Friday, November 6, 2009

Farming in Space

In previous posts I’ve described plans for space habitats which include allowances and techniques for the closed system recycling we will need to establish self-sustaining life in space.

This post describes in some detail just what is needed, and where I found the information. Fundamentally, we will need to provide recycling for nearly everything in space.

Humans breathe oxygen which is plentiful in the form of oxides and silicates, but rare as the free element in space. We each need a bit less than a kilogram of oxygen per day (0.83kg on average, more under high work loads). Note that processes such as smelting metallic ores use heat and a reducing agents such as carbon to turn metal oxides into the metal plus oxides such as CO2. Green plants turn the CO2 into carbon (or carbohydrates), freeing the oxygen, and the carbon can be fed as raw material into the smelter. The only net products of smelting are metal (such as iron) and oxygen.

Humans consume water for drinking, food growth and preparation, cleaning, even entertainment. Since hydrogen is in short supply (until we can gather and use the resources of a comet, in which case we’ll have more than enough to throw away), we’ll need to carefully recycle water. Each human drinks or eats about 2.6 kg of water daily; more is needed for hygiene and growing crops, a lot more.

Working humans need to eat, on average, about 2500 Calories per day (some estimates suggest 2000, my wife’s diet suggests 1500). This can be provided by about 540 grams (dry weight) of food (50g protein, 70g fats, 420 carbohydrates).

The total consumption of a human is about 4 kilograms of oxygen, water, and food per day, and it should come as no surprise that that the same human produces 4 kilograms of waste (in the form of carbon dioxide, exhaled water, sweat, urine, and feces) per day.

Also unsurprising is that green plants (such as algae) can take those 4 kg of wastes and produce the needed food and oxygen with only the addition of energy in the form of light.

The complications arise from the need for a balanced, nutritious, and tasty diet containing all of the essential amino acids and fatty acids, and from the fact that we do not digest all portions of plants. Cellulose (fiber) is undigestible,  and we don’t even attempt to eat most of the plant material of a crop (stems, roots, leaves, bark).

The ecosystem we create in space must be perfectly balanced. When plants produce food for us to eat, they simultaneously produce the exact amount of oxygen needed to metabolize that food. But they also produce those stems, roots, leaves, etc., and excess oxygen to match. All of that extra plant matter must be fed to some other animals (such as rabbits or goats), or to fungi, or to bacteria, or burned. The excess CO2 must be captured and fed back to the growing plants of the next crop, because we don’t exhale enough CO2 to feed the plants, only enough to grow the food we ate – a fraction of the total plant material.

Note that we can meet all of our dietary needs by growing a variety of algae such as blue-green algae including spirulina, and chlorella.

Somewhat surprising is how little water is needed to grow adequate volumes of algae – as little as 6 to 10 liters per person. This is due to the extremely high growth rates of alga under optimal conditions.

However, an algae diet is not only boring, it doesn’t taste good. It is likely to only be used for long space missions where space and payload is at a premium, and even then at least 4 varieties must be cultivated to meet our dietary requirements.

Whether we are growing alga or traditional crops, much of human waste is not readily usable as fertilizer. Portions are, and some bacteria excel at producing nitrates out of urea and ammonia. But much solid waste cannot be so easily processed. Luckily, a technique is available to solve the problem: a Supercritical Water Oxidizer applies high pressure, modest temperatures, and oxygen to burn the carbohydrates to water and CO2, freeing nitrates and mineral salts in the process. This is called the Zimmerman Process.

The process boils down to:

  1. feed CO2 and light to growing plants
  2. harvest human-edible feedstuffs
  3. feed much of the rest to animals such as rabbits, goats, and chickens, as well as vegetarian fish such as tilapia.
  4. burn the rest of the plant matter to produce CO2 and ash (which is fertilizer)
  5. feed food byproducts (and table scraps) to animals such as chickens or pigs (which when harvested produce still more byproducts)
  6. use that Supercritical Water Oxidizer on animal and human wastes to convert them back into CO2 and fertilizers for the plants.
  7. Condense water out of the air for drinking, and recycle irrigation water (which holds excess fertilizers) for plants.

Remember, as we are growing crops we need to feed them extra CO2 (much more than humans exhale), and store the excess oxygen they produce; we’ll restore the balance when we burn the crop residues and wastes. The “burning” doesn’t have to be an open fire. Feeding crop residues to goats counts as burning, as does using the plant matter as a reducing agent in the production of iron – both produce CO2 and free the water in the carbohydrates.

What crops should we grow? In general, dwarf varieties of grains, beans, and vegetables will satisfy most of our needs. We’ll have bread and pasta from wheat, rice, soybeans, oatmeal, lettuce, tomatoes, melons, potatoes, sweet potatoes, onions, herbs, etc.. I’m sure we’ll grow strawberries and other fruits, and eventually our parks will also serve as a source for nuts and fruits such as apples. I’m also quite certain we’ll grow grapes for wine, barley and hops for beer, coffee, and tea. Some human appetites insist on being satisfied.

That does leave the question of space. Just how big must our farm be? According to T.A. Heppenheimer’s excellent book Colonies in Space, the answer is derived from existing studies and experiments in high-intensity farming. Using dwarf varieties that have also been selected for short planting-to-harvest times, using interplanting (sowing the next crop before the current one is harvested), and optimizing CO2, water, light, and humidity, Heppenheimer calculates that 60 acres of farmland will support ten thousand people. This is only about 25 square meters per person.

I propose to average less than half that efficiency and allocate 64 square meters per person to include space for crop tending, to support a greater variety of foods, and to allow some extras to feed goats (for milk, cheese, and meat), rabbits (for meat), and chickens (for eggs and meat). Note that crops don’t require high ceilings; a single meter is good enough (on average), yielding a volume requirement of 64 cubic meters per person. This is less than the 100 cubic meters per person of living space I recommend in Designing a Space Habitat, where I also recommend about 33 cubic meters of workspace volume and an equivalent amount of overhead.

In that previous post, I assumed that 3 levels of living space would be allocated for the farms, but that may not be the best option. Rather, the farms are the primary source of waste heat. All that light energy ends up as heat and must be dissipated. The end caps of our cylinder expose a great deal of surface, so it makes the most sense to place our primary heat sources – the farms - adjacent to them. Using 8 meters along both end caps as our farms provides 64 cubic meters per person, independent of the size of our habitat (as long as we use the Kalpana geometry). Many plants need little gravity, indeed aquaculture (raising algae and fish) may require none, and these may be placed near the center. I expect that we’ll place livestock near the outer rim, as their needs for gravity are likely to mirror our own.

A future post will describe the lighting needs of the crops, and the technologies we’ll use to provide it.


michael Hanlon said...

N-P-K, the first acronym we learn in agricultural education. It stands for Nitrogen, Potassium and Phosphorus, the three essential elements required for plant growth

Nitrogen. Does this come from the asteroid? It is usually in nitrate form, as you mention that the "Zimmerman Process provides, and usually accounts for 50% of the tonnage of applied nutrients. On earth it is abundant in the atmosphere and intrudes into the soil. It is essential in most root systems to enable the release of all the other elements for use by the root uptake system. We apply tons of it just to make sure it's available because in nitrate form, it washes away easily, eventually finding its way back into our atmosphere.
I cannot calculate or estimate the tonnage of N you would need. The only figure to go on is 60sq m(5yds by 12 yds=15 ft by 36 ft =540 sq ft) per person for crops. That's a good number 'cause you can go to your nearby garden center and see that most bags will be good for that area size, those being twenty-five pound bags. If you have 400 people to grow for you will need approximately 10,000 lbs (5 tons ) of "STARTER" fertilizer. That will start to boost the atmospheric amount available. You will probably have to infuse this amount for the first ten cycles neaning you need to import (from the rock or here) 100 tons of just nitrates. I think that's a drawback to the process you cite, it does fine here but is untested from a 'zero to begin with' situation.
There are other methods you can employ to make the nitrogen more available and reduce your infusion amount) in the soil (or is this going to be aqua-culture?) And that is to rely heavily on legumous crops (peas) They have nodules on their roots which have the sole purpose to release nitrogen into the soils air. Peas grow quickly and many varieties have edible pods, too. Lots of N gets tied up in those nodules, so, recycling them means keeping them away from livestock feed. A good step but now you have multiple recycling efforts going on. It also removes material that was intended for Zimmerman. Some of the other legumous crops can be sprouted and eaten that way. Further, I have heard of fermenting the sprouts for nutricious livestock feed/change in diet.
That's about all the apples in my tree for now. I will think more on this essential aspect of a sucessful habitat.

michael Hanlon said...

I've scanned over your earlier posting about designing a habitat ans see that you intend phases of development and you self debated the soil v. aquaculture issue , deciding on aqua. Those answer some of the obstacles I had build. Back to the crop growth nutrient need.
Back to the nutrient requirement for proper plant growth. I mentioned that there would be a need for 100 tons per 400 people. That figure could be drastically reduced if liquid nitrogen is delivered to the factory floor Doing so, allows you to not have to lift all the trace elements in the nitrate forms. Or, a compromise can be made when after sufficient trace elements have been delivered, switch to shipping the nitrata thermoses (OOO, if the thermoses are large enough you could use them for rest chambers!)
You still need those other 100 tons of P & K though! The potassium is the same as potash which is the ash ingredient you mentioned. It still has to be delivered before you can begin to cycle it back and forth fom nutrient to waste and repeat. Have any of the thousands of rock pix taken or the actual visits made to asteroids given any clue as to % composition Potassium or Phosphorous?

michael Hanlon said...

I think this will jump ahead on your topic scedule plan but I don't see a negative bringing it up now, so's I don't forget it later. I've just made an entry at the SciAm "planetary bombardment..." blog about it and I felt It should also gain space here, it being a wonderful idea and all.
Economically, having a tremendous mass nearby in space can allow a bebnefit to extra-terran exploration that would require a lot of fuel boosting to space. Since there would be a "factory with trained technical people availableat the habitat or on the rock, the mass along with any probes can 'fall' into the Earth's or Moon's gravity well and at a trajectory determined point, separate from each other, the rock coming back to orbit, and the probe being shot out of the "gravity gun" toward its target. The habitat sould be able to sell its position in he skies and its quality of personnel in this venture.

michael Hanlon said...

All agricultural growth efforts require the control of media ph levels. Most growth operations tend to remove alkalynity from the soil leaving it acidic. That means you need to keep raising the soil ph toward a neutral 7.0 (depending on crop). This is usually accomplished by adding calcium carbonate (limestone)to the media.
Limestone is a product of oceanic decomposition of life forms. It is unlikely it will be found on an asteroid. You will need to have it delivered. The amount required usually equates to the amount of nitrates needed, so, there's another 100 tons of boosting required.
I think tissue growing vats (Industrial sized Petri dishes) are the best option. At Thanksgiving you can have a real meal delivered to your inhabitants at only a few hundred pounds.

Stephen D. Covey said...

Actually, our asteroids will have abundant quantities of potassium, phosphorus, sodium, calcium, and most other elements needed in trace quantities.

It will be trivial to produce lime (calcium oxide) from the asteroid, even calcium carbonate (although that ties up some very valuable carbon). Our farms are not likely to use soil because we would lose the root mass from our recycling efforts. Also, all that acidification of soil comes from the bacterial decomposition of organic matter. We'll have more efficient ways to recycle roots, stems, and leaves (crop residues).

The only elements in relative short supply that we need for our space habitat are hydrogen (water), and nitrogen (as in ammonia and nitrates). Water is likely to be more abundant than nitrogen because it binds with so many minerals.

However, there is a good chance that we'll need to ship large amounts of ammonia to the habitat, at least until we can capture the resources of a comet, extinct comet, or carbonaceous chondrite asteroid (which should have nitrogen in abundance). We'll need enough to produce the crops we'll eat, but we will not need as much fertilizer as on Earth because (a) we won't lose it to runoff, and (b) we'll carefully recycle what we have.

Nitrogen is an essential part of proteins and much of life's machinery. Nitrogen compounds readily dissolve in water, and on Earth end up in the ocean.

Humans do not need free nitrogen (although it is 80% of every breath we take). We would (probably) not fill our air with nitrogen, and any bacterial process that released free nitrogen would be a negative (but possibly unavoidable), as we would need to capture that to recycle it back into fertilizers and ultimately into proteins.

Assuming efficient recycling (which we'd better learn how to do), a plant is about 2% nitrogen (dry weight), and an animal (such as a human) is 3%-3.5% nitrogen by weight. We'll need about a half-ton of carbon (per person) for our crops, and about 50 kilograms of nitrogen (which we can get from 60 kilograms of ammonia).

If Apophis is as bone-dry as the Moon, we'd need to supply (for each person) 50 kg of nitrogen, 500 kg of carbon, and 500 kg of hydrogen. We would ship the nitrogen as ammonia, the carbon as methane, and still need to ship a lot of liquid hydrogen - about 325 kg of it. Hopefully, Apophis (or another accessible asteroid) will provide the bulk of these elements, so that we'll only have to ship people and tools (including lights).

michael Hanlon said...

We interupt this dialogue to introduce some impotant breaking news on the 'get to space' front.

Moving up out of the Earth's gravity well is the problem we initially face in getting beyond the Earth for habitat. To date we have used two methods: Chemical (that we are all aware of that took us to the moon) and bouyant (which has lifted payloads to the fringes of our upper atmosphere in H & He filled balloons). On many drawing boards are magnetic rail guns (that could inexpesively boost masses to orbit via attractive impulsion). What all these methods have in common is they achieve delta x's by imparting energy resulting in a lift force.

Where the energy comes from is a design decision. Pick the source of energy and build the machine which captures and converts it to force. I have just thought of another method to develop force that has not been investigated and may actually be the best way for living tissue to make the trip. This will get complicated and I've never written about it before. Please bear with me.
I'm sure you've all seen whales breaching the ocean's surface. Is it an attempt to fly? Probably not, but if they had jet packs ignited at the right moment, they might. When you hold a gas bubble under water there is a bouyant force pushing up on the bubble. The deeper the bubble, the greater the force. I know, you think this is going to a vast underwater ocean bubble launching facility. No, there are drawbacks to having sea level being your launch altitude.
I propose a deep well excavated into a high mountain site filled with water. Down in the well bottom a pointed nosed launch sling could carry a craft along with it. Bolt the nose section in the well's bottom, Keep pumping gas into the bubble structure until the desired pressure is built up. Upon release, the nose will push the water aside in front of it and gain velocity. At the surface, the two parts disengage and our craft would be moving upward with some velocity. Now add some small chemical boost component to maintain the v's until orbit is achieved. The cost? What does it cost to build some gas pumps to deliver thousands of pounds of pressure?. This is like the rail launch system only using Archimede's principles instead of Gauss'.

hitssquad said...

"What crops should we grow? In general, dwarf varieties of grains [...] We’ll have bread and pasta from wheat"

You haven't read Wheat Belly, have you?

Kaboom3009 said...

A couple of issues, the habitat's breathing environment will need an inert gas to lower fire risk, Nitrogen is a good one as it is used elsewhere and is going to infiltrate the system anyway.
One of the NASA web pages covers the caloric needs for one astronaut and they use 2000 to 3000 calories for moderate to heavy workload in space in zero G.
As for farm field overhead height I would suggest walking room so you dont have to use robotic harvesting machines... which if they break down in the middle of the field have to be crawled to to repair.
As for ammonia, if you provide for tilapia and chickens and algae early on you provide 3 enjoyable food stuffs and one distasteful one. while all 3 work together to feed each other and provide the nutrients and chemicals needed for other resources.
Launching the needed stuffs into orbit with your idea is a Novel way to do it. Another option might be to use the weather balloon type idea and float the object to the stratosphere and then launch from there. This has the advantage of allowing for less propellant as the required burn time is now much less as well the needed delta V for achieving orbit.
The N-P-K needs could also be partially met by growing plantains in the habitat since plantains provide all the needed nutrients for human consumption and the waste materials can be converted into the required fertilizers in one process or another. So you now have 4 good food items and 1 bad one.. add in the goats and you also increase the manufacture of propellants via methane or you increase your CO2 content. If you ship dry ice up when payloads have room you allow for a way to increase the CO2 for the gardening zone. If you hold off on the green plants during initial build-up and store excess CO2 as dry ice you can start with a surplus that might be sufficient to maintain the high level of CO2 needed for plant growth until the population maxes out.
As these are older posts we now know the moon does have all the materials and ingredients needed to allow for an alternate location of material supply including water and the various metals.
If one of the plants concerns in rooting then g rowing willow will allow for a natural root starter and can be continuously recycled back into the needed nutrients as well as be used for a natural pharmaceutical. The same goes for plants like clover which can also be fed to rabbits; again increasing our good foods and limiting the less visually pleasing foods.
The Ph levels can be controlled to some extent by rotating to crops that need higher acidity levels, asparagus, thus reducing the Ph. If we grow Dwarf pines we can increase acidity and provide a form of vitamin C with Pine tea and Charcoal for purification. The charcoal can then be used to purify water and remove odours from the air and be used to resupply the CO2 needed for plant growth.

Kaboom3009 said...

I forgot to ask ... where did you get your oxygen per day value ? I would be interested in reading more about that, if you could post a web site or some such would be great ;)

The NASA papers I have read suggest closer to 3 liters of water per person not including hygiene or cooking. (oops 2.6Kilos is almost 3 liters) Could the "burning" of the inedible plant stuffs be achieved with a solar furnace, since the sun is a ready source of free energy? we also shouldn't forget about the human need for over packaging, all that can be recycled as well for various compounds.

Stephen D. Covey said...

The figures for human requirements (food, water, oxygen, etc.) come from the 1977 NASA study, "Space Settlements: A Design Study". It is on-line at
One of the key points I wanted to emphasize is that humans only consume about 25% of most crops, yet we have to grow the entire plant. This means that as the plant grows, it produces 4 times the oxygen consumed by the humans being fed by the crops, and 75% of the plant matter produced must (in the end) be burned with that excess oxygen (could be "burned" by goats or chickens, of course).
As crops are grown, they need more CO2 than we exhale, and they produce more oxygen than we can consume. That must be balanced out.