Moss Survived Nine Months Outside the ISS — What That Means for Space Habitats

Researchers in the Tanpopo 4 mission attached spores of the moss Physcomitrium patens to the exterior of the International Space Station for 283 days (about nine months) to test whether Earth-based life can withstand real space conditions. When returned to Earth, the spores remained highly viable, offering the first direct evidence that bryophytes can survive long-term exposure to open space and still germinate.

What the Team Did

The experiment focused on spores kept inside their sporophyte capsules — a resilient life stage whose outer cell layers protect against drying, radiation and temperature swings. Laboratory tests showed these encapsulated spores were up to 1,000 times more resistant to ultraviolet (UV) radiation than brood cells, and retained strong germination after extreme-temperature trials (see results below).

Key Results

• Returned germination: Spores exposed on the ISS germinated at 86%, compared with 97% for control samples kept on Earth.
• Lab temperature resilience: 80% germination after 30 days at −80 °C and 36% after 30 days at 55 °C in controlled tests.
• Main hazard: UV radiation was identified as the largest contributor to reduced germination (about an 11% drop versus controls).
• Chlorophyll loss: All exposed samples — even those protected by UV filters — showed roughly 20% chlorophyll degradation, implicating intense visible and infrared (IR) light in unfiltered space as a likely cause.

What This Means

These results demonstrate that resilient plant life stages can survive long-term exposure to space. The experiment is notable for being the first confirmed return-to-Earth viability test for bryophytes after direct space exposure. Based on the pre-exposure baseline and the nine-month result, researchers built a simple survival model that extrapolates a potential survival time of roughly 5,600 days in space. However, the authors emphasize that this projection is speculative because it relies on only two data points and therefore carries substantial uncertainty.

Implications For Space Missions

Moss and other bryophytes are considered "pioneer" species: they can help form substrate and conditioners for future plant life. Their advantages for off-world habitats include tolerance for low light, compact growth habit, simultaneous oxygen production and CO₂ fixation, and potential roles in bio-regenerative life support systems on lunar or Martian bases. This experiment strengthens the case for including hardy species like moss in early-stage space agriculture and ecosystem-design experiments.

Limitations And Next Steps

While encouraging, the findings are preliminary. The survival model needs more intermediate time points and experiments that isolate specific factors (UV, visible/IR light, vacuum, temperature cycling, and microgravity) to quantify their separate effects. Future experiments should also test additional species, growth stages and protective strategies that might be used in habitats or in-situ resource utilization on the Moon and Mars.

Bottom line: Moss spores survived long-term exposure on the outside of the ISS with only modest reductions in viability, highlighting both the resilience of certain plant life stages and the need for follow-up research before relying on them for life support in space.