Maciej Cegłowski has written a history of going to the bathroom in space, and without quite seeming to try, it is one of the better arguments about Mars I have read. It opens on the oldest strategy of all: the astronaut's low-residue breakfast of steak and eggs, which "pays tribute to the oldest and wisest strategy for going to the bathroom in space, which is to do everything possible to avoid it." From there it is a catalogue of what happens when avoidance runs out. Gemini crews holding on for days in a capsule the size of a phone booth. The Apollo collection device astronauts rejected because you could imagine "just turning your whole body inside out in it." The Skylab toilet bolted to a wall so a crew member could "defecate like Spider-Man." A shuttle flight that grew a mass of frozen urine on its hull and had to worry it might shear off and hole the heat tiles. It is very funny, and the humor is a delivery system for a serious claim: that the grandest vision we currently allow ourselves, becoming a species that lives on more than one world, runs aground first and hardest on the most mundane fact about a body, which is that it produces waste and cannot be argued out of it.
The toilet is a synecdoche, and he chooses it precisely because it is unanswerable. You can wave a great deal away with enough money and enough will, but you cannot wave away four astronauts producing, across a seven-hundred-day surface stay, what he puts at "three to four tons of highly septic waste," for which NASA has set itself the design goal of keeping it "sequestered for fifty years," on a planet where, as he dryly notes, "NASA has trouble building structures that can last 50 years on Earth." The reason it lands is that it forces the vision to meet its own mechanism. The romance of Mars is arrival: the red planet, the boot print, the multiplanetary destiny. The mechanism of Mars is a level-four biohazard shed you have to get right on the first attempt, tens of millions of miles from the nearest person who could fix it. Every grand story has a layer it would rather you not examine, and the surest way to tell whether the story is a plan or a fantasy is to walk past the artist's rendering and go look at the part it finds embarrassing.
But I want to turn his own evidence over, because it points somewhere he does not quite follow it. Every problem in his catalogue was, at an earlier moment, exactly as unanswerable as the Mars toilet looks now. Separating waste from a body in freefall was a showstopper, until suction and a seat only a few inches across. Reliable collection was hopeless, until the wall mount and the "nine good 'data points' for the fecal collector" wrung out of two days of parabolic flights. The frozen urine nearly downed a shuttle, and then it did not, because they stopped venting it that way. Read forward instead of backward, the history of the space toilet is not the story of an impossible problem. It is the story of a genuinely disgusting one solved incrementally, badly and then less badly, by people willing to take apart used waste canisters from the space station and catalogue what they found. The toilet does not prove that Mars cannot be done. It proves something more specific and more useful: that Mars is not a heroic problem. It is a maintenance problem.
That distinction is the thing worth carrying out of the essay. Keeping a body alive somewhere it was not built for turns out to be almost entirely maintenance: not the flag and the footprint but the biofilms, the filters, the odor he says is still not honestly solved, and the two years the whole apparatus must sit uncrewed in orbit without its plumbing rotting, a condition he likens to "getting a camper van or vacation home prepped for winter," except that it has never once been tried and a mistake kills everyone. The romance sells the arrival. The reality is the upkeep, and the upkeep is where all the difficulty actually lives. This is not a fact about Mars. It is the standard shape of the distance between a vision and its cost, and the reliable tell of a vision that is mostly romance is that it cannot bear to discuss its own maintenance. The people selling the destiny are, structurally, the least interested in the plumbing, which is exactly why going to look at the plumbing is the honest move.
And then there is the detail I cannot put down, which is the proposed answer to the waste. You roast it. Low-temperature roasting cooks the three or four tons of sewage down to a dry, sterile char; the char can be pressed into tiles; and the tiles can be stacked around the habitat as shielding against the cosmic radiation that would otherwise cook the crew. The thing that protects the pioneers is their own excrement, compressed and baked and stood up as a wall. "Such are the glories of going to Mars!" he writes, and the joke is doing more than deflating. That one object, the radiation shield that is also the sewage, is the truest thing in the piece. The glory and the waste are not opposites that a sufficiently inspiring vision can hold apart. On a long enough mission, in a hostile enough place, they are the same tile. The romance and the plumbing were never two stories. They were always one, and only one half of it was ever going to keep you alive.