§ 6.6 Ecological Issues and CELSS
To date, when humans have gone to space, they have brought with them all the air they needed to breathe, their water and their food. All wastes created were either flushed into space or returned to Earth in their original form. (The water astronauts drank was often a byproduct of electric power generation by chemical means - hydrogen-oxygen fuel cells.)
Gaseous wastes were recycled by machines -- carbon dioxide was processed to produce oxygen, by "physical-chemical" processes.
In order to become self-sufficient in space -- independent from Earth -- we will need to grow our own food in space. We can use machines to recycle urine and water vapor in the air to produce drinkable water, but it will eventually become more desirable and economical to recycle our human wastes naturally rather than only by machines, and to do so naturally in conjunction with food production. Machines would be used only to sterilize and purify water that has already been cycled through the artificial biosphere.
On Earth, animals breathe in oxygen (O2) from the air and breathe out carbon dioxide (CO2) as a waste. Plants absorb this carbon dioxide from the air, and using the energy of sunlight plus water and materials from the soil and air produce sugar, starch and other things -- based on a process called photosynthesis. Plants emit oxygen as a waste. That completes the animal-plant cycle. In this cyclic manner, animals and plants are mutually dependent upon each other. Plants produce both food and oxygen for animals. In turn, animals produce carbon dioxide for plants. In addition, animals produce excrement wastes which enrich the soil. Dead plants also enrich the soil and are not wasted.
This natural cycle can be moved to space, in whole or in part.
Early experiments in the 1950s and 1960s focussed on recycling air using algae, not food crops. Flat tanks of algae were put under artificial light in order to absorb carbon dioxide that humans had exhaled in closed chambers, and emitted the oxygen for the humans to breathe. It was found that each human required about 8 square meters of algae for equilibrium. (The algae tanks were generally stacked as shelves so that they took much less than 8 square meters of floor space.)
More recent research has expanded this to include production of edible food, and recycling of human excrement wastes and dead plant wastes in the food cycle.
In the early years of space colonization, we will use a combination of natural systems and machines. We can always import pure oxygen and water from asteroidal materials, as well as carbon dioxide if we wish. It's not necessary to produce a completely closed system. However, it is important to maintain healthy and highly productive crops, which requires waste management and recycling skills.
The technologies required may be broken down as follows:
This field of study -- regenerative life support systems -- is called "Controlled Ecological Life Support Systems (CELSS)" (also called Closed... instead of Controlled... though "closed" is probably not attainable for awhile). There is a wealth of information from various institutions around the world on this topic, including papers presented at conferences dealing with lunar and asteroidal materials utilization.
CELSS is the focus of most of the rest of this webpage.
Note on the word "biosphere"
In some circles, the word "biosphere" is used instead of CELSS to refer to large closed systems. However, just as frequently, the word biosphere refers to Earth or one of Earth's ecosystems, not to space based CELSS systems. For example, if you search databases for the word "biosphere" you will get a lot of hits on remote sensing of the environment on Earth by NASA satellites, and the Mission to Planet Earth (MTPE) program. But you will also get hits on Biosphere 2, Bios-3, and Biosphere J, all CELSS experiments for space colonization.
A better database search word is "CELSS" (for Controlled Ecological Life Support Systems ... or alternatively Closed Ecological Life Support Systems).