The rooms where spacecraft are built are among the most sterile environments on Earth. Workers enter through airlocks, wear full-body suits, and the air itself is filtered to remove particles measured in fractions of a micron. For decades, NASA’s planetary protection teams have scrubbed these facilities with extraordinary care, trying to ensure that nothing living hitches a ride to another world. And yet. Inside the cleanrooms used to assemble hardware for the Mars 2020 mission, researchers kept finding fungi. Resilient, persistent, apparently unbothered by conditions designed to kill them.
Now a study published in Applied and Environmental Microbiology suggests at least one of those fungi, a species called Aspergillus calidoustus, might be capable of surviving not just the cleanroom decontamination process but the entire journey to Mars itself.
Researchers led by microbiologist Kasthuri Venkateswaran, formerly of NASA’s Jet Propulsion Laboratory, took 27 fungal strains isolated from the Mars 2020 assembly facilities and subjected them to a comprehensive battery of simulated space horrors. Intense ultraviolet radiation. The thin, carbon dioxide-rich Martian atmosphere at roughly 6 millibars (compared to Earth’s 1,013). Temperatures dropping to minus 60 degrees Celsius, the mean annual surface temperature of Mars. Six months of neutron radiation mimicking the galactic cosmic rays that pummel any spacecraft on an interplanetary cruise. And the standard dry-heat sterilisation protocol that NASA uses on spacecraft components. Most strains failed quickly. A. calidoustus did not.
A Mission Simulation, End to End
What makes this study unusual is its end-to-end design. Previous work had exposed fungi to one or two space-relevant stressors. This is the first to walk a single organism through every stage of a Mars mission, from the assembly floor to the surface. “Microbial survival is not determined by a single environmental stress but rather by combinations of stress tolerance mechanisms,” Venkateswaran said.
The results were, to put it mildly, uncomfortable reading for planetary protection specialists. A. calidoustus survived 24 hours of continuous Martian solar ultraviolet irradiation, emerging with cell counts reduced by perhaps three orders of magnitude but not eliminated. It survived six months of neutron radiation equivalent to roughly 775 days inside a deep-space spacecraft, retaining about 43% viability. Under NASA’s standard dry-heat reduction protocol at 125 degrees Celsius, two hours were needed to cause a three-log reduction in viable cells; at 150 degrees, five minutes finished the job, but NASA guidelines typically operate between 110 and 126 degrees. Lethal? Only when the researchers combined Martian ultraviolet irradiation with cooling to minus 60 degrees simultaneously. That combination killed A. calidoustus completely.
“This does not mean contamination of Mars is likely, but it helps us better quantify potential microbial survival risks,” Venkateswaran said. The finding, he added, is partly a diagnostic tool: “Together, these investigations help refine NASA’s planetary protection strategies and microbial risk assessment approaches for current and future space exploration missions.”
What the Spores Actually Looked Like
The fungus’s resilience is carried in its conidia, the asexual reproductive spores that Aspergillus and related moulds produce in vast quantities. Conidia are not spores in the bacterial sense, but they share some of the same virtues: tough outer walls, low metabolic activity when dormant, the ability to survive desiccation. Under scanning electron microscopy, the researchers could watch what various Martian conditions did to A. calidoustus conidia in real time. UV irradiation caused sporadic ruptures and surface irregularities. The Martian atmosphere added deep pitting and scarring. The combined assault left only fragments. And yet a measurable fraction of conidia pulled through conditions that should, on paper, be universally lethal. The Martian regolith (the loose surface soil) added an odd wrinkle: mixed with conidia, it seemed to buffer some of the atmosphere’s toxicity, though it offered no protection against radiation. Researchers reckon this may reflect the thermal conductivity of fine soil particles rather than any protective chemistry, but the mechanism isn’t fully resolved.
A Gap in Planetary Protection
The study’s broader implication is for how NASA and other space agencies think about bioburden. Current planetary protection protocols quantify cleanliness by counting aerobic bacterial spores, particularly strains of Bacillus. The logic was reasonable: Bacillus spores are tough, well-studied, and easy to enumerate. But this approach rather assumes that if you’ve killed the bacteria, you’ve killed everything else. The new findings suggest that assumption deserves scrutiny. In the Mars simulation experiments, A. calidoustus outperformed Bacillus pumilus SAFR-032, the gold-standard space-resistant bacterium, under several conditions.
There’s also a terrestrial dimension here that goes beyond astrobiology. Aspergillus species are already known to cause respiratory infections in immunocompromised patients, and A. fumigatus (a close relative, also isolated from the International Space Station’s HEPA filters) became notorious during the COVID-19 pandemic as a secondary infection in severely ill patients. Members of the genus have previously been shown to survive pasteurisation and heat treatments used in food safety and pharmaceutical manufacturing. The possibility that A. calidoustus-level resistance exists in strains contaminating clinical or food production environments is, perhaps, worth worrying about.
The researchers are careful not to overstate what they’ve found. “Microorganisms can possess extraordinary resilience to environmental stresses,” Venkateswaran said, and that cuts both ways: the existence of a tough fungus in a cleanroom doesn’t mean Mars is about to be colonised by Earth moulds. Mars’s surface is not merely cold and irradiated; it’s also drenched in reactive perchlorate compounds that would likely destroy organic molecules over time. Whether A. calidoustus could actually establish itself rather than just survive in transit remains entirely unknown.
But planetary protection has always been about quantifying risks that may be vanishingly small yet carry consequences that are permanent. Contaminating Mars with Earth biology before we’ve had a chance to look for Martian biology would be, to use the technical term, a catastrophic mistake. What this study offers is a more complete picture of what might actually slip through. Fungal conidia, it seems, deserve a place on that list. The cleanroom protocols written for bacteria may need a rethink, and fairly urgently, given that Mars Sample Return missions are already in planning. The question is not whether A. calidoustus could survive the journey. It probably could. The question is what we intend to do about that.
DOI: https://doi.org/10.1128/aem.02065-25
Frequently Asked Questions
Could a fungus from Earth actually colonize Mars if it arrived on a spacecraft?
Surviving the journey and actually establishing on Mars are very different challenges. A. calidoustus can endure the radiation, cold, and low pressure of simulated Martian conditions, but Mars’s surface is also saturated with reactive chemicals, particularly perchlorates, that would likely destroy organic molecules over time. Whether any Earth organism could reproduce and persist in that environment remains unknown; what this study shows is that the fungus could potentially arrive alive, which is what planetary protection teams most need to plan around.
Why do NASA’s current sterilization methods focus on bacteria rather than fungi?
The bacterial-spore standard was established because Bacillus spores are well-characterised, extremely resistant, and straightforward to count and enumerate. For decades, killing Bacillus was treated as a reliable proxy for killing everything else. The new research suggests that logic has a gap: A. calidoustus not only survived the standard dry-heat regime used on spacecraft components but outperformed the benchmark bacteria under some simulated Martian conditions, implying that fungi have been underestimated as a contamination risk.
Is the same type of fungus a health risk to people on Earth?
Aspergillus species, the genus A. calidoustus belongs to, are already recognised as pathogens in immunocompromised patients, and became prominent during the COVID-19 pandemic as a source of secondary lung infections. The study notes that Aspergillus can also survive heat treatments used in food safety and pharmaceutical manufacturing. Whether A. calidoustus specifically poses a clinical risk at meaningful scale is a separate question, but the genus’s tolerance to sterilization processes is relevant well beyond space exploration.
How did researchers simulate conditions on Mars in a laboratory?
The team built a compact chamber fitted with a magnesium fluoride viewport that transmits ultraviolet wavelengths down to 120 nanometres. Inside, a solar simulator replicates the Martian UV spectrum, a custom gas mixture of mostly carbon dioxide recreates the thin Martian atmosphere at around 6 millibars, and a liquid-nitrogen cooling system brings temperatures to minus 60 degrees Celsius. Samples dried onto spacecraft-grade aluminum discs were then exposed to these conditions in various combinations, some for up to 24 hours, making it one of the most comprehensive Mars simulation experiments conducted on biological samples.
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