With Nasa's Artemis countdown already begun, humans will soon be returning to the Moon – this time to establish a manned base. For any moonbase project to succeed long-term, astronauts must be able to grow their own food.
The lunar soil, known as regolith, poses significant horticultural problems, being essentially a powder in which it is difficult to grow plants. The Moon is also characterised by temperatures that can reach as high as 127°C during the day, then fall to as low as -173°C at night.
Addressing all of these issues is the LunarPlant international project, headed by NTNU Social Research and the Centre for Interdisciplinary Research in Space (Ciris).
Norwegian researchers are also part of the LunarPlant project, with the independent research organisation Sintef working in collaboration with the VTT technical research centre in Finland to develop a substrate that functions as a supportive collar for growing plants.
Sintef researcher Galina Simonsen and her colleagues believe that it will be possible to grow food plants on the Moon, provided that a suitable solution can be found that addresses the rational use of available resources, combined with sufficient light and an artificial atmosphere, and a replacement for fertile 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," Simonsen said. "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 is used by some horticulturalists, but rockwool is not a sustainable material on the Moon.
"Sending rockwool to the Moon could cost up to 20 million Norwegian krone [£1.5m] 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."
The Sintef and VTT researchers have developed a cellulose-based substrate, produced from plant waste, that functions as a supportive collar for the growing plants and potentially makes a good alternative to soil and rockwool.
This substrate could first be used as insulation material for the secure transport of vital and sensitive equipment that has to be carried from Earth to the Moon. On its arrival, it can be reused as a growing medium. 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," Simonsen said.
"Its application is limited to only certain types of plants. Only a few plants can be cultivated hydroponically, such as tomatoes, cucumbers, strawberries and salad vegetables. Others, such as root vegetables, cannot be grown using this method."
Simonsen added: "You may already have heard of hydroponics. 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 kilogrammes of ice. 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."
Growing plants in water requires that the water contains sufficient nutrients. As the LunarPlant project is based on the circular use of resources, the 'fertiliser' that will be used on the Moon will be provided by the astronauts themselves in the form of human waste – i.e. their urine.
This 'liquid gold' – already a celebrated solution deployed by many Earth-based gardening enthusiasts to make their flowerbeds bloom – is the name given to urine diluted in water. However, it is not recommended to use urine as a fertiliser for plants intended for human consumption. The researchers are addressing the challenge of identifying how to use this natural resource safely in space.
"Barriers linked to the use of urine as a fertiliser include the strict regulations governing the use of human waste in food plant cultivation," Simonsen said. "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 'liquid gold' must be analysed carefully and accurately so that safe threshold values can be determined 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 (the lunar soil)," said 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."
The substrate is essential to the process, but it is also important to know how the water it contains behaves. For example, some plants do not respond well to having their roots sitting in water.
"The water must not be allowed to stagnate," Simonsen said. "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."
The researchers borrowed the concept of multiphase flow modelling from the oil industry to better understand the issues.
"Technologies used for oil and gas transport have proved to be transferable," said 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."
The LunarPlant research also holds potential benefits for growing plants in hostile regions on Earth.
"This method of cultivation can be applied anywhere," said Simonsen. "Urine contains phosphorous, which is a non-renewable resource, and rockwool, which is currently used in a number of situations, is not biodegradable."
With the projected 2024 date looming for the next Moon landing, research into how astronauts can survive on the lunar surface has gone into overdrive.
In May last year, scientists from the University of Florida demonstrated plants grown in soil taken from the Moon for the first time.
In 2021, a team of engineers developed an experimental zero-gravity fridge that could allow astronauts to chill food, giving them access to longer-lasting and more nutritious food. The fridge also raises the possibility of drinking a cold beer in space.
Meanwhile, the tricky issue of building habitats on the Moon in which astronauts can live also has researchers and engineers focused on the properties of lunar regolith.
The use of astronaut urine has once again been identified as a possible solution to this problem, as the urea in urine could be used as a plasticiser in the construction of concrete structures.
Failing this, scientists have also discovered lunar pits and caves on the Moon where it's always 'sweater weather', potentially allowing humans to live and work in comfortable steady temperatures.
