For over 1,000 Earth days, a car-sized rover named Perseverance has been driving around the surface of Mars. It was designed to study Jezero, a crater roughly 28 miles in diameter and thought to have once been full of water. The rover, equipped with a drill and several test tubes, collects samples from the atmosphere, rocks and soil that best represent the site’s geologic diversity.
For now, Perseverance is caching these test tubes either on the Mars surface or inside its rover body. Researchers at NASA hope to bring those samples back to Earth one day and look for signs of ancient life.
Upon sample return, however, any extraterrestrial microbes that may have hitched a ride could become new pathogens on Earth. NASA already has protocols for sterilizing specimens that come from outer space; humans have previously brought back samples from the Moon, four asteroids, solar wind atoms and the tail of a comet.
Spacecraft headed towards new celestial worlds also must be cleansed of Earthly microbes, which could quickly spread and jeopardize the search for extraterrestrial life. As astronauts set their sights on returning to the moon and going even farther, finding a sterilization method that is faster, less costly, less cumbersome and not too harsh is more important than ever.
But these existing methods have plenty of room for improvement and will not be appropriate for crewed missions that might take place in the future. Current technologies include dry heat and vaporized hydrogen peroxide, but both have demonstrated damage to sensitive equipment.
One of the people working to address that challenge is Támara Revazishvili, a researcher at the University of Florida Emerging Pathogens Institute (EPI) who is also a potential recipient of Mars material if NASA decides to invest in a sample return mission. She is leading EPI’s partnership with SurfPlasma, Inc., a tech company with an idea for space sterilization that has received funding from NASA.
Their device, called an Active Plasma Sterilizer (APS), looks like a simple box – but it’s far from it. It uses plasma to generate its own ozone, which causes an oxidative burst upon contact with a microbe’s outer surface, creating a hole. Riddled with enough holes, the microbe loses its surface shape and dies.
“It works well because it penetrates everywhere without breaking anything or damaging sensitive equipment,” Revazishvili said.
The APS consists of Compact Portable Plasma Reactors (CPPRs), which are distributed throughout the APS so that the generated ozone can easily flow without needing a separate mixing device. This allows decontamination to happen more quickly with less ozone and, crucially, helps the device stay portable.
In EPI’s laboratories, Revazishvili measures the APS’s effectiveness at killing microbes. She recently completed the first phase of testing and published the results in the journal Nature. The study, which was designed with a potential Mars sample return mission in mind, used Bacillus subtilis and Escherichia coli as test organisms.
E. coli was chosen for its ability to reproduce even in unfavorable conditions, while B. subtilis was chosen because it can form spores. That ability allows microbes to spread far and wide, which could be particularly devastating if an outbreak occurred on a new planet. Both B. subtilis and E. coli are found in soil and vegetation, and both can contaminate food.
“They are concerned about the health of the astronauts. Not just the equipment,” Revazishvili said. “Many of the pathogens we test have been found on their clothes before.”
They successfully sterilized E. coli and B. subtilis on four different materials common on space missions—aluminum, polycarbonate, Kevlar (material used for astronauts’ spacesuits) and Ortho-fabric (material used for astronaut suits)—within 30 minutes.
In the next phase of testing, Revazishvili will look at three new pathogens. The first is Geobacillus stearothermophilus, which she described as the gold standard for showing the effectiveness of sterilization. They have an additional membrane, which allows them to survive for many years and makes them difficult to kill. Any technique that manages to kill 100% of G. stearothermophilus is very effective at sterilizing, Revazishvili said.
She will also look at Aspergillus fumigatus, a fungus found on Earth that is typically resistant to most treatments. Two things that are capable of killing it, Clorox and radiation, would be dangerous to use in a closed spaceship and on a crewed mission. A. fumigatus disperses countless spores into the air each day, making it a good candidate for testing the APS’s ability to sterilize fungus.
Finally, Revazishvili will test Deinococcus radiodurans, a Gram-positive bacterium known as one of the most radiation-resistant organisms to exist. It has even been found in nuclear reactors and, in August 2020, was shown to have survived for three years in the International Space Station.
There are still many years to go before a possible Mars sample return mission. But so far, results point to APS as a promising option to help sterilize spacecraft and safely bring extraterrestrial material down to Earth.
Written by: Jiayu Liang