At the European Astrobiology Meeting this week in Edinburgh, Scotland, Ralf Moeller from the German Aerospace Center gave a presentation about a novel sterilization method that could be used to kill bacteria stowing away on spacecraft sent to other planets. These “hitchhikers” are a critical concern for planetary protection—which seeks to avoid contaminating other worlds with terrestrial life as well as preventing possible alien organisms from reaching Earth on returning spacecraft.
The sterilization methods most commonly used today are based on ultraviolet irradiation and chemical sterilizing agents. No method is 100 percent effective, and large numbers of hitchhikers survive space travel, which Alberto Fairén and I recently pointed out in the journal Nature Geoscience in connection with planetary protection policies for Mars exploration. Current sterilization methods selectively kill certain microorganisms by exposing them to the kinds of environmental stresses that microbes would experience on Mars. In other words, organisms that wouldn’t survive on Mars anyway are killed before they leave Earth. And life that might survive on Mars would most likely also survive the sterilization measures.
Another common sterilization method—autoclaving or heating a spacecraft component or Mars soil sample beyond a temperature that organisms can survive—is either impractical or too expensive. It can’t easily be done with whole spacecraft, some instruments are heat sensitive, and it’s especially problematic for sterilizing samples returning from Mars.
Moeller’s proposal for sterilization—using low-temperature plasma (electrically charged gas)—is therefore a promising alternative because it occurs at a low temperature (even room temperature), does not involve toxic chemicals, and can be done within a minute or less. Moeller’s research group has shown that plasma sterilization is very effective at killing even the spores of the soil bacterium Bacillus subtilis. Many microbes, when they encounter detrimental environmental conditions, go dormant and form these hardy spores, which can become viable again when conditions improve. Not only does plasma sterilization neutralize active microorganisms, it also stops this revitalization of spores to a very large degree. The research presented in Edinburgh this week advances earlier work done by Satoshi Shimizu and co-workers on sterilizing spacecraft components.
If the technique can be applied to whole missions, it would be a big advance for planetary exploration and astrobiology.