In space, as was first noted in the ad campaign for the movie Alien, no one can hear you scream. What prompts that screaming in the Alien franchise and other space opera sci-fi is typically terror, dismemberment or larval-monster intestinal occupation. But more mundane issues make real-life astronauts want to scream. Because in space, everybody can smell your gas.

Space, when done with people living together in close quarters, stinks. Best-selling author Mary Roach catalogues the rank unpleasantness of space travel in her new book Packing for Mars: The Curious Science of Life in the Void (W. W. Norton, 2010). Her introduction’s first words encapsulate, if you will, the situation: “To the rocket scientist, you are a problem. You are the most irritating piece of machinery he or she will ever have to deal with.”

We humans are incredibly demanding because of our hunger and thirst—and the messy, odoriferous products of our satiety. Humans are the reason the space shuttle needs toilets. And humans in zero gravity are the reason the toilets have rearview mirrors. (You can find the details in Roach’s book or spend a moment to think about the various ramifications of weightless evacuation.)

The Mercury, Gemini and Apollo spacecraft didn’t even have toilets—rudimentary devices were used for docking and capture, ruling out the chances of anybody boldly going. Showers are also too tricky to deal with out there. So it’s no surprise that the interior of your average spaceship quickly winds up smelling very, very bad. Think of a car ride with three immature guys for whom “pull my finger” is considered a droll example of classic wit. Now imagine spending a week in the car with the windows rolled up.

In an interview, Roach recounted her gentle interrogation of Apollo 13 hero James A. Lovell about the stank: “So when the capsule came down and those frogmen came and they opened the hatch, what was that like for them?” she asked him. “And he said, ‘Well, it was ...’—then his gentlemanly instincts took over, and he said, ‘It was quite different than the fresh ocean breezes outside.’ But elsewhere I saw him describe it as like living in a Porta Potty.”

On shorter jaunts into space, some of the stuff humans produce is simply chucked from the ship. The physiologically confusing term of art for one such action is the “urine dump.” And, believe it or not, space pee is pretty. “A number of astronaut memoirs mentioned how these flash-frozen droplets, illuminated, would look like this silvery snowstorm,” Roach told me. “I think three different astronauts mentioned how beautiful the urine dump was.”

For manned Mars voyages, however, recycling is mandatory. As on the International Space Station now, treated urine would be a treat, sort of. “Once the salt is taken care of and the distasteful organic molecules have been trapped in an activated charcoal filter,” Roach writes in the book, “urine is a restorative and surprisingly drinkable lunchtime beverage. I was about to use the word unobjectionable, but that’s not accurate. People object. They object a lot.”

Nevertheless, urine is the easy part. On a Mars trip, the captain’s log, if you will, presents a whole different set of problems—and possibilities. “Hydrocarbons are good radiation shielding,” Roach told an audience at a Manhattan bookstore just before our conversation. “So the thinking is that on the way to Mars, you would use your food to line the interior of the module that you’re living in. And at NASA, they have this device where you could make tiles that would contain fecal material. It’s like an Easy-Bake Oven. And on the way back, you would line the capsule with the new tiles. So you’d fly to Mars in a can of food, and then you’d fly home in a can of poop.” Only she didn’t say “poop.”