Perhaps you saw the news recently about astronauts in the International Space Station eating their first home grown lettuce? It's just a beginning, but in the future, could they grow all their own food and get all their oxygen from plants? A little known series of experiments in Russia in the 1960s through to the 1980s suggests that they could. The research continues to this day, and we may see the first steps towards such a system taken in space in the near future.

The crew of the ISS eating their first home grown lettuce in space. Could they grow all their own food and get all their own oxygen from plants?

We probably won't recreate the tropical jungles and other mini ecosytems of Biosphere 2 in space until we can build much larger habitats than the ISS. But biosphere 2 was designed for studying ecosystems in miniature, not as a prototype space station greenhouse.

We need to look instead at ideas for lunar or martian greenhouses. Those are space habitats, in vacuum or near vacuum conditions like the ISS. So anything that worked there would also work in orbit.

Actually in the 1980s, scientists were looking into the possibility of a Controlled ecological life support system, for future space stations in orbit around the Earth, for instance here is a series of papers on a conference on the topic in 1984. Though they went for a mechanical system in the end; for a while a biological system seemed a distinct possibility for future space stations.

Not many know about the series of experiments done by the Russians in BIOS-1, 2 and 3, in the sixties and seventies, continuing through into the eighties to study the potential for space habitats like that. These produced all the oxygen and nearly all of the food for a crew of three, in experiments of up to 180 days from a surprisingly small volume of habitat of 315 cubic meters. That included 237 cubic meters set aside for growing crops. 

Biosphere 2, which is what most people think about when you suggest a closed habitat. Useful for studying closed ecosystems, but not practical for a near future space habitat

Bios 3 - facility in Siberia which was used for a series of ground breaking experiments in closed systems for space habitats in the 1960s through to the 1980s. And still exists today.

This is a model of the habitat. As you see there are three rooms devoted to growing crops ,and one room which was the crew quarters. The crops provided all their oxygen and nearly all their food, in a series of experiments in Russia, for a crew of 3. Longest test was 180 days.

They grew ten different crops, including dwarf wheat which they used to make all their own bread. Only 13 square meters of growing area was needed, per person for 40-45% of their food requirements. All wastes were recycled, with no malodour.

They were by no means the only ones working on this. But they got closer to 100% recycling than anyone else in the field at the time.

They recycled all their wastes, with no malodour, and kept healthy. They needed some extra food supplied, such as dried meat, but  all the bulky food such as carbohydrates, they grew themselves, and nearly everything else. They baked grains harvested from dwarf wheat and made all their own bread, for instance, from that small growing area, as well as greens, radishes, beets for sugar etc. It sounds like a healthy and tasty diet.

DOES THE ISS HAVE ENOUGH VOLUME TO GROW ITS OWN FOOD?

The ISS has a total volume of 32,333 cubic feet, or 915 cubic meters. Nearly three times the volume of the BIOS-3 experiment. So that is enough to grow nearly all the food for eight people at least, including living space for the crew, and provide all their oxygen from the plants. And the BIOS-3 experiments weren't particularly optimized for volume, as you can see from the model - so you could probably grow food for many more in the volume of the ISS. 

So you could grow nearly all their food, easily in the ISS volume, and probably a much smaller volume than that. 

Of course I'm not suggesting that we turn the ISS over to crop growing in space. They need that space for other things, but this preliminary rough calculation is promising enough to look at this more closely.

Also don't think of it as like your allotment or garden or house plants. There would be no pests in space, no insects at all except the ones you take up with you. And the plants would be grown in sterile conditions using aeroponics with their roots dangling in moist air supplied with nutrients. This is a mature and practical technology on Earth and it's already been shown to work in space. The system would be largely automated with minimal work for the crew.

RELIABILITY OF PLANTS

The crew of the ISS have had many issues with their machines for generating oxygen. The Russian Elektra, and the US OGS for splitting water to make oxygen, and the Sabatier system for recycling CO₂ have all had issues, needing to be fixed, and sometimes not functioning for long periods of time. The astronauts often rely on "top up" oxygen from Earth in other forms, including oxygen air tanks and solid oxygen generators. And the system isn't yet closed. Even with the Sabatier system when it is working properly, they only recycle 50% of the oxygen, and the rest has to be supplied from Earth as water. It's not a mature technology yet.

They continue to research into this. Currently the aim is to increase the efficiency until they recycle 75% of the oxygen. But the Russian algae system in the 1970s already achieved 100% recycling of oxygen. So, is there any potential for looking to algae to solve these problems?

Unlike machines, plants and algae always work and don't need to be repaired. Your dwarf wheat won't suddenly break down and need parts shipped from Earth to keep it making wheat grains and absorbing CO₂ and producing oxygen. And your green algae will never break down either, it's as reliable as brewing beer or using yeast to raise bread. Nature, through evolution, has sorted that all out millions of years ago. All you need there is reliability of the lighting, plumbing and pumps; a rather lower level of technology.

Here is a discovery channel program about growing plants using aeroponics in space.

GETTING STARTED - ALGAE FOR OXYGEN

Though crops would be what you want to grow in the future, you would probably start with growing algae for oxygen, as the Russians did.

In the early BIOS-1 experiment they had already shown that you can produce all the oxygen you need for one person from just 20 kg of water and algae (that's 0.02 cubic meters), spread over 8 square meters of surface area.  

That's small enough so you can consider flying it in even a small module, perhaps even a Bigelow inflatable BEAM module would be large enough.

The inflatable BEAM module, from Bigelow aerospace, which will fly to the ISS later this year is 4 meters long, 3.2 meters in diameter and has 16 m³ living volume, and weighs 1.36 metric tons.

The total volume for all the algae and water, for a crew of six, would be 0.12 cubic meters. Most of the space would be to supply it with lighting and make sure there is plenty of exposed surface area for growing. The lighting for an algae bioreactor can be supplied in many ways, for instance the algae flow in tubes, or you insert light pipes into the solution.

If you had, say, 0.25 cubic meters for each square meter of algae for the lighting, pumps etc, you could fit 48 square meters of surface area into a volume of 3 by 2 by 2 or 12 cubic meters. I haven't been able to find out the details of the system the Russians actually used in their early green algae experiments, but it can't have taken up a huge amount of volume, probably less than that.

The air inside the growing containers would be moist, and this is condensed to supply drinking water for the crew.

The Russian experiment used three 6 kW xenon lamps providing 200 - 300 watts per square meter of the cultivar. That's 18 kW per person, so for a six person crew, that's 48 kW, which is a lot. The ISS has maximum power of 120 kW and is often in darkness. 

But with modern LED lights you could reduce those requirements to a tenth. 1 LED watt per square meter is roughly as good as 10 conventional light watts per square meter.

For instance, just checking commercially available high efficiency grow lights available to aeroponic / hydroponic growers as of writing this, August 2015: this High Efficiency Full Spectrum SMD LED Plant Grow Light -  uses 20 watts of power to illuminate 0.2 square meters, i.e. 100 watts of power needed per square meter. It is recommended for crops that require bright sunlight such as lettuces here: Top 10 Best LED Grow Lights, and so seems roughly comparable. That would be 800 watts for 8 square meters, or 4.8 kilowatts for the light needed for algae for a crew of six. Or if they grow all their own food, three times that, 14.4 kW.

Using modern high efficiency LED grow lights like this a crew of six could provide all their own oxygen with 4.8 kW. Or if they grow nearly all their own food, they would need 14.4 kW.

By comparison the Russian Elektra electolysis unit, when it was working, needed 1 kW to supply all the oxygen for a crew of 3 or 4. (I can't find the figures for the US OGS - if anyone knows do say in the comments). The green algae needs more electricity, but it is 100% recycled, needs no resupply of water from the Earth, and also absorbs the CO₂ as well.

Here is an ESA video about the idea of using Spirulina to produce oxygen in space.

Spirulina is better than the Chlorella used in earlier experiments because it is edible unlike the almost inedible Chlorella.

Spirulina which has been harvested for food in Africa and South America for centuries. This image is credit ESA. It could produce all the oxygen and some of the food for astronauts on board the ISS. Experimental algae bioreactors using Spirulina will fly on the ISS in the near future. See the Melissa project.

NEXT STAGE - FOOD

More space needed for food because you need head room for the crops. But still, not a huge amount. With the BIOS-3 experiments they had a total of 237 cubic meters set aside for growing crops. But it is clear the experiment wasn't set out to be optimized for volume as they only grew the crops in a single level.

With 13 square meters of growing area per crew, conveyer belt system, growing wheat, sedge-nut, beet, carrots, and other crops, ten crops in total, they reduced the daily substance requirements for the crew for dry food from 0.924 kg to 0.208 kg, for oxygen from 1.22 to 0.35 k, and didn't need drinking water or water to hydrate the food at all so that was a saving of 5.133 kg a day for water.

So that's only a little more than was needed for the oxygen. It's clear from the photographs that they weren't optimizing for volume with lots of spare headroom above the plants and just one layer of crops in the room.

There seems plenty of space for three or four layers if you had them in trays. The wheat was used to make bread. So, that 79*3 cubic meters is very much an over estimate.

I can't find an estimate of the total volume needed for the crops themselves if it was used in a space station with minimal overhead space above the crop.

"Wheat plants of various ages showing the "conveyer" approach that was used in the Bios experiments, Young wheat plants are in the foreground, with more mature plants toward the back. The aisle between benches is narrow (to leave as much space as possible for the crops). The post, with some environmental sensors attached, further obstructs the aisle. Crew members planted various herbs and other special plants in the corner and next to the wall to the left, space that would otherwise be wasted." photo from here

In her 2006 masters thesis Living in Space, for the Swedish Physics in Space program at Uppsala university, Maria Johansson compared the Russian BIOS-3 with Melissa and the current ISS system.

The main difference between BIOS-3 and MELiSSA is that in BIOS-3 the inedible plant wastes are burnt in an oven - returning CO₂ to the air which is then used by future generations of crops to make more food. In MELiSSA the wastes are decomposed by micro-organisms in biological reactors, basically, they are composted and used as nutrients for the algae and hydroponically grown plants.

She found that BIOS-3 and the ISS system used about the same power and that BIOS-3 breaks even with the ISS on equivalent mass after two years. MELiSSA takes longer to break even, 7.5 years because BIOS-3 has a lower startup mass and less supply mass. But the MELiSSA system is more controllable than BIOS-3.

She assumes that light is provided using light collectors, with a mass penalty of 338 kg per kW, and 46 watts per kW used for tracking for the solar collectors. Her figures for average power needed are.

ISS: 1.2 kW
MELiSSA 4 kW
BIOS-3 0.92 kW

So in this situation if you have solar collectors instead of LED lighting, BIOS-3 actually uses less power than the ISS systems.

Then the mass for the system is

That's not including the modules needed for the growing volume though.

Most of the mass for the BIOS-3, which is mainly aeroponics, is for the oven (1.305 tons) and the condensors to produce light (4.225 tons).

For MELiSSA as a basically hydroponics system the water takes up most of the mass at 8.89 tons, and much of the rest is taken by the centrifuges at 2.8 tons.

For ISS, though nothing particularly stands out by way of mass, the hygene water supply system is the most massive at 0.706 tons.

As for state of readiness,

ISS - systems already in use, but not yet totally reliable, needs occasional repair with new components from Earth and the ability to "top up" with oxygen when the systems fail.
BIOS-3 -system not designed for micro-gravity so would need to be redesigned to be used in space.
MELiSSA system - currently being actively developed but should be considered at an early stage of development still.

THE MATHEMATICS BEHIND IT

It might surprise you to know, but actually the main waste product of a human, apart from water, is not feces. It's CO₂.

You produce only 0.03 kg dry mass of feces a day. But you exhale 1 kg of CO₂ every day. And breath in about 0.84 kg of oxygen a day. For details see Design Rules for Life Support Systems (NASA document).

A crew member needs about 5 kg total per day.  Much of that is water which is recycled already in the ISS. Apart from the oxygen, the main other non water component of the crew needs is dried food solids, 0.62 kg per day.

If you can capture that CO₂ and turn it back to food, you can save about 1 kg per person per day. But you also save on resupply of oxygen. as well, so as we'll see, the total saving is more than 1 kg per day.

HOW IT WORKS ON THE ISS

The oxygen is produce by electrolysis of water. Originally this was done by the Elektron unit which requires 1 kilowatt to supply the oxygen needs for a crew of 3 or 4. So 2 kilowatts for a crew of six. They also have the US Oxygen Generating Unit. And several other ways of creating oxygen.

This oxygen can be recycled using the Sabatier reaction CO₂ + 4 H₂ -> CH₄ + 2 H₂O

This reacts the hydrogen waste product from the water electrolysis with the carbon dioxide to produce methane and water. The methane is then vented to space. To recover all the water that was originally split by electrolysis, then all the hydrogen would need to be combined back with the oxygen from the CO₂. Since half the hydrogen is wasted in this reaction, then only half the oxygen can be recovered (unless you supply the extra hydrogen needed from the Earth). That means also that you have to vent half the CO₂ to space.

The ISS has a Sabatier system installed and when it is working can achieve near to 50% recovery of the oxygen to water so that it can be reused again.

OTHER TECHNOLOGICAL SOLUTIONS

The methane could alsobe split at high temperatures by pyrolysis, or recovered in other ways, making an almost complete cycle, with only carbon as the waste product. You could also split the CO₂ completely to C and O₂, by using laser light: Making oxygen before life.

You can also use the Bosch reaction CO₂ + 2H₂ -> C + 2H₂O which, unlike the Sabatier reaction, recycles all the oxygen to water in one go, but this requires higher temperatures in range 450 to 600 °C and is technically harder to do in space.

There are several other ways you could improve on the Sabatier reaction.

At present NASA has sponsored four projects, with $2 million each, with the aim to boost recovery of oxygen from 50% to 75%.

100% RECOVERY OF OXYGEN WITH PHOTOSYNTHESIS

With plants, the basic equation is
6CO₂ + 12 H₂O -> C₆H₁₂O₆ + 6O₂  + 6 H₂O

There the atoms shown in bold are the same atoms on both side, so technically the oxygen comes from the water rather than the CO₂, but the end effect is the same.

The end effect is that all the CO₂ gets used up and an equivalent amount of oxygen is produced. So you get 100% recovery of the oxygen from the CO_2.

Techy aside. You might think that the plants are splitting the CO2 - but no - actually the oxygen comes from splitting water.

The oxygen from the CO₂ goes into the glucose, and into water. Indirectly that H₂O from the CO₂ might well end up as O₂ if it gets taken up for photosynthesis later on, but it is not split directly.

This has been proved by using isotopes to tag the oxygen in the CO₂ and the water.

See Plants Don't Convert CO₂ into O₂ and Page on lamission.edu (see page 5 there for a useful diagram of how it works).

HOW IT WORKS WITH FOOD AND ALGAE

What matters is the harvest fraction, how much of the plant growth from the exhaled CO₂ gets turned into food and so can be eaten again.

With typical harvest foods, then half of the plant material is edible, and the other half is wastes. You may be able to go higher than that, for instance with edible algae, 100% of the mass is edible.  But, for purposes of this calculation, to show how it works, let's suppose that half of the plant material is edible.

In a closed system, nearly all the dried weight of the material in the plant comes from the exhaled CO₂ of the crew (apart from trace elements). And that exhaled CO₂ is the main carbon containing waste product from eating the food. So, if you can supply all the oxygen for your crew from plant growth, you supply half the food requirements, because half of it gets turned into food and half of it becomes plant wastes. 

If you supply all the food, the plants will supply double the oxygen requirements, so then the food wastes need to be incinerated or composted and turned back to CO₂ to close the loop - that extra CO₂ is needed to continue with plant growth. That's why MELiSSA relies on bioreactors (basically high tech composting) and BIOS-3 uses ovens to burn the CO₂.

If you can grow all the food in space then you save 0.84 kg for oxygen per day, and 0.62 kg for the dried food solids for a total saving of 1.46 kg per person per day, or 3.197 tons per year for a crew of six.

If you can produce all your oxygen only, then that saves 1.8396 tons per year.

Currently the ISS is able to produce about half of the oxygen it needs, when the Sabatier system is working. And when it isn't working, all of that mass has to be provided from the Earth.

PAYLOAD SAVINGS

Three tons per year is a significant amount of extra payload for an interplanetary mission or a long duration deep space mission or mission to the Moon. And it's a considerable saving for a long duration space station also.

If a station like the ISS had 100% recycling of oxygen and food, it could save a little over 3 tons per year. The ISS launched in 1998 and had its first resident crew in 2000. The crew fluctuated, but for our calculations let's suppose the future station has a crew of six.

The ISS is expected to last until 2028 now, if so you could save nearly 90 tons of mass over that time period (nearly six times the mass of the Destiny module) and since Progress has a payload capacity of 2.35 tons, you'd save around 38 Progress launches (28*3.197/2.35), if you had close to 100% recycling of oxygen and food.

If you recycle just the oxygen, you'd still save over 51 tons over a 28 year lifetime of a space station.

In future, the modules are likely to be lighter, e.g. the Bigelow aerospace inflatable modules, the BA330 is only 20 tons for 330 cubic meters of living area.

Proposed BA330 from Bigelow - mass of 20 tons, and internal volume 330 cubic meters. By comparison, the Destiny module is 14.5 tonnes for 106 cubic meters of pressurized volume.

Future habitats will have more space available for growing crops for the same mass.

FASTEST PAYBACK FOR ALGAE

With oxygen from algae the payback could be much faster. The BEAM module weighs 1.36 metric tons. The startup mass for an algae bioreactor for a crew of six is negligible, fraction of a ton. And the payback is 1.8 tons a year.

So you'd pay back both module and the bioreactor within a year. This would seem to make it worthwhile even to send an algae bioreactor module to the ISS to use as its main system for oxygen, in the near future, if the MELiSSA experiments prove that the method works in space. And as a bonus, the algae they use can be recycled as food.

While with the more complex systems, the payback time is longer, but still, it could be well worth doing, even with the limited lifetime left for the ISS especially with larger inflatable modules like the BA330. These modules could be detached and re-used in future space stations when the ISS is de-orbited.

Anyway that's looking rather far ahead. So far we don't have any bioreactors or biological closed systems ready to fly into space. MELiSSA is the system being developed most actively at present. From Maria Johansson's figures, it would seem that a system based on BIOS-3 would also deserve attention as a competing system, since it seems it could use less power, less startup mass, and require less input mass to the system each year.

FUTURE PROMISE

Engineers tend to see machines as more reliable and controllable. But in practice the machines on the ISS for generating oxygen and recycling CO₂ have gone wrong often and been out of order for long periods of time.

So far, we have no machines that are anything like as reliable as, e.g., yeast, or algae, or dwarf wheat. If you can make sure there are no insect pests, or crop diseases, and keep it sterile, easy to do with aeroponics, then you have nothing to go wrong.

There is no need to get resupply of components from Earth to fix your algae. And even if something goes wrong, say, the lighting or pumps fail and the crop dies back before they can be repaired - well, so long as you have some seeds and some algae still alive, it can regenerate itself, you can grow your algae and crops back again, as they found out with the BIOS-3 experiments. Try doing that with machines. So it seems to have a lot of future promise.

HUMAN WELL BEING AND HAPPINESS

As a fringe benefit, if we can grow food in space, this is likely to lead to a happier crew. We aren't machines, and most human beings enjoy having plants around and growing plants.

First, there's the taste. Fresh food, lettuce leaves and tomatoes picked from the plant, and bread you bake yourself, from wheat you grew yourself tastes much better than food that is dried and reconstituted, which is all you'd have otherwise in a long duration journey.

Also most people enjoy having plants around and tending plants.

It's true that you can survive fine without plants. If you are a prisoner in solitary confinement, you have no choice, and may find that you adjust fine to your situation. And many hermits in the past, and even today, spend years on end in caves and other confined small places, without any plants or much of anything except blank walls, and come out of their retreats happy. It is the same also for the crew of yachts on long distance voyages. And they are happy in those situations.

However, that's not for everyone. And having plants around in the spaceship seems likely to contribute to overall happiness and well being of the crew. This is often mentioned as a fringe benefit in the literature.

A happy crew will make better decisions, and are more likely to come up with inspired and creative solutions to problems, and so may be better at completing mission objectives. And in any case, all things being equal, surely it's better to go for a solution that is more enjoyable for the crew.

Especially on long duration missions, far from Earth, where their plants in their spaceship may be the only green thing there is for many light minutes, many millions of kilometers, in all directions. Even on the far side of the Moon, the green plants in their spaceship may be their one direct tangible link with the ecosystem of the Earth which they can no longer see in the sky.