L'Affaire Siloxane

In the early years of the International Space Station, water needed to keep the crew alive had to be delivered by Space Shuttle at a cost several times its weight in gold. By 2005, over 9,000 kilograms of the stuff had been flown up from Earth to keep astronauts hydrated, while a further 7,000 kilograms of treated urine were sitting in orbital storage tanks, waiting to be processed.In November 2008, the Water Processing Assembly arrived on the ISS to realize the great dream of space exploration: boiling astronaut pee. The 800 kilogram Urine Processing Assembly would help take the station from a 45% to 80% water reuse rate. For the first time in the history of space flight, astronauts would be substantially recycling their water in an orbiting habitat.In June 2010, thirteen months after the Water Processing Assembly went online, excessive levels of total organic carbon began to show up in the astronauts’ drinking water. Total organic carbon is a non-specific measurement that warns the crew about a contaminant being present, but gives them no clue to its identity.Graph of total organic carbon in reprocessed ISS water, 2010-11. The red arrow points to the safety limit of 3ppm.When the space station was being designed, NASA had set the safety limit for total organic carbon at 3 parts per million, based on a worst-case scenario where formaldehyde got into the drinking water. By summer, the weekly trend in organic carbon was rising steadily and on track to exceed this threshold in December. At that point, NASA would either have to send up fresh drinking water or bring the crew back home.There is no provision, then or now, for doing analytical chemistry on the space station. If you have a mystery substance, you need to put it into a returning Dragon or Soyuz capsule and wait for a lab on Earth to identify it. Astronauts and cosmonauts collect regular environmental samples, but whatever is in those samples only gets analyzed when that archive is brought down to Earth. So it wasn’t until September that a Soyuz capsule finally landed with the summer’s trove of water samples, which were quickly sent to the Food and Water Analysis lab in Houston.There chemists confirmed the total organic carbon reading, but to everyone’s surprise couldn’t identify a specific contaminant in the samples.

Whatever was in the water was not on the watchlist of several hundred chemicals that ISS engineers had anticipated might find their way into the station’s water system. In fact, the mystery substance wasn’t even in the lab’s vast reference library of mass spectra.It took colleagues at Boeing, working from a newer reference library, to identify the mystery contaminant as dimethylsilanediol, or DMSD.Dimethylsilanediol (C2H8O2Si), first of its name, destroyer of life support systems.DMSD belongs to a family of compounds called siloxanes, molecules that contain a silicon-carbon-oxygen bond and occupy a kind of middle ground between organic chemistry and beach sand. Siloxanes (also called silicones) are common ingredients in cosmetics1, contact lenses, fake boobs, caulks, packaging, and all kinds of personal hygiene products, where they’re used to make things feel smooth and slippery. It’s siloxanes that give deodorant and hair conditioner their slick texture, and the same property makes them a popular industrial lubricant.Manufacturers like siloxanes because they are cheap, stable, nontoxic, and unreactive, at least until they come into contact with something expensive aboard the space station. In ISS life support stories, siloxanes play the role of the meek character in an Agatha Christie novel who has been in the mansion the whole time, but who no one ever suspected had enough moxie to be the murderer. We will meet them again.In this early brush with siloxanes, NASA greatly underestimated the compounds’ appetite for inflicting havoc. To confirm that DMSD was the culprit in the months-long excursion in organic carbon, chemists in Houston synthesized a pure reference solution of the stuff to calibrate against. They were happy to find that their state-of-the-art gas chromatograph/mass spectrometer was sensitive to DMSD, showing strong and clear peaks in every ISS water sample they looked at.Unfortunately, the instrument also showed strong and clear DMSD peaks in everything else, including unrelated environmental samples from Earth and blank sample runs of distilled, deionized water. The chemists destroyed three expensive gas chromatographs before realizing that the tubing in their instrument was also made of siloxane.

Once injected, the DMSD would happily dissolve into the walls of the chromatography tube and stay there, contaminating every future measurement the instrument made.After devising an alternate analytical method that didn’t obliterate their lab equipment, the chastened chemists set to work figuring out how much of this DMSD stuff an astronaut could drink in a day without dying. They were still working on an answer when, to everyone’s surprise, total carbon readings on the space station dropped back to normal levels and stayed there, as if nothing had happened.Since that episode, there have been at least five more spikes in total organic carbon in the station’s drinking water, all traced to DMSD. By NASA logic, this has turned siloxane contamination from a critical anomaly to a familiar behavior that can be modeled and planned against. The agency even boasts about its fight against siloxanes as an achievement of the space station, which is a little like bragging that your clifftop mansion helped further humanity’s understanding of erosion by falling into the sea. Where do space siloxanes come from? Sleuthing has shown the main sources of siloxane vapor on the space station are antiperspirants, wet wipes, lotion, and leave-in hair conditioner. About a gram and a half of the stuff evaporates every day into the cabin atmosphere. There, helped by ionizing radiation from space, it decomposes to form the diol (DMSD), which is highly soluble in water. This compound collects in the water condenser, passes through the treatment chain mostly intact, and from there enters the clean water supply.The dramatic spikes in total organic carbon observed aboard the ISS turned out to be a buffering artifact2. A more chemically active substance would bind to the ion-exchange medium in the water filtration beds and stay there. But DMSD binds weakly and can be kicked out by basically anything else. When a filtration bed is first installed, DMSD will begin to accumulate on the fresh resin, with no sign of it in the output water. But after some months, when the filter medium has saturated with DMSD, other substances will start to displace it, creating the signature rapid rise in organic carbon. Once all the DMSD that collected in the filtration bed has eluted out into the water supply organic carbon readings again drop to near zero. If a filtration bed is replaced, the process repeats.

Along the way, DMSD costs the space station a fortune. Each year a set of replacement multifiltration beds (which weigh 50 kilograms and have a three year design life) must be flown up from Earth. NASA could try just ignoring the stuff. But there’s always a danger that DMSD-induced spikes in total organic carbon could be masking a rise in a different, more serious contaminant. And having this stuff in the output water does cause other problems. Siloxane has graduated to a known nuisance whose main effect is to shorten the life of the multifiltration beds in the water system and require the cabin heat exchanger (a 70 kilogram piece of metal) to be flown down annually to have its hydrophilic coating reapplied. What makes dealing with siloxanes difficult is that they’re so inert. It’s easy to get reactive contaminants out of the life support loop, but siloxanes pass lightly through most of the various filters and ion exchangers. The only thing they seem to like to react with is catalyst beds and a costly and delicate hydrophilic coating on that heat exchanger. And when they do finally react, they do so by depositing a layer of glass, very effectively killing any reactive surface. It was a combination of DMSD and dimethylsulfone (from astronaut urine) that did in the space station’s experimental Sabatier reactor after only 1,800 liters of throughput. After a fruitless search for some substance that might be able to sequester DMSD in water, NASA decided to attack the siloxane problem in the air phase, capturing the various siloxane vapors before they could hydrolyze into the diol and enter the water supply. In 2015, they replaced some of the rectangular HEPA air filters in the station with special siloxane-scrubbing filters packed with activated charcoal.While these new filters reduced the concentration of siloxane vapor in the air, they also led to a mold outbreak. After running the new filter system for two and a half years, NASA has had to retreat to a hybrid solution—the filters are now half charcoal, half HEPA. This keeps mold counts down while capturing at least some atmospheric siloxane. At present the agency is testing a new filtration system to put in front of the heat exchangers, to try to protect them, and continuing to try to cut down on siloxanes at the source level.

There are probably people at NASA now whose entire career has been built on siloxane control. But the status quo remains unsatisfying.It’s interesting to imagine how the siloxane story would have played out if it was first encountered on a mission to Mars. On the ISS, the initial rise in total organic carbon in 2010 came about 13 months after the station started recycling water. Assuming that every Mars-bound crew would be spending a few months on a shakedown cruise near Earth, that ten month mark would just about coincide with the ship’s arrival at Mars. There, the crew would put their spacecraft in a dormant state and move as a group to the surface habitat, with its own water recycling system, resetting the clock for the siloxane problem. A few months into their 17 month surface stay, organic carbon in the water would start to rise again, as DMSD started to elute from the ion exchange beds. Within half a year, the crew would face the difficult choice of whether to abort to orbit, swap out equipment, or accept the elevated levels of a mystery contaminant in their drinking water.Regardless of their choice, once they got back to the orbiting spacecraft, they would see the rise in organic total carbon readings there resume, from the initial DMSD spike that was interrupted by their descent to the Martian surface. From the perspective of the crew, the same ominous problem would seem to be following them from orbit to the Martian surface and back. If the astronauts had analytical equipment on board, like a gas chromatograph/mass spectrometer, the situation might grow even muddier. They would have trouble identifying the siloxane peak in their water samples, since it was not even listed in NASA’s reference database on Earth. And by the end of the hunt, enough DMSD would have dissolved into the tubing of their gas chromatograph to introduce a spurious signal to all future sample runs, causing endless potential confusion about its source. A common pattern in aviation accidents is that efforts to fix a non-fatal problem rapidly snowball into a life-threatening situation, putting the crew in an unfamiliar part of the flight regime and eroding situational understanding to the point where they start to make serious mistakes. It’s easy to imagine how the siloxane issue, though ultimately harmless, might have prompted some bad choices on such a high-stakes mission with low margins for error.