Salad grown in urine using hydroponic techniques could be on the menu for astronauts dwelling on the moon as part of the Artemis programme.

Over the next six years, humans will be returning to the Moon for the first time since Apollo 17 in 1972. This time they will be staying at a crewed base, and if the project is to succeed, they must be able to grow their own food.

Norwegian researchers are in the process of making this possible using hydroponics - the science of growing plants without using soil, by feeding them nutrient minerals dissolved in water. The results could also make it easier to grow food in inhospitable environments on Earth.

Lunar soil - or regolith - is essentially a powder which lacks the structure needed to grow plants. As if this was not enough, the Moon is characterised by temperatures that can reach 120 degrees celsius during the day, and fall as low as -130 degrees celsius at night.

Galina Simonsen from research company SINTEF and her colleagues believe that it will be possible to grow food plants on the Moon, and are working on the international LunarPlant project. She says meeting this challenge requires a rational use of available resources, combined with sufficient light and an artificial atmosphere. There is also a need to find a replacement for fertile soil.

“You may already have heard of hydroponics”, says Simonsen. “This is a method of growing plants in water, which is entirely possible if the water contains sufficient nutrients. The use of this method is essential to the success of this project.

“Radar data indicates that the Moon’s polar regions hold more than 600 billion kilograms of ice”, she explains. “This is enough to fill about 240,000 Olympic-sized swimming pools.

"It is much less than we have on Earth, but will be enough to enable humans to maintain some level of activity. The ice will be melted to form water which will be used to cultivate food plants.”

However, growing plants in water requires that sufficient nutrients are made available. The fertiliser available on the Moon will be provided by the astronauts themselves, in the form of urine, because it is necessary for the mission to adopt circular use of resources. One of the challenges for the project is to find out how to use this essential resource safely.

“Barriers linked to the use of urine as a fertiliser include the strict regulations governing the use of human waste in food plant cultivation,” explains Simonsen. “In addition, the handling of human urine is generally unpleasant, combined with the odour, and the fact that it releases long-lived organic environmental toxins and trace metals.”

Plants grown in diluted urine must be analysed carefully and accurately so that safe threshold values can be identified, with a view to approving their use as a food source. Moreover, the plants themselves have to contain sufficient nutrients.

“It may be possible to extract some nutrients for plant growth from the lunar regolith,” says Simonsen," but these are somewhat meagre. Urine can provide nitrogen, potassium and phosphorous.

"If you are cultivating salad plants, you can also grow other edible plants that can assist with the regulation of both the water quality and nutrient balance in the system,” she says.

Soil support

Another aspect of the LunarPlant project is focusing on soil. Or, to be more precise in the case of the Moon, the lack of soil. Soil on Earth not only provides nutrients – it is also the habitat in which the plants live.

“We’re trying to find out how we can get the plants to grow without collapsing”, says Simonsen. “This involves identifying a growing medium that enables plants to develop a root system that gives them adequate support.”

In order to stand upright, plants prefer to have something solid in which to drive their roots. Currently, Rockwool - a stone-based mineral fibre - is used by some hydroponic horticulturalists, but it is not a sustainable material, at least not on the Moon.

“Sending Rockwool to the Moon could cost up to [US$1.9 million] per kilo,” explains Simonsen. “For this reason, it is important that we can use a material that is entirely circular.

"It has to be light and multifunctional. In other words, a material that can first be used for a purpose other than that as a growing medium.”

Cellulose substrate

In collaboration with the VTT technical research centre in Finland, the researchers have developed a cellulose-based substrate that functions as a supportive collar for the growing plants.

The cellulose is produced from plant waste and the substrate can initially be used as insulation material for the secure transport of vital and sensitive equipment to the Moon. So far the results have been promising.

“We observe that the substrate doesn’t break up in the aqueous growing medium. Its components are also plant-friendly and free from any chemicals that may have a negative impact on plant growth or food safety,” says Simonsen.

However the system's application is limited to a few plants that can be cultivated hydroponically, such as tomatoes, cucumbers, strawberries and salad vegetables. It is also important to know how the water it contains behaves. For example, some plants do not like getting too wet.

“The water must not be allowed to stagnate”, says Simonsen. “There must always be adequate water flow.

"Both air and water have to be transported efficiently through the material in order to ensure healthy plant growth. The substrate must be sufficiently robust to support a fully developed plant and at the same time enable the roots to grow unobstructed.

“Technologies used for oil and gas transport have proved to be transferable,” says Simonsen. “The methods we apply for fluid hydrocarbon transport in major installations can be transferred to the mechanisms working in minute structures such as these plant substrates.

“Our aim is to construct a digital model that simulates the different factors that influence the behaviour of the substrate. This will enable us to run simulations under conditions that are identical to those on the Moon, including the effect of weightlessness.”

Food on Earth

The research could play an important role in providing food here on Earth, as the technology can also be used to grow plants in hostile environments on this planet.

Simonsen says, “This method of cultivation can be applied anywhere, and is particularly important in the context of resource utilisation. Urine contains phosphorous, which is a non-renewable resource, and Rockwool, which is currently used in a number of situations, is not biodegradable.”

The research is headed-up by the Norwegian University of Science & Technology (NTNU) Social Research and the Centre for Interdisciplinary Research in Space (CIRiS).