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Telescopes and space missions

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03 Apr 2018
This article first appeared in the April 2018 issue of Physics World

To survive far away from Earth, astronauts need to have green fingers. But the science of growing plants in extraterrestrial environments has a way to go, as Jon Cartwright reports

It is known as the Habitat: a white geodesic dome, perched on the northern flank of a red mountain. Inside, there is sleeping room for a crew of six, plus a small kitchen, laboratory, bathroom and an airlock. Outside, apart from a solar array, is very little except reddish dust and rocks. Virtually no flora or fauna. No sign of civilization.

This isn’t Mars, though it’s a fair simulation, even if it’s not quite Matt Damon in the Hollywood movie The Martian (above). This is the Hawaii Space Exploration Analog and Simulation (HI-SEAS) – a small research site on the desolate Mauna Loa volcano on the island of Hawaii, one and a half hours’ drive from the nearest town. Designed for investigations into how crew members can live in close quarters for long periods of time, HI-SEAS is not a terrestrial version of a Mars base in all respects – the inside is more like an apartment, and the water, though strictly rationed, is not recycled. But in terms of day-to-day life, says Lucie Poulet, a former crew member, “we’re close to being the first Martians”.

Despite bold pronouncements by NASA, the Chinese space agency CNSA and private enterprises such as SpaceX, the prospect of sending humans to Mars is still littered with challenges. For HI-SEAS, the psychological toll is a key area of study, but Poulet – who is now completing a PhD at the Université Clermont Auvergne in Clermont-Ferrand, France – is interested in a more basic requirement.

We’re talking food.

On any space mission the amount of stored food is limited, which means that very long missions away from Earth’s orbit will always require a way of growing fresh produce. According to Poulet, an astronaut needs a minimum mass of 800 g of food (measured as dry matter) per day to survive; for a mission to Mars lasting several years, that amount could rise to over a tonne. Multiply that mass by the number of crew members and you can see why a renewable source of plant matter is vital for long-term space travel.

Lettuce alone

Photo of HI-SEAS

While confined in HI-SEAS for four months in 2014, Poulet managed to grow some lettuces and radishes, although her crop primarily existed for study purposes and was far from enough to survive on. Indeed, growing food on a hostile planet, or in the zero gravity of space, is no mean feat. Outside the Earth’s protective magnetosphere, ionizing radiation is a major factor. Ever since 1946, when NASA sent a V-2 rocket carrying seeds of maize into space, scientists have been interested in the effects of radiation on plants.

Generally speaking, radiation can prevent seeds from germinating, or stunt the growth of those seeds that do manage to germinate, by damaging proteins or DNA. But the situation is complicated, as space is home to various forms of high-energy radiation and some species are more resilient than others. In 2008, for example, an international team of astrobiologists working under a project called EXPOSE arranged for seeds of tobacco and other species to be attached to the outside of NASA’s International Space Station (ISS) for one and a half years, some with shielding from ultraviolet rays, some without. When the seeds were finally recovered, 60% of the shielded seeds germinated, compared with a mere 3% of the unshielded seeds.

Clearly radiation is to be avoided, but this brings complications when it comes to supplying seedlings with light for photosynthesis. There is no transparent material that can be made thick enough to totally withstand high-energy radiation, so on a planetary base any natural sunlight would have to be redirected by parabolic mirrors and optic fibres to shielded or underground greenhouses. More likely would be some form of artificial lighting. The relatively recent development of efficient, broad-spectrum light-emitting diodes (LEDs) helps here, but even LEDs require some energy, which is usually in short supply.

Growing without gravity

Photo of Zinnia flowers on the ISS

Even so, it may be the travelling to and from moons and planets that brings the biggest issues, due to zero or micro-gravity. On Earth, gravity encourages surface water to be absorbed evenly into the soil; when that force is absent, surface tension becomes dominant, and water tends to congregate around a plant’s roots. Counter-intuitive though it may sound, this is not a good thing: roots need to respire, and a root smothered in water will drown. NASA has tried to solve this problem with an artificially created growth medium borrowed from professional baseball fields. Known in the sporting trade as “turface”, the medium consists of slow-release fertilizer, calcined (baked) clay and crushed granite, all of which is sieved to improve aeration.

A similar issue to water distribution arises with a plant’s leaves, as scientists have long known from tests on the Russian space station Mir and on the ISS. Without gravity, there is no natural convection (cold air has no weight in order to sink) and so leaves suffer from a stagnated atmosphere, unable to transpire and unable to receive new carbon dioxide for photosynthesis. What’s more, waste gases begin to accumulate, including ethylene, which inhibits growth. Artificial circulation can ease matters, but according to Poulet, the problem gets worse when lots of plants are packed in tightly. Even apparently slight inhomogeneities in air flow can make a large difference to local carbon-dioxide and ethylene levels, and hence plant growth. “If you have a big space, it’s really hard to have the same flow everywhere,” she says.

Technology will help to solve these matters. But a deeper challenge, and one that particularly draws the attention of physical scientists, is predicting how plants will respond in subtly different conditions. An engineer by background, Poulet has been developing a theoretical model for plant growth in low gravity that contains some 30 parameters, including temperature, humidity, air pressure and carbon-dioxide levels. The model needs to be validated with real, species-specific data, although such data aren’t that easy to come by. In one recent experiment Poulet and her colleagues took a parabolic flight in order to collect specific data on gas exchanges and temperatures of spinach leaves between gravity accelerations of zero and 2g.

Once validated, the computer models can make real-world predictions, and in this field, those predictions can have important consequences. “If there is a lighting failure [aboard a spacecraft], for example, how much oxygen will be lost?” asks Poulet. “How much biomass will not be produced? Will the crew survive, or will they starve to death?”

A bit of a mouthful

Photo of Oscar Monje working on the APH

Poulet and her colleagues still have some way to go to make such life-and-death predictions. In the meantime, scientists elsewhere are learning how to make the greenhouses themselves more efficient. For the past three years, NASA has operated a plant growth system on the ISS known as Veggie. Developed by the aerospace company Orbitec in Madison, Wisconsin, in the US, Veggie consists of a tray of six “pillows”, each containing several seeds in an aerated growth medium, beneath a bank of coloured LEDs. To initiate growth, astronauts syringe into a wick in each pillow a meagre 100 mm of water, then they monitor the seedlings carefully.

In August 2015, thanks to Veggie, astronauts on board the ISS ate freshly grown food for the first time – red romaine lettuce. The meal helped to demonstrate that the crop did not contain harmful levels of contaminants, although it was only a few mouthfuls. Gioia Massa, a scientist working on the Veggie project at NASA’s Kennedy Space Center in Florida, believes there are still many challenges for space food production: effective watering, maximizing space, preventing the growth of plant diseases, and finding the optimal conditions for growth, nutrients and flavour. Water can mostly be recycled on spacecraft, and on Mars levels could be topped up by extracting it from below the planet’s surface. “I don’t see any insurmountable obstacles,” Massa says.

It is not all about space – sometimes results can have a bearing on Earth-grown crops. Although they have a microgravity focus, models similar to Poulet’s contain enough parameters to help scientists understand how agriculture will be impacted under other extreme conditions, such as climate change. And last year, biologist Hideyuki Takahashi of Tohoku University in Japan and others reported a potentially very useful insight from an ISS experiment: the roots of cucumbers grown in space curl towards a water source placed to one side in the growth medium. On Earth, plant roots tend to grow downwards with gravity, and in fact Takahashi’s group found that gravity, when present, was able to override the cucumber’s unusual water-seeking tendency, too. But the researchers believe that it may be possible to genetically engineer cucumbers and other vegetables so that a water-seeking tendency is dominant. That could enable them to grow better on Earth when water is scarce (New Phytologist 215 1476).

Veggie futures

Plant research in space is consistently advancing. Last October, a new NASA–Orbitec growth unit was installed on the ISS: the Advanced Plant Habitat (APH). Unlike Veggie, the APH is fully enclosed and has some 180 sensors and controllers for nearly every aspect of the environment. It only requires astronauts to initiate growth; thereafter, the system is monitored and controlled remotely by scientists back at Kennedy. “The APH will not only be a high-quality plant-physiology instrument for better understanding space plant growth, but also it will allow researchers to understand what parameters need to be controlled for space crop production, and how to control these in microgravity,” says Massa.

The APH is not necessarily a vision of the future, but how to get there. “A future space garden will likely be more controlled than the Veggie chamber on the ISS – which uses the ISS environment and additional lights and fans to grow plants – but it will likely be less controlled than the APH, which is a scientific tool to understand plant growth,” Massa continues. “What we learn from both of these will allow us to better optimize space plant gardens…We hope to provide the crew with the capability to regularly supplement a packaged diet with fresh, nutritious, delicious produce – no matter where they live and work.”

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