By The Artful Age Editorial Team, 2 September 2026
A patch of human skin the size of a pencil eraser carries roughly a million bacteria. That number holds whether the skin belongs to a toddler, a grandmother, or an astronaut sealed inside a spacesuit on the way to the Moon. And it is the reason NASA researchers reported in August 2026 that the next crewed landing near the lunar South Pole may leave behind something more durable than bootprints — a thin biological signature vented from the suits themselves, settling into shadowed terrain where temperatures sit below minus 200 degrees Celsius.
The concern is not science fiction. It comes from planetary protection researchers looking at a simple problem. Spacesuits leak. Human skin sheds constantly. The Moon’s polar craters are among the coldest, most stable places in the inner solar system. Put those three facts next to each other and a question falls out: how long could a hitchhiking microbe last in a shadow that has not seen direct sunlight for an immense span of geological time?
The million-microbe eraser
Start with the skin itself. The human epidermis is a landscape of ridges, oil wells, and sweat vents, and a peer-reviewed review of the skin microbiome puts the density at roughly one million bacteria per square centimetre — about the area of a pencil-eraser tip. That is an average, not a constant. Culture-based counts across the body span several orders of magnitude, from a few thousand per square centimetre on dry forearm skin to the full million in humid folds. The oily forehead runs hotter than the dry forearm. The damp crease behind the knee is its own city.
Writing in Forbes in August 2026, Rutgers evolutionary biologist Scott Travers put the total skin population on the order of a trillion bacterial cells, spread across roughly two square metres of surface, before counting the fungi, mites, and viruses. Most of them are harmless. Many are helpful — Staphylococcus epidermidis, Cutibacterium acnes, various Corynebacterium species — organisms that outcompete pathogens, tune the immune system, and keep the skin’s pH slightly acidic.
The bacteria do not sit still. People constantly shed skin cells, and each of those flakes carries passengers. In a sealed room, the shed accumulates in the air and on surfaces. In a spacesuit, it has nowhere to go except the suit itself — and, eventually, the suit’s vents.
Why the ratio matters
For decades textbooks repeated a striking claim: bacterial cells outnumber human cells ten to one. That figure was revised downward to roughly 1:1 by Ron Sender, Shai Fuchs and Ron Milo of the Weizmann Institute of Science, writing in PLOS Biology in August 2016. Their estimate for a 70 kg reference adult: about 38 trillion bacterial cells alongside about 30 trillion human ones. The ratio wobbles depending on the hour of the day, the meal just eaten, and, yes, whether the person has recently visited the bathroom.
For lunar mission planners, the exact ratio matters less than the raw output. A single astronaut on an eight-hour surface excursion is a mobile source of billions of microbial cells. Some die from desiccation inside the suit. Some are trapped by filters. But the suits are not hermetically sealed — they cannot be. They must vent carbon dioxide, moisture, and heat. Anything venting out carries the air the astronaut has been breathing and shedding into.
What NASA is actually worried about
The specific concern is the Artemis programme’s planned landings near the lunar South Pole. The region is scientifically valuable precisely because of its permanently shadowed regions, or PSRs — crater floors tilted so their bottoms never receive direct sunlight. These floors reach some of the lowest temperatures in the inner solar system, cold enough to trap and preserve volatile compounds.
A team led by planetary scientist Prabal Saxena at NASA’s Goddard Space Flight Center put numbers on the risk. Writing in Science Advances on 19 August 2026, the researchers modelled five human-associated organisms — three bacteria and two fungi, all of them previously recovered from spacesuits, payload capsules, or the exterior of the International Space Station — against high-resolution temperature and illumination maps of three Artemis III candidate regions: Nobile Rim, Connecting Ridge, and De Gerlache Rim. Their framing of the problem is precise: microbial survival between consecutive spacewalks has the potential to affect scientific operations by increasing the organic contamination baseline in exactly the places Artemis most wants to sample.
The survival numbers are shorter than the science-fiction version and more awkward than the reassuring one. All five microbes could survive for at least 24 hours in certain areas of the polar terrain. Aspergillus niger, a mould familiar from damp bathrooms and HVAC ducts and the most UV-resistant organism in the set, held out for around a week in roughly three per cent of the mapped ground, and tolerated some sunlit exposure. The survivable pockets the team mapped range in scale from a crater floor kilometres wide down to the shadow cast by a single boot print.
Two caveats matter. The study modelled survival, not growth: nothing in the lunar polar environment is expected to let these organisms reproduce. And the shadowed floors are not perfectly dark. The team’s ray-tracing showed that light scatters off crater rims and boulders, so even permanently shadowed ground receives some indirect ultraviolet — which cut the survivable area inside one modelled PSR near De Gerlache.
The venting problem
Spacesuits work by maintaining a pressure differential. Inside: a controlled oxygen atmosphere at reduced pressure. Outside: hard vacuum. To keep the astronaut cool, the suit’s life support system manages thermal regulation through sublimation, venting waste heat into space. That vapour plume carries whatever aerosols happen to be riding the air inside — including skin flakes, respiratory droplets, and the microbes attached to both. Airlocks do the same thing on a larger scale every time a crew cycles through.
The numbers are small per minute but nontrivial over a moonwalk. Multiply by a crew of two, several excursions per mission, and multiple Artemis flights to overlapping candidate sites, and the cumulative biological deposit near a landing zone starts to look measurable. That accumulation, rather than any single hardy organism, is what the contamination baseline is meant to track.
The organisms most likely to persist are the ones already selected for toughness. Bacillus species form endospores — dormant, dehydrated cells with thick protein coats that shrug off radiation, desiccation, and extreme cold. Spores have survived years of exposure to the space environment aboard orbiting platforms. A crater floor shielded from most direct solar UV is a gentler environment than the exterior of a spacecraft in low Earth orbit.
Why skin bacteria travel so well
Skin microbes are unusually good at surviving hostile conditions because the skin itself is a hostile condition. It is dry, salty, mildly acidic, and periodically doused in sebum and sweat. The species that thrive there have robust cell walls and stress responses built for temperature swings, osmotic shock, and long stretches without nutrients.
Add to that the fact that skin bacteria are constantly being displaced. A ZME Science report on hand dryers collects the evidence on how readily particles come off wet skin and into circulating air: one 2015 study found jet dryers can disperse 190 times more virus particles than paper towels, flinging them as far as three metres. A spacesuit’s ventilation system is a milder version of the same physics: airflow across skin, particles lifted into circulation, some fraction eventually reaching a vent.
The pattern is not new to space biology. Where humans go, microbes go with them.
The extremophile precedent
Earth offers plenty of evidence for how far this can be pushed. Bacteria have been recovered from deep ocean trenches, acidic hot springs, ancient salt deposits, and radiation-intense industrial sites. Microbes have shown remarkable tolerance for conditions that would destroy most complex life.
None of those environments quite match a lunar PSR, which combines vacuum, cryogenic cold, and radiation exposure at the crater rim. But the trend line is clear. Every time researchers assume a limit, some organism turns up past it — which is precisely why the Goddard team argued the Moon could double as a natural laboratory for testing survival limits that are hard to reproduce on Earth.
What makes the lunar case different is the contamination direction. On Earth, extremophiles evolved in place. On the Moon, they would arrive as immigrants — carried, unintentionally, by the humans studying the very ice deposits that make the South Pole scientifically interesting in the first place.
Why the ice complicates things
The permanently shadowed craters are an Artemis science target because they contain water ice, potentially ancient, mixed with volatiles that could reveal how the inner solar system got its water. That ice is also the resource future missions hope to mine for drinking water, oxygen, and rocket propellant.
Human organic material in that setting creates two problems at once. First, it corrupts the science — any organic signature detected in a sample becomes suspect. Was it native to the Moon, delivered by a comet four billion years ago, or shed by a boot seal last week? Andrew Needham, the Artemis contamination-control scientist for lunar samples at Goddard and a co-author on the study, framed the point as a matter of establishing what was there before we arrived, because the same question will be asked far more sharply at Mars. Second, if polar ice is ever melted and processed for life support, anything dormant inside it enters the habitat’s water system.
The Committee on Space Research classifies the Moon as a Category II body for planetary protection — meaning documentation of contamination is required but sterilisation is not, with enhanced documentation expected for the polar regions and PSRs specifically. That framework was written when the Moon was assumed to be biologically irrelevant. The polar ice discoveries of the past two decades have started to strain the assumption.
The scale of a bootprint
Apollo astronauts left roughly 96 bags of human waste, food packaging, and hardware on the lunar surface between 1969 and 1972. Those bags have been sitting in near-vacuum, cycled between roughly 120 Celsius in daylight and minus 170 Celsius at night, for over half a century. Whether anything inside them is still viable is one of the questions a future sample-return mission might answer. Researchers have proposed retrieving one to find out.
An eraser-sized patch of skin, one million bacteria. A gloved hand brushing a suit seal, thousands of shed cells. A four-hour moonwalk, an unmeasured but non-zero cloud of vented aerosol drifting downslope toward the coldest sink on the surface. The bookkeeping adds up quickly, and it adds up again on the next excursion.
The astronauts themselves are not the problem. The million-per-eraser figure is a feature of being alive, not a failure of hygiene. Skin without its microbial community is skin that gets colonised by something worse. The same population of skin microbes that concerns lunar planners is what keeps the astronaut’s own body functioning during the flight.
What the shadowed craters actually look like
Picture a bowl carved into rock, with a rim tall enough that the Sun — which barely rises above the horizon at these latitudes anyway — never crests it. The floor is dust, boulders, and, in patches, frost. There is no wind. There is no weather. A footprint pressed into that dust could last, geologically speaking, for a very long time. From orbit, these regions register as black voids against the sunlit terrain: patches on the Moon that no camera has ever seen in natural light, because the light does not reach them.
Radar and neutron spectrometers have mapped hydrogen concentrations across the polar terrain, indirect evidence of ice. The Lunar Crater Observation and Sensing Satellite confirmed it in 2009 by crashing a spent rocket stage into Cabeus crater and reading the vapour plume. The results were unambiguous: water, and along with it, methane, ammonia, hydrogen sulfide, and other volatiles frozen into a chemical record older than most terrestrial rock.
The strange arithmetic of contamination
Sterilising a spacecraft is possible. The Viking landers were baked at 112 Celsius for 30 hours before launch in the 1970s, and every subsequent Mars lander has followed some version of the protocol. Sterilising a human is not possible. The microbiome is not a coating that can be scrubbed off — it is a living part of the organism, embedded in pores and follicles, replenishing itself hourly.
Which leaves engineering and sequencing. Better suit seals. Higher-efficiency vent filters. Sample-collection protocols that assume the sampler is itself a contamination source. A characterised baseline of what the crew brought, taken before the first sample is lifted, so that later detections can be sorted rather than argued over. Some planetary protection scientists have suggested reserving particular PSRs for robotic study until the most contamination-sensitive science is done.
The alternative is not a Moon quietly seeded with immortal spores. It is something less cinematic and harder to undo: a growing organic background at exactly the sites chosen for their pristineness, laid down a vent plume at a time, until the question of what is lunar and what is ours stops having a clean answer. The organisms themselves may only last days. The ambiguity they leave behind lasts as long as the samples do.
The eraser on a pencil is a small object. Held up against the black interior of a polar crater, it is smaller still. But it carries a million passengers, and Artemis is going to make that trip more than once.
See: Original Article
