfriend. She does, however, point out that male astronauts have a diaper alternative that fits directly onto their anatomy. In the way of Starbucks, where a small is termed a “tall,” the men’s devices come only in L, XL and XXL.

On a long-duration Mars trip, urine would have to be recycled, which is not as vile as it sounds. “I will tell you sincerely and without exaggeration that the best part of lunch today at the NASA Ames cafeteria is the urine,” Roach writes, adding that after purification and desalination, it tastes like Karo syrup. Her husband, however, doesn’t share her keenness and protests when she stores her urine in their refrigerator.

The strongest parts of “Packing for Mars” chart the American space effort during the cold war. Roach deals less knowingly with the situation today, when space is, as the private entrepreneurs say, a place, not a program. While investigating zero-gravity sex, she mentions that Robert Bigelow, the founder of Budget Suites America, plans to build an orbiting hotel. But she doesn’t convey that NASA itself has begun spurning Big Aerospace boondoggles (like Orion) in favor of shoestring alternatives (a contract with tiny, upstart SpaceX for cargo flights to the International Space Station).

Just as Roach refuses to grapple with grief, she also plays down spaceflight’s greatest danger: radiation, for which no cost-efficient shielding has yet been engineered. Linked to brain damage and rapid-onset leukemia, it could quickly devastate a Mars crew. In contrast to excrement and sex, which have dedicated chapters, radiation surfaces in a scattershot, piecemeal fashion. Roach states that astronauts are classified as “radiation workers” because they receive such high doses. She tells us that cosmic rays — high-energy heavy ions from outside our solar system — can be damaging to cells, and that hydrogen compounds (not metal spacecraft hulls) are required for shielding. But she never directly addresses the radiation from solar flares, and makes a joke about a brilliant idea that, in my view, deserves a chapter of its own: on a Mars mission, the astronauts’ solid waste (rich in hydrocarbons) could be wrapped around the crew quarters to protect against cosmic rays.

At the book’s end, after more than 300 pages of debunking the romance of spaceflight, Roach herself buys into that idea, making a misguided, emotional pitch for a $500 billion human Mars mission — at the expense of cheap, reliable, robotic missions. I am not impervious to sentimentality. I felt a surge of tenderness when Roach described the “unlikely heroics” of a patch of moss on Devon Island: “something so delicate surviving in a place so stingy and hard.”

Yet compared with the irradiated void of space, a frozen rock in the High Arctic is as cozy as a baby’s crib. Packing for Mars, Roach has shown, can be entertaining here on Earth. But no way are humans ready to make the actual trip.