None of us would be alive today without plants, and if humans want to survive beyond Earth long-term, we'll need to bring our leafy greens with us. Eventually, astronauts are going to have to become space farmers.
Space gardens in the movie Sunshine.
First, there's the obvious argument for growing plants in space: unless we want to pack enough shrink-wrapped protein bars to last until the end of time, we'll need a self-replenishing food supply. But there's even bigger reason to bring plant life with us into the cosmic void, and it has to do with turning our spaceships into ecosystems. We need plants to conserve and recycle resources, helping maintain a breathable atmosphere and a drinkable water supply.
Much easier said than done. In order to build a space greenhouse that doesn't eventually kill us, we first need to understand how plants make a living and how they have shaped our planet. We then need to deal with some of the strange challenges of life in a sealed environment.
How Plant Life Made the Earth Liveable
There's a lot on planet Earth we take for granted. Oxygen to breathe. Water to drink. Invisible chemicals in our atmosphere that keep Earth warm, others that prevent the sun from frying us. Earth is our cosy blue marble, but life on this planet hasn't always been such a picnic. In fact, for much of its history, the world we call home was a miserable wasteland covered in a thin, asphyxiating atmosphere and awash in sterilising UV radiation.
Today, forests like the Amazon exchange massive amounts of CO2 and oxygen with the atmosphere.
It was the ancestors of plants — cyanobacteria using the same photosynthesis machinery — that first started transforming the Earth into something habitable for animals like us. Beginning some 2.3 to 2.7 billion years ago and lasting for nearly a billion more, the cyanos filled our atmosphere with oxygen, a waste product of photosynthesis, as they soaked up carbon dioxide and sunlight to make sugar. When more complex plants came on the scene roughly half a billion years ago, they took up the photosynthesis torch and began shaping our atmosphere more dramatically, drawing down larger amounts of carbon and spewing out even more oxygen.
That's all ancient history, and you might wonder what it has to do with space gardens. The answer is, everything. Photosynthesis keeps our atmosphere in balance. It's the reason we live on a world that doesn't run out of oxygen and where the CO2 we exhale doesn't accumulate to the point that everybody asphyxiates. (The oceans help draw down CO2 as well, and over much longer timescales, so does Earth's crust). These may seem like big, abstract concerns, but rest assured, if we stuck you in a sealed metal can, they wouldn't be.
Of course, that's to say nothing of the fact that photosynthesis forms the base of nearly every food web on our planet. If you've never thanked a plant for keeping you from choking and starving, you can go do so now.
Clearly then, plants offer some serious advantages — without them, our space habitats would need an artificial means of replenishing their atmosphere, which would cost precious energy and resources. But plants are a double edged sword. They keep Earth's biosphere in balance, but in a more confined space, the gases they exchange can get out of whack quickly.
Which is to say, we can't simply stick a bunch of tomato plants and grow lamps in the cargo bay and expect our spaceship's environment to run smoothly. Closed environments need to be approached very scientifically, as two cautionary tales help illustrate.
Biosphere 2 and Mars One
Perhaps the most ambitious (certainly the largest) closed ecosystem humans have ever conceived is Biosphere 2, a 3.14-acre Earth system research complex located in the desert outside of Tuscon, Arizona. Built in the early '90s by the now-defunct company Space Biosphere Ventures, this massive glass and metal greenhouse complex, filled with representative "biomes" from across the planet, was intended to serve as a simulation for future space habitats. It was constructed as a materially closed system, meaning there was no exchange of atmosphere or water with the outside world, only sunlight.
A Biosphere 2 living experiment began in 1991, when eight men and women sealed themselves inside the complex with nothing but simple tools. The plan was to grow all of their own food and survive off the land for as long as possible. It was an unmitigated train wreck.
Throughout the two years that the living experiment lasted, CO2 levels within the habitat fluctuated by as much as 600 parts per million every day, thanks to the drawdown of carbon during sunlight hours by photosynthesis, and its subsequent release at night through plant metabolism. (The concentration of CO2 in Earth's atmosphere today is just north of 400 ppm). CO2 also varied seasonally with light availability, reaching peak concentrations of 4500 ppm in the winter and 1000 ppm in the summer. Most of the complex's vertebrate life and pollinating insects died, while populations of greenhouse ants and cockroaches exploded. Morning glories threatened to choke out all other plants. Filtrations systems clogged, and unexpected condensation made the desert soggy.
Worst, oxygen concentrations in the facility fell steadily, from a healthy 20 per cent at the outset to 14.5 per cent at 16 months in — roughly equivalent to oxygen levels at an altitude of 13,400 feet. When the half-starved Biospherians began suffering from sleep apnea and chronic fatigue, the management group decided to intervene and artificially boost the O2.
The reason for the oxygen shortage was unclear at the time, but later studies would demonstrate that the culprit was probably decomposer microorganisms in the soil. When the Biospherians built the system, they included the most organic-rich soils possible to give the plants their best chance at survival. But the fertile soils also harboured large populations of oxygen-consuming microbes. This issue — and many of the experiment's other problems — might have been prevented had the system been designed with greater scientific oversight.
And yet, the efforts of the intrepid humans who sealed themselves inside Biosphere 2 weren't a total waste, because we learned an awful lot. Biosphere 2's failure became an important cautionary tale, underscoring how easily closed ecosystems can spiral out of control if the initial conditions of the system are not carefully measured and aligned. (After more than a decade of inactivity, the facility was acquired by the University of Arizona in 2007, and has since been repurposed as a state-of-the-art Earth science laboratory).
But Biosphere 2 is also massive, and far more complex than anything we might hope to build on the first or second generation of space habitats. In space, we'll probably grow plants without dirt, so we won't have to worry about invisible soil critters mucking things up. We certainly (knock on wood!) won't have to deal with cockroaches. Surely a little plot of potatoes and soybeans won't crash and burn like this gigantic Earth-in-a-bottle did?
Well... let's not be too sure. Take Mars One, for instance. One of the world's most hyped (and haphazard) space colonisation initiatives, Mars One became a household name in 2012, when the company announced to the world that it'd be shipping humans off to the Red Planet in the mid-2020s to live out the rest of their lives in small, inflatable bubbles. Even more ambitious than Mars One's timeline, however, is the company's plan to have its colonists grow 100 per cent of their food on site. Unfortunately, this plan has all the makings of a catastrophe that Biosphere 2 did, and for surprisingly similar reasons.
The proposed Mars One colony, a series of inflatable habitats.
Mars One — perhaps concerned that other groups will steal its great ideas — has refused to tell us much of anything about the technical details of its proposed mission. This prompted a group of MIT PhD students to publish their own feasibility analysis last year. Using all of the information available on the company's website, the researchers calculated the resources needed to sustain the colonists and ran models to evaluate the stability of the habitats over time.
It wasn't pretty. First off, the study found that the cropping space inside the habitat Mars One was proposing wasn't nearly large enough to meet the caloric and nutritional demands of the crew. As the mission was designed, everyone would probably starve. If they didn't starve, model simulations showed that the crops would, over time, produce unsafe levels of oxygen, which could cause the atmosphere to spontaneously combust (also bad). Counteracting the extra oxygen, the atmosphere would be rather swampy, because plant transpiration would increase the relative humidity to 100 per cent. (Still wanna sign up? Just send 'em a check!)
But the MIT researchers took things one step further. Rather than simply showing us the many ways Mars One was doomed, they redesigned the mission in order to keep the astronauts alive.
We've talked a lot about what can go wrong. What does a successful space farming colony look like?
How A Closed Ecosystem Could Work
We've gotten a taste of how space habitats can spiral out of control when we start trying to introduce ecology. To make a closed ecosystem work, we have embrace and take advantage of that closed part. That means putting things in separate compartments, and controlling and measuring the fluxes between them.
For Martian colonists to grow their food locally, the MIT researchers proposed placing the crops in a separate enclosed chamber, effectively decoupling their use of resources from that of the crew. An oxygen removal system would siphon O2 from the plant chamber into a dedicated tank for later use. The CO2 exhaled by the crew would likewise be collected and sent into the crop unit to feed the plants. Ideally, robotic food processors would be deployed so that the crew never had to interact with the garden at all.
Environmentally controlled gardens, such as MIT's CityFarm, offer a template for how we should be structuring our space gardens.
Making such a design work will mean getting a number of different technologies up to speed. Oxygen removal systems have been used extensively on Earth, but space-rated versions don't yet exist, and hardware can fail in strange ways after liftoff. (In 2011, NASA published a report on hypothetical technologies to help recycle the ISS's oxygen supply, but as of 2015, astronauts are still dependent on regular shipments of O2 in cryogenic tanks). CO2 removal systems, which use a synthetic rock called zeolite to scrub carbon from the air, are already in use on board the ISS. While these scrubber systems currently vent CO2 to space, they could be repurposed to shuttle the gas into a growth chamber.
Water is another big issue. Plants and humans both require lots of it, and, like everything else, blasting water out of Earth's gravity well is wildly expensive. Astronauts are currently using a variety of methods to recover and recycle water on the ISS: condensing it out of the air, recollecting urine, heating and filtering the stuff to make it potable again. A process called the Sabatier reaction, wherein hydrogen gas and CO2 are combined to form water, is also being explored by NASA.
Biology can help us do even better. Biofiltration systems that use plants, fungi and microorganisms to remove excess nutrients, heavy metals, bacteria and viruses from water are cropping up everywhere. Such technologies are not yet well developed for space life. But in a draft technology roadmap published this past May, NASA identified "bioregenerative food systems" as an R&D priority. Hopefully, that's a sign that research on how to integrate gardens into our space habitats to maximise their resource recycling potential is coming.
Of course, even if we do our damndest to recycle everything, we're still going to have to accept small amounts of resource loss. Even the best sealed compartment will leak a little bit of atmosphere, and more will vent every time we open the airlocks. Ship parts will get old and need to be replaced. Trace metal shortages could cause nutrient deficiencies among the crew. Replenishing resources in the depths of space will probably mean mining metal and water-rich asteroids, or, on a planet, extracting resources in situ. NASA, Planetary Resources and Deep Space Industries are currently developing these technologies, but space-based resource extraction tech still has a long way to go.
Making space gardens, and more broadly, space ecosystems work, starts with the right conceptual approach. Measure everything. Waste nothing. Recycle as much as possible. If done properly, our gardens could be oxygen farms, CO2 recycling centres, nutrient and metal recovery plants, and water purification systems. But if we fail to pay careful attention to habitat design, plant life could be our undoing out there.
The Biospherians were lucky to be inches away from a plentiful source of fresh air. Millions of miles from Earth, we better get our space ecology right.