If humans are going to go to Mars, or mine asteroids, then recycling is going to matter. And that means recycling everything — including human waste.

NASA has put some effort into solving the problem, because recycling is such an essential part of building a spaceship that can get people to Mars or anywhere else. Interplanetary missions won’t be able to get supplies from Earth. Resources will be limited, and that means “closing the loop” — you can’t afford to throw away anything, not even human poop. Any spacecraft design has to take that into account.

“You have to start with a life support system and build a spacecraft around it,” says Marc Cohen, president of Astrotecture, a consulting firm that specializes in space architecture.

The Story So Far

First a few facts about human poop. A healthy person produces about 128 grams of feces per day, or about 46.7 kilograms (102 pounds) in a year, according to the medical literature. For a mission to Mars that might last two to three years, a crew of six (as posited in The Martian) would generate 300 pounds of feces each.

In the Apollo era, the toilet was a plastic bag attached to the astronaut’s butt

In the Apollo era, the toilet was a plastic bag attached to the astronauts’ butts with an adhesive. Urine was collected with a condom-like device and vented to space. Famously — or infamously — the last Mercury flight in 1963 actually suffered system failures because the urine collection bag leaked. Clearly, the bags didn’t work. Floating human waste is also a health hazard, since one can inhale tiny bits of urine or feces as they float around.

Enter Don “Doctor Flush” Rethke, a retired engineer from Hamilton Standard, now UTC Aerospace Systems. Rethke goes way back with NASA; he worked on life support for the Apollo 13 mission. He designed a commode that takes in urine and feces separately. It used suction — essential because in zero-g, liquids turn to spheres and float around, and solid waste won’t just fall into the bowl. Urine was collected in a cup-like contraption, while the solid stuff was sucked into a container and exposed to the vacuum — effectively freeze-dried and compressed. “We called them fecal patties,” Rethke says.

A variation of his design is on the International Space Station, with two big differences: one is that the urine is now treated so that the water can be removed and reused, and the other is that the new system doesn’t freeze-dry the feces. (The ISS recycling system also takes in moisture from the air, which is largely astronauts’ sweat and exhalations.) As for the solid waste, during the shuttle era it was just brought back. On the ISS, it’s stored in plastic or metal containers. When those fill up, astronauts load them onto a used Russian Progress vehicle, unlock it from the ISS, and let it fall to Earth to burn in the atmosphere, along with the rest of the ISS’s garbage. (Think of that the next time you see a meteor shower.)

Throwing feces out an airlock is not an option, for a couple of reasons. One is that anything jettisoned from the spacecraft won’t go very far away without a substantial push. So if you throw something outside, it will simply follow your trajectory — any waste thrown “away” would follow you all the way to Mars. Pushing it away would mean something like opening an air lock with some air still in it, to provide a kind of explosive decompression. That would waste air.

Then there’s that trajectory problem — even if the waste moves some distance away, blocks of it might drift to various points around the ship, entering unpredictable orbits. (During the shuttle and Apollo eras, it wasn’t unusual for the spacecraft to meet clouds of urine-ice crystals that had been vented previously.) Dumping out a container behind the spacecraft is, as a result, quite dangerous. “When you near your objective you’re going to make a sudden stop,” says John W. Fisher, of NASA’s Ames Research Center, who has written several papers on recycling waste in space. “If you slam on the brakes, it’s going to hit you in the rear end.” A pound bag of anything hitting a decelerating spacecraft can pack a lot of force.

The second problem is that some human feces — now freeze-dried in space — would probably settle back on the ship; absent a substantial push, the turds will just hang around. The poop, now in a powdery, crystalline form, would get on the windows, says Fisher. It would foul optical sensors as well. Unlike bird droppings on a windshield, there’s no way to squeegee it off.

So you have to store it, Rethke says. In the early days of the shuttle commode, they thought of refrigeration to keep the bacteria from growing. “That takes energy, and you have to back it up with a redundant system,” he says.

Besides, throwing feces away is actually the last thing space crews want to do — there’s too much useful stuff in it. About 75 percent of it is water, along with bacteria from our guts and human cells. Some 80 percent of the solid mass is organic molecules, which means compounds containing carbon. About a quarter of that is bacterial biomass, another quarter is protein, another is undigested plant matter (mostly the fiber), and a smaller percentage is fat. Organic chemicals and water are like gold in space.

On Mars, human poop, at the very least, would make a good fertilizer to grow food, Rethke says. “I would put it into a mushroom patch — let Mars take care of it.”

Reuse, Recycle

Human feces aren’t the only thing you need to recycle. People produce a lot of garbage. All this adds complexity to the problem of recycling and reuse. Any machines for doing that have to be light, because launching anything into orbit is pricey, thousands of dollars per pound. Those machines also have to be small, because there’s only so much room in a space module. And they have to work reliably and be easy to fix, because there’s no calling for help between Earth and Mars.

Jay Perry, lead aerospace engineer for environmental control and life support systems at NASA’s Marshall Space Flight Center, says designing such systems is complicated. Take urine, for example: separating water from urine is relatively straightforward on Earth, but in a zero-gravity environment, the situation changes.

For example, weightless astronauts’ bones lose mass and density, since there’s no loading on them. This is why current astronauts on the ISS have a strict exercise regimen. The bone mass gets excreted as calcium in then gets into the urine. That places a limit on how much water can be pulled out, because eventually the remaining stuff is a concentrated brine, “unpleasant stuff to deal with.” A 2013 study by United Technologies Aerospace Systems noted that the calcium forms small kidney stones, which can clog up the valves on toilets.

Human feces pose similar challenges, both because of zero gravity and figuring out which chemicals you want to save. In addition there’s the question of the necessary energy and the complexity of the system you want to build. The United Technologies study, for example, noted that current space toilets use machines to compress the poop. That adds complexity — instead, the study proposes a manual lever, which requires no power (except that provided by the crew member’s arm).

While there are a lot of useful chemicals in poop, separating every one of them isn’t easy. Chemical toilets and septic tanks would be useless. Chemical toilets don’t really work because the very compounds used to break down waste would still need to be sent up with the astronauts. You’d also need hundreds to thousands of gallons of that blue-dyed stuff for a years-long journey, and most of it is water — effectively you’d be adding tons of water that would only be used in toilets, which isn’t very efficient. Septic tanks depend on gravity to work — and you still have to store the feces somewhere.

Rethke says he favored using natural biodegradation; simply allowing the fecal material (and whatever else — “menstrual waste, vomitus, it’s all in there”) from the commode to ferment in a metal container with some activated charcoal to stop the odors. The container could release gas — almost all would be carbon dioxide — which the spacecraft’s scrubbers could handle well enough. He even built such a device. “I put it on my desk for several months,” he says. “Nobody noticed.” Once astronauts get to Mars, the stuff in the containers could be fertilizer. The down side is the storage — the volumes would start to add up.

weird as it may sound, poop may make for good radiation shielding

Weird as it may sound, poop may provide good radiation shielding. In space, there are two sources of ionizing radiation that could harm astronauts. One is the background of galactic cosmic rays (or GCR). The other is a solar storm, known as a “solar particle event” or SPE. Both consist of charged particles, mostly protons.

These sources of radiation are less of a problem for ISS astronauts because they are still inside the Earth’s protective magnetic field. But once astronauts leave that field, the SPE could cause acute radiation sickness, while cosmic rays increase the risk of cancer.

The most efficient shielding is solid hydrogen because the element more easily deflects flying particles. But solid hydrogen isn’t available outside of a gas giant, and liquid hydrogen is difficult to handle, needing high pressures, cryogenic temperatures, or both. The next best thing is water, which has lots of hydrogen in it, or polyethylene. Metal shielding like lead, which provides good protection against gamma and X-rays, is actually worse than no shielding at all, because the protons hit the atoms in the metal and create cascades of other particles, creating even more harmful radiation.

Jack Miller, a nuclear physicist at Lawrence Berkeley National Laboratory, along with Michael Flynn and Marc Cohen of NASA’s Ames Research Center, conducted an experiment funded by a grant from NASA to see how well human waste would work as radiation shielding. He and his colleagues couldn’t use real feces; instead they used a simulated poo made out of miso, peanut oil, propylene glycol, psyllium husks, salt, urea, and yeast. The goal was not to exactly duplicate the actual chemicals in feces; they wanted something roughly like it that held water and absorbed radiation and particles similarly.

They put it in a particle beam to see how well it absorbed the energy of flying protons. The beam was about as energetic as particles typically found in space. The fecal simulator absorbed a measurable amount of the energy, and the team found that the thickness matters. Too thin and the problem gets worse for the same reason that metals are bad shielding — the spaceborne particles make cascades. However, they were able to calculate that a fecal shield about 8 to 11 inches thick would cut down the radiation dose a lot. That was a good result, though Miller noted that the situation is more complex.

Remember, there are two kinds of radiation in outer space: the SPEs and the background radiation from cosmic rays. Cosmic rays carry five times as much energy as SPE particles do, and they’re the ones that can increase the risk of cancer. (NASA rules say the increased risk to astronauts shouldn’t be more than 3 percent above the general population.) The fecal simulator wasn’t as good at stopping those, but that was expected. “The energy of GCR is so high it will punch through just about anything,” Miller says. “So you try to balance getting the risk as low as reasonably achievable.”

You can’t simply put the feces in sealed bags or metal containers

Another issue is that you can’t simply put the feces in sealed bags or metal containers because the CO2 and other gases they generate could make them explode, absent some “breathing” mechanism as in Rethke’s vision of making fertilizer. So sterilizing the waste might be a good idea.

To do that, some proposed systems effectively burn the waste, without oxygen present, a process called pyrolysis. This also allows for more immediate use of the water. Advanced Fuel Research, a company in East Hartford, Connecticut, is exploring a variation called torrefaction (which takes less energy to do than straight-up pyrolysis). The waste gets heated to around 550 degrees Fahrenheit, (300 degrees Celsius). What’s left is something compact and dry, mostly carbon. At the same time it retains a lot of hydrogen.

Rethke notes one trade-off with pyrolysis or torrefaction is what to do with the leftover carbon. “If it’s a brick that’s one thing,” he says. “But powder is harder.” Remember there’s no gravity, so any particles are going to float around and could foul air intakes. So you’d need some way of compacting the carbon to store it.

Torrefaction has other challenges too, says Michael Serio, the president of Advanced Fuel Research. (He’s authored two papers on the subject, and has more work — involving bird and dog manure — forthcoming.) While some materials will reduce to ash, others won’t. Cotton, for example, contains hemicellulose, which doesn’t break down as well. “A cotton T-shirt would just look like a burned T-shirt,” he says.

One could just make all the waste into bricks, Serio says. You take all the garbage — food wrappers, human waste, everything — and heat it up enough to melt it into a brick. This reduces volume and detoxifies the waste. That’s good for making partial radiation shields or even, Serio says, bricks for a Martian (or Lunar) habitat. Serio is working with other companies to see if there’s a way to build some kind of heated recycling into a commode itself. The big challenge would be making it compact and fast enough so that it doesn’t put the toilet out of commission for extended periods.

These recycling technologies are all promising enough. Cohen, though, expressed some frustration at the way NASA has approached funding. Cohen, a co-investigator with Miller and Ray Flynn of Ames on the radiation shielding experiments, says there has been little development beyond simple demonstrators. NASA isn’t planning a Mars mission explicitly — the closest they’ve come is a road map. “There’s been such deep cutbacks it’s difficult to get anything funded,” he says.

Even so, NASA will have to come up with something if the agency is serious about going out of Earth orbit — even if only to return to the Moon. “What NASA would like is you drop a bag of poop into a canister — maybe process it right below the commode,” says Serio.

Rethke added that whatever system is in place also has to have built-in redundancy and some way to fix it. Natural bacteria, he notes, do a fine job of breaking stuff down, don’t need complex machinery to operate, use no electricity, and produce some very useful chemicals in the process. (Carbon dioxide, for instance, can be “burned” with hydrogen to make methane and water.) That’s one reason he likes natural biodegradation. “It’s all about how much power to use for reclamation, versus storage, versus the weight,” Rethke says. “I like to keep things simple.”

Correction: Due to an editing error, the word “each” was dropped from a sentence about how much poop a crew of six en route to Mars would produce; each astronaut would produce 300 pounds of feces — not 300 pounds total. We regret the error.