How do you feed astronauts on extremely long journeys?
European astronomers have calculated the amount of onboard space needed to feed the crew of a multi-generational ship – one of the many challenges of journeying to planets orbiting stars other than the sun.
“With our current technology, it is not feasible to reach an exoplanet in less than several centuries of travel,” explains Frédéric Marin, astronomer at the Astronomical Observatory of Strasbourg in France, who led the study.
To reach even the closest exoplanet – Proxima Centauri b – would take about 6300 years.
This is well beyond the lifespan of any astronaut, and so such spaceships must be multi-generational.
“The original population would grow old and die,” says Marin, “leaving their descendants to continue travelling.”
Marin and his team are interested in how human populations may evolve and survive in the resource-limited environment of space – especially when the journey is one-way.
They previously calculated the minimum size of the initial crew to ensure that their descendants arrive at the other end without genetic disorders. Turns out, just 98 crew members are needed to ensure a 100% success rate for a 6300-year journey.
All about the numbers
How do you feed astronauts if the journey lasts millennia? Dried food stocks aren’t an option because they would take up far too much space and their vitamins would deteriorate over time. Astronauts on the ISS currently consume about 1.8 kilograms of packaged food per day; to feed crew this way over centuries would add up to millions of tonnes of cargo.
The best and most obvious option is to grow crops and raise animals aboard the ship – essentially, to run a farm. This would not only produce fresh food, but also recycle nutrients and faeces, generate oxygen, and continuously purify the air – making the spaceship into a closed ecological system.
Space agriculture systems are already being tested. The Lada greenhouse on the ISS has been growing plants since 2002 in microgravity.
But spaceships have limited space, so just how much would need to be dedicated to farming?
Marin and colleagues used a numerical code called HERITAGE to tackle the question. They estimated the annual caloric intake of the crew aboard, then combined this with parameters set by modern farming techniques to predict the size of the space required.
For a crew of 500 people with omnivorous diets, just 0.45 square kilometres would be enough to feed them all. This assumes that fruits, vegetables, starch, sugar, and oil would be produced using aeroponic techniques, which allow plants to grow without soil, while conventional farming would produce meat, fish, dairy, and honey.
Marin says that this size compares well with a prediction by the Food and Agriculture Organisation of the United Nations, which states that half a hectare of land per person is the minimum amount of agricultural land necessary for sustainable food security, with a diet including meat.
“Our results go a step beyond by including aeroponic processes and considering the effect of microgravity,” says Marin.
Mike Dixon, an independent researcher at the Controlled Environment Systems Research Facility at the University of Guelph in Canada, points out that previous studies have actually come up with smaller land requirements per person.
Just 75 square metres each, he says, would “suffice to provide all the functions of a vegetarian life support system mostly driven by the food requirement – water recycling, O2 production and CO2 scrubbing being more than double the human life support requirements by that much plant material”.
The larger requirement calculated by this new study may be to produce the non-vegetative food requirements – which, Dixon says, “seems a bit excessive”.
Briardo Llorente, synthetic biologist at Macquarie University in Australia, and not involved in the study, notes that “in the future, we will most likely develop crops and also microbial-based food to entirely replace the production of animal-based food. Substitution of ‘food animal protein’ with engineered plants and microbes is indeed foreseeable.”
He also points out that increasingly productive crops are currently being developed.
“It would have been interesting to see estimations that take into account what we can estimate the crops of the future will be,” he says.
Land area is not the only issue facing food production in a spaceship. Plants have previously been shown to grow more slowly in space. Although the lighting, temperature, humidity and carbon dioxide of Earth can all be mimicked, gravity and radiation are harder to control and have not yet been extensively studied.
“It seems that we need to learn more about how plants – and humans – can deal with radiation damage, along with several other things such as growing in microgravity, before planning for a long voyage,” says Rupesh Paudyel, a plant scientist at the University of Leeds in the UK, who was also not involved in the study.
Paudyeul adds that “we also need to consider where plants would get nitrogen and phosphorus and other vital nutrients. These key nutrients are essential for plants and they are unable to fix them, and in deep space travel, we need to consider how these can keep their supply steady for the plant to carry on growing.”
What about water?
“How do you feed astronauts?” may not even be the biggest question. This new study also does not touch upon perhaps the most important requirement for life – water. Food might be easy, but creating a sustainable water cycle on a spaceship is immensely more complicated. This is a problem that Marin and his team hope to address in the future.
Still, no estimation is perfect and Marin and his team take a good crack at it. The numerical tool used to make these calculations, HERITAGE, is especially interesting. It is the first of its kind – a code entirely focused on multi-generational ships, investigating their mathematical, biological, demographical and statistical feasibility.
“The code contains many biological, demographic, anthropological, physical and chemical data that has been carefully collected from human sciences as well from Mir and ISS studies,” explains Marin.
It uses the Monte Carlo method, a mathematical approach that employs repeated random sampling to test all possible outcomes of a scenario – such as the evolution of a space crew over the generations.
In this study, Marin and team achieved their predictions by looping their code one thousand times, which is the equivalent of simulating the requirements of one million humans.
“There are still many more steps to be taken in order to provide a realistic simulation of a global generation ship,” says Marin.
Next, he and his team aim to push HERITAGE to higher grounds by including population genetics and mutation.