On Earth, “rot” refers to the biological decomposition of organic matter. This process is driven by living organisms, such as bacteria, molds, and yeasts, which break down food into simpler compounds, causing spoilage. The unique and hostile conditions of space, however, fundamentally alter the environment required for this type of biological decay to occur.
What Causes Food to Rot on Earth?
Food spoilage is a microbiological process requiring a specific set of conditions to thrive. Microorganisms multiply by consuming the organic material in the food. They require a sufficient level of moisture, measured as water activity, to support their growth and metabolism. High water content makes foods like fresh meat and produce highly susceptible to rapid spoilage.
Microbial growth also depends on temperature, with most spoilage organisms flourishing at warmer, room temperatures. Neutral pH levels, common in many meats, also favor bacterial growth. Preventing rot on Earth involves methods like refrigeration or dehydration, which remove these necessary conditions for microbial life.
The Lethality of the Space Vacuum
The environment of open space, outside the protective hull of a spacecraft, provides conditions that are lethal to the microorganisms that cause rot. Space is a near-perfect vacuum, which immediately causes any exposed water within the food and the microbes to boil and vaporize. This extreme loss of pressure and moisture results in the rapid desiccation, or drying out, of the biological material.
The immediate removal of moisture stops all biological processes, effectively halting microbial activity. The lack of an atmosphere also means that temperatures fluctuate wildly, from intense heat in direct sunlight to deep cold in the shade. This rapid cycling, combined with the vacuum, prevents any sustained biological function, ensuring that the agents of decay cannot survive long enough to cause true rot.
Physical Changes: Sublimation Versus Biological Decay
When food is exposed to the space vacuum, biological rot is replaced by sublimation. Sublimation is the physical process where a solid, such as ice, turns directly into a gas, bypassing the liquid water phase entirely. Because the space vacuum is intense, any frozen water within the food immediately sublimates away, similar to how freeze-dried foods are created on Earth.
This vacuum-induced freeze-drying transforms the food into a sterile, dry, and often brittle husk. The food does not decay biologically; instead, it becomes a preserved object, retaining its structure but losing its moisture. The resulting material is chemically and physically altered, but microbial rot, like mold growth or putrefaction, is absent.
The Controlled Environment of the Space Station
Within a pressurized and temperature-controlled habitat like the International Space Station (ISS), the potential for rot returns because Earth-like conditions are recreated. The ISS maintains a stable internal environment with acceptable pressure, temperature, and humidity for the crew, which allows microorganisms to exist. Consequently, if fresh food were not properly handled, it could spoil just as it would on Earth.
To mitigate this risk, the majority of space food is pre-processed using preservation techniques such as freeze-drying, thermostabilization, or irradiation to give it a shelf life of at least one year. While stored food is highly stable, a limited supply of fresh produce is occasionally sent up. This fresh food must be consumed quickly due to the lack of dedicated refrigeration. Instances of spoiled food packages have been recorded on the station, confirming that microbial decay is possible when the necessary environmental factors are present.