Space farming

On the International Space Station (ISS), fresh food is the ultimate luxury. Each pound of food costs around $10,000 to send to the ISS, so the focus is on supplying highly processed food containing the maximum number of calories and with a long shelf life. What small amount of fresh food is sent up, including carrots and apples, is quickly devoured.

But what if the astronauts could grow their own fresh food on board the ISS? That is the idea behind Nasa’s Veggie plant growth system, which arrived on the ISS in April 2014. The Veggie system is a square, plastic chamber, 11.5 inches wide and 14.5 inches deep, with air pumps at the top and a bank of red, blue and green LEDs. Several ‘pillows’ containing seeds in growth media – a mixture of controlled-release fertiliser and calcined clay – are placed into the chamber. These pillows are irradiated with light from the LEDs, exposed to a flow of air from the air pumps and supplied with water from a reservoir beneath.

As a first test, Nasa astronauts on the ISS successfully grew red romaine lettuce from May to June 2014. Unfortunately, though, they weren’t allowed to use these leaves as part of a tasty salad, because of concerns that the lettuce could be contaminated with pathogenic microbes. Instead, the lettuce was frozen and packed for return to Earth, where Nasa scientists are investigating whether it’s safe to eat.

‘We’re working very hard to get permission for the crew to consume what’s grown in Veggie,’ says Howard Levine, chief scientist at Nasa’s Kennedy Space Center Utilization and Life Sciences Division in Florida, US. ‘It’s possible that in the next year or so they may be able to consume the food that they grow.’ While beginning with lettuce, this food will also include other ‘pick and eat’ vegetables, including dwarf tomatoes, cabbage and spinach.

Still, even when Nasa astronauts are given the go-ahead to eat what they grow in the Veggie system, it’s only ever going to be able to produce a tiny supplement to their main diet. But the Veggie system is likely just the beginning. As astronauts begin to spend longer periods in space at greater distances, the ability to grow their own food will become increasingly important, as it will be impractical to supply them regularly from Earth. This will certainly be the case if and when we begin to establish bases on the Moon, Mars and other bodies in the Solar System. Indeed, our ability to grow food on other planetary bodies may even be essential for humanity’s long-term survival.

In truth, plants have been going into space for even longer than humans: seeds of wheat, pea, maize and onion were carried on Russia’s Sputnik 4 in 1960, a year before Yuri Gagarin became the first man in space. Plants have since been grown on every space station in low-Earth orbit, with the model plant Arabidopsis thaliana grown on Russia’s Salyut 7 and vegetables and wheat grown on Mir.

These efforts have accelerated since the construction of the ISS, which has played host to a variety of plant growth experiments in many different plant growth systems, including the Lada greenhouse unit and the Advanced Biological Research System. In contrast to the Veggie system, which was specifically designed to grow food for eating, the aim of these systems is to investigate how plant growth and reproduction are affected by the space environment.

The most obvious aspect of space likely to affect plant growth is the lack of gravity, although there is actually a small, residual ‘microgravity’ caused by orbiting around the Earth. Compared with the surface of the Earth, plants in space are also exposed to higher levels of radiation and greatly reduced magnetic fields, both of which could impact their ability to grow and reproduce.

The amount of space-borne radiation, in the form of cosmic rays and solar wind, reaching the surface of the Earth is minimal, as it is blocked by the Earth’s magnetic field and atmosphere. The ISS, however, receives around 20 centisieverts (cSv) of radiation/year, and may even be exposed to 10cSv over just a few days during an intense solar storm.

Nevertheless, microgravity has the largest short-term effect on plant growth and development. ‘Gravity is a huge force that shapes life on Earth, especially plants, which are exquisitely attuned to gravity as they make their growth direction and development decisions,’ says Robert Ferl at the University of Florida in Gainesville, US, who has studied plant growth on the space shuttles and the ISS.

These and other studies over the years have found that nutrient uptake and gas exchange in plants are adversely affected by microgravity, as are cell growth and seed development. Radiation, on the other hand, is likely to have longer-term effects on plant genomes and these will probably only become apparent when multiple generations of plants are grown in space, which hasn’t happened yet.

Despite the adverse effects of microgravity, what these studies also show is that plants will grow in space, albeit not always as well as on Earth. Furthermore, it turns out that many of the adverse effects of microgravity are actually indirect effects and thus can be resolved with technology. For example, the reduced rate of nutrient uptake and gas exchange is due to the fact that convection doesn’t take place in microgravity: warm air doesn’t become more buoyant and rise to displace cooler air.

As a result, there is no natural air flow around the plants and their roots, causing a layer of dead air to surround the plant, reducing nutrient uptake and gas exchange. This can be resolved quite easily by simply pumping air around the plants and through any growth medium, which is why the Veggie plant system contains air pumps.

Some of the other adverse effects on plant growth are nothing to do with microgravity or radiation at all, but are to do with the non-ideal growing conditions found in space stations. Foremost among these is that artificial light is not as good as sunlight for conducting photosynthesis, hence the Veggie plant system’s use of a bank of red, blue and green LEDs to replicate more closely the wide range of wavelengths found in sunlight.

The Veggie plant system demonstrates that, with the right kind of support, edible plants will grow quite happily in space. The question now is how to make this process as efficient as possible, such that the maximum amount of edible biomass can be produced for the minimum amount of inputs, especially inputs brought up from the Earth. One obvious way to do this is to grow each new generation of plants from seeds produced by the preceding generation, but this will require investigating in detail the long-term effects of space-borne radiation on plant genomes.

This is why two new plant growth systems on the ISS, Biolab and the European Modular Cultivation System (EMCS) contain centrifuges. By spinning the plants, these centrifuges can replicate the gravitational force (1g) on the Earth’s surface, offering scientists the ability to study the effects of space-borne radiation in isolation.

As well as replicating 1g, the centrifuges can also produce fractions of a g, known as partial g. This means they can replicate the gravity found on the Moon, which is a sixth of a g, and Mars, which is a third of a g. It is on these planetary bodies where space farming could really take off, because future inhabitants of bases on these bodies will have no option but to grow their own crops; fortunately, this should be easier to do on the Moon or Mars than in space.

‘Many phenomena that you can observe at microgravity aren’t nearly as pronounced when you have partial gravity,’ explains Levine. ‘As long as there is some level of gravity, for instance, you can still get convection.’

Besides producing food, these crop plants will also need to soak up carbon dioxide, produce oxygen and recycle water, providing an essential component of the critical life support systems on these bases.
One of the most advanced of these systems is known as the Micro-Ecological Life Support System Alternative (MELiSSA). MELiSSA consists of five compartments:  three contain microbes that break down solid waste into fatty acids, nitrates and carbon dioxide, which are then used as nutrients for the growth of photosynthetic cyanobacteria and higher plants in the fourth and fifth compartments.

MELiSSA is a collaboration between several different groups under the aegis of the European Space Agency. A group from the University of Guelph in Canada, led by Mike Dixon, has been handed responsibility for the higher plant compartment, reflecting Canada’s experience of growing plants in inhospitable locations.

‘The next worst place after a snow bank in Canada to try to grow a plant has got to be the moon,’ says Dixon.

What their work has already shown is that the food requirement trumps everything else. ‘It’s food that drives the equation,’ explains Dixon. ‘The food needed to sustain all the life support requirements of a single individual would take between 60 and 80m² of plant growing area. Now, that provides more than twice the oxygen, fresh water and CO₂ scrubbing capacity that a single human would require.’

They have also determined the optimum mix of characteristics that crop plants grown in the higher plant compartment should possess. These include a short cultivation cycle from seed to seed, high productivity, natural resistance to pathogens, small plant size, high levels of adaptability to expected space conditions, and high nutritional value. Crops that possess these characteristics and are therefore currently being considered for MELiSSA include wheat, potato and soya beans.

Most recently, Dixon and his team have been exploring ways to grow these plants as efficiently as possible. They have developed novel lighting systems based on highly efficient LEDs to enhance photosynthesis and thus plant growth, and novel sensor technologies to monitor the concentration of nitrates, sulphates and potassium in the nutrient solutions that feed the growing crops.

Growing crop plants in such nutrient solutions, known as hydroponics, is the favoured approach for life support systems such as MELiSSA, especially as water in the form of ice is available on the Moon and Mars. But there is another option;for it turns out that both the Moon and Mars possess a form of soil that plants may be able to grow in.

On Earth, soil is formed by the action of both physical weathering and living organisms on rocks. While living organisms are distinctly lacking on the Moon and Mars – at least as far as we know – physical weathering does occur in the form of impacts by meteorites, chemical reactions and, in the case of Mars, water flowing in its warmer past. As a consequence of this weathering, the surfaces of the Moon and Mars are both covered in sand-like material that scientists are increasingly realising can be classed as soil.

Analysis of samples brought back from the Moon and in situ analysis of rocks by the Mars rovers, have revealed that these soils contain all the nutrients required for plant growth apart from reactive nitrogen, which on Earth is mainly derived from organic matter.

This fact recently inspired a team of Dutch scientists, led by Wieger Wamelink at the Wageningen UR research centre in the Netherlands, to try growing a range of different crop plants, including tomato, rye, carrot and cress, on replicas of lunar and Martian soil (PLOS ONE, 2014, 9, e103138). These replica soils were based on volcanic soils from Hawaii.

They found that almost all the plants would germinate and grow in the replica soils, despite the lack of reactive nitrogen, with some even forming seeds; however, all the plants grew much better in the Martian soil, probably because it contained higher levels of carbon. These soils could also easily be improved by adding the missing reactive nitrogen. On the Moon or Mars, this could be done by adding human waste. On Mars, it could also be done by adding bacteria that can fix atmospheric nitrogen, although the Martian atmosphere is very thin.

Nevertheless, Wamelink now plans to explore this possibility. ‘Special attention will be paid to nitrogen-fixing bacteria, since nitrogen content in the soils is low for agricultural purposes and bacteria in combination with legumes could partly solve this problem,’ he says.

Even better soils may be found on asteroids, especially carbonaceous asteroids, which contain carbon, nitrates and phosphates. Indeed, Michael Mautner at Virginia Commonwealth University in Richmond, US, has already grown various plants, including asparagus and potato, in material from carbonaceous meteorites.

Based on these findings, Mautner calculated in a recent paper that all the known carbonaceous asteroids could yield enough biomass to support 1bn people for the future habitable lifetime of the Solar System (Planetary and Space Science, 2014, 104, 234).

Which just goes to show that what started out with a single lettuce could end up ensuring that billions of our space-faring descendants could one day in the future always be able to enjoy fresh food.

Jon Evans is a freelance science writer based in Bosham, West Sussex