In 2015, Japan exposed bacteria outside the ISS for 3 years; some clumps survived, while thicker ones were estimated to last longer

In 2015, a team of Japanese scientists attached some dried clumps of bacteria to a panel and launched it onto the outside of the International Space Station. They left it there for three years. According to a study titled ‘DNA Damage and Survival Time Course of Deinococcal Cell Pellets During 3 Years of Exposure to Outer Space’ published in Frontiers in Microbiology, when the panel was eventually opened in 2018, the bacteria within the more compact clusters were found to be alive. The project was called Tanpopo, meaning "dandelion" in Japanese, and it was led by microbiologist Akihiko Yamagishi and his team from Tokyo University of Pharmacy and Life Sciences.

The bacterial species that was selected for this research was Deinococcus radiodurans. CNN has reported that scientists have given this organism a nickname: Conan the Bacterium. In the Guinness Book of World Records, it is listed as the most radiation-resistant life form, capable of surviving doses of radiation that are about 3,000 times higher than the amount that would kill a human being. It was first discovered decades ago in a can of meat that had been irradiated to sterilize it.

Why send bacteria into space on purpose


It wasn’t by accident, and that was the whole point of it. The scientists set out to investigate the controversial theory known as panspermia, which states that life can be transported from one planet to another by riding aboard microorganisms floating about. And if that’s possible, then life wouldn’t have to arise independently everywhere it exists. Earlier versions of this idea, called lithopanspermia, imagined microbes tucked safely inside rocks and shielded from the sun’s radiation as they floated through space. A 2013 study published in the journal Origins of Life and Evolution of Biospheres, by members of Yamagishi's laboratory, had already investigated the effects of heavy ion radiation, vacuum, and dramatic shifts in temperature on several Deinococcus species on Earth, and in it, they coined the term "massapanspermia" to describe bacteria surviving in space simply by forming a mass, or clump, rather than hiding inside rock.

What happened to the clumps after three years

To test this hypothesis, the researchers placed dry samples of Deinococcus bacteria in little wells made out of aluminum plates, making pellets of different thicknesses. These plates were then exposed to vacuum, radiation, and extreme temperature swings for one, two, or three years at a time on an exposure panel outside Kibo, the Japanese space station module. Within three years, all the clusters that were larger than 0.5 millimeters had at least some bacteria living within them. While the surface layer of cells had died out, in death it seems to have served as a shield, protecting the cells underneath from the worst of the radiation.

From survival data collected during year one, year two, and year three, the researchers extrapolated that a pellet just above half a millimeter in thickness could survive anywhere from 15 to 45 years on the ISS. Using that same survival curve, they separately estimated that a denser, roughly one-millimeter-diameter colony would have enough protection to endure the space environment for up to eight years. Both figures come from the study's own extrapolation of its three-year dataset, not a direct measurement, since the experiment itself only ran for three years.

What this means for the search for life on Mars

The timing is also consistent with something else. A trip between Earth and Mars through the shortest path can take anywhere between a few months and two years, which comfortably fits into the survival period that has been established in this research. This has significant consequences for how space agencies plan missions. If Earth bacteria can indeed survive such a journey, then spacecraft need to be thoroughly cleaned before launch to avoid the same risk. NASA's own planetary protection documentation notes that the Perseverance rover, which launched for Mars in July 2020, underwent extensive pre-launch cleaning for exactly this reason, though a sterilized, engineered spacecraft is a different scenario from microbes surviving naturally on their own, which is what the Tanpopo results speak to.

An open question, not a closed case

None of this proves that panspermia actually happens. Survival within an artificial device mounted on the space station is worlds away from making a real interplanetary journey, which involves not only traveling in space but also safely launching and landing gently on another world without burning up or shattering on impact. However, the findings add a new piece to an old puzzle. The Tanpopo findings suggest that if life ever did make that trip, it might not need a rock or a spacecraft to shield it, clinging to itself may be enough.