Engineers face an omnipresent but invisible enemy: microbial life in the cleanrooms where spacecraft are put together. All surfaces are scrubbed at scalding temperatures and soaked in disinfectant. The goal is simple: to ensure that no organisms on Earth end up elsewhere in the cosmos by accident.

But a new study published in Applied and Environmental Microbiology suggests that some organisms may be far more difficult to eliminate than previously believed. It shows that not only can some fungi survive sterilization, but they can also survive the complete round-trip to Mars.

The keys to solid planetary protection are as follows: how well they withstand sterilization, how well they can withstand conditions in space, and what happens when they reach other realms beyond Earth.

To do that, the scientists examined fungal strains collected from NASA spacecraft assembly cleanrooms, the most stringently cleaned spaces on our planet. Strikingly, 23 of these strains proved resistant to ultraviolet radiation, a major sterilization method used in the construction of space hardware.

One of the most resilient species was Aspergillus calidoustus.

The team used advanced simulation facilities to recreate the extreme environments of space flight and the Martian surface. They were blasted with powerful ultraviolet light similar to that from the Martian sun; exposed them to lower atmospheric pressures and a Majorly carbon dioxide-dominated atmosphere; put in contact with Mars, a replica of dusty surface soil called regolith; irradiated, which gave birth over an extended period using neutrons mimicking deep-space ionizing radiation, and a topic for sterilization tactics used on spacecraft.

The findings were striking. Under simulated Martian solar irradiation, the conidia, specialized asexual spores, of A. calidoustus persisted up to 1,440 minutes regardless of desiccation pressures (low pressure), a chemically distinctive atmosphere, and surface dust present on Mars.

Even more alarmingly, the fungus was also resistant to neutron radiation and to dry-heat microbial reduction procedures used to sterilize materials intended for launch.

But the fungus was not invincible. The two extremes would have to be combined, intense radiation and cooling, producing an equilibrium state at −60°C, about the average surface temperature on Mars.

This finding underscores an important lesson: No single stressor drives the death or survival of microbes; rather, it is combinations of multiple stressors or factors.

“This does not mean contamination of Mars is likely, but it helps us better quantify potential microbial survival risks,” said microbiologist and study leader Kasthuri Venkateswaran, Ph.D. “Microorganisms can possess extraordinary resilience to environmental stresses.”

He added, “Microbial survival is not determined by a single environmental stress but rather by combinations of stress tolerance mechanisms.”

Nevertheless, this work is the first complete analysis of eukaryotic microbial survival (including multicellular organisms such as fungi) throughout an entire Mars mission, as previous studies have focused only on bacteria.

This entails everything from pre-launch cleaning and preparation to exposure to the radiation-filled vacuum of space to contact with Martian environmental conditions during robotic exploration.

The chronic exposure used in their experimental design, alongside realistic simulation systems, has provided the researchers with a “biologically meaningful model” of long-term microbial risks during spaceflight.

This suggests a shortcoming of existing protocols for planetary protection. Most sterilization is developed and validated for bacterial spores, which have been the traditional hardest contaminants to kill.

Fungi, for their part, deserve just as much attention.

The resilience of fungal conidia to several space-relevant stresses indicates that they can act as forward contaminants, organisms transported via spacecraft capable of surviving on other worlds.

At the same time, this complication to the search for extraterrestrial life raises questions about whether Earth-based microbes would inadvertently alter an alien environment.

Furthermore, the study’s implications extend beyond Mars exploration. This ability of fungi to withstand conditions we would categorize as space-like may also make contamination control more difficult for industries that require tight sterilization, such as food production, pharmaceuticals, and health care.

Gaining insight into how species such as A. calidoustus thrive in extreme habitats could improve the safety of many sectors.

The field of planetary protection becomes more difficult as humankind pursues ever more ambitious plans to reach Mars and deeper into the solar system.

Journal Reference:

Atul M. Chander, David J. Burr, Severin Wipf, Rubern Nitsche, Gretchen Fujimura et al. Survival of NASA-cleanroom microbial isolates under simulated space and Martian conditions. Applied and Environmental Microbiology. DOI: 10.1128/aem.02065-25