Space
Life Support And The Closed Loop Problem
Keeping people alive in space means recycling air and water rather than carrying them, and the difficulty of closing those loops sets the limit on long missions.
A crew consumes oxygen, water and food continuously and produces carbon dioxide, waste heat and waste. Every additional day of mission duration multiplies those quantities unless the loops are closed.
Mass is the reason recycling exists
Carrying consumables is straightforward for short missions and impossible for long ones, because the required mass grows directly with crew size and duration.
Recycling equipment has a fixed mass and power draw regardless of duration, so beyond a certain trip length it always wins.
Calculating that break-even point for each consumable is the central design exercise in life support architecture.
Carbon dioxide removal runs continuously
Exhaled carbon dioxide accumulates quickly in a sealed volume and becomes dangerous well before oxygen runs short, so removal is the most time-critical function aboard.
Systems use materials that adsorb the gas and are then regenerated, either venting it overboard or feeding it into a process that recovers the oxygen it contains.
Recovering that oxygen rather than discarding it is one of the largest levers available for closing the air loop.
Water recovery is the most closed loop in practice
Water is reclaimed from humidity in the cabin air, from washing and from urine, then filtered, distilled and treated to a standard suitable for drinking.
Recovery rates are high but never complete, since some water leaves in waste and some is consumed by chemical processes, so resupply remains necessary.
Reliability matters more than efficiency here, because the equipment must run for years with maintenance performed by the crew using it.
Food remains almost entirely open
Growing food in space at a scale that meaningfully offsets supply requires substantial volume, lighting power and crew time, and results so far are supplementary rather than substantial.
Experiments aboard orbital stations focus on plant biology and on crew wellbeing as much as on caloric contribution.
For missions beyond low orbit, food is therefore still carried, and its mass is a fixed cost that recycling does not reduce.
Contamination is the failure mode that matters
A closed environment concentrates whatever is in it, including microbial growth in water lines and volatile compounds released by materials and equipment.
Monitoring air and water quality, and designing systems that can be cleaned and repaired in flight, occupies as much engineering attention as the recycling processes themselves.
Materials are therefore selected partly for what they release into the cabin over years, and spares for the water and air systems account for a meaningful share of what resupply missions carry.
