The Plastic Ceiling - Opportunities and Limitations of Microbial Carbon Recovery in Crewed Spaceflight

Abstract The International Space Station operates on an open resource model: consumables are delivered from Earth, used once, and disposed of by atmospheric re-entry or venting. While the water cycle is substantially closed and oxygen partially so, virtually all elemental carbon in food, clothing, and packaging is lost, a paradigm incompatible with the autonomy and planetary-protection demands of deep-space operations. Microbial biotechnology has repeatedly been proclaimed a route to carbon-loop closure, yet the recovery capacity of resupply-derived waste has never been quantified. Here, we deliver an empirical carbon-accounting record covering 25 years of stable ISS operations, over which logistics resupply carried approximately 87 metric tons of carbon to low Earth orbit. An accompanying Bio-Process Sizing Framework converts user-defined operational inputs into projected carbon-recovery capacities and required reactor volumes under the physical constraints of aqueous bioprocess design, cross-checking what physics permits against what extant biology delivers. Across every scenario the framework resolves, the ceiling is set by the composition of the waste stream: packaging polymers bound what biology can recover.