For humans to survive off Earth, we’ll need plants to grow there, too.
Earlier this month, tiny green plants sprouted on the moon.
The plants arrived as cotton seeds, tucked inside of Chang’e 4, a Chinese spacecraft that had landed, in a historic first, on the far side of the moon, the side that never turns toward Earth. The seeds came with the comforts of home: water, air, soil, and a heating system for warmth. Huddled together, the seedlings resembled a miniature, deep-green forest. A hint of life on a barren world.
And then, about a week later, they all died.
Lunar night had set in. Without ample sunlight, surface temperatures near the spacecraft plummeted to –52 degrees Celsius (–62 degrees Fahrenheit). The sprouts’ heating system wasn’t designed to last. The plants froze.
Outer space, as you might expect, is not kind to plants, or people, or most living things, except maybe for tardigrades, those microscopic creatures that look like little bears. If you stuck a daisy out of the International Space Station and exposed it to the vacuum of space, it would perish immediately. The water in its cells would rush out and dissipate as vapor, leaving behind a freeze-dried flower.
China’s experiment marked the first time biological matter has been grown on the moon. (There is biological matter on the moon already, in what NASA politely refers to as “defecation collection devices.”) But plants have blossomed in space for years. They just need a little more care and attention than their terrestrial peers.
The first to flourish in space was Arabidopsis thaliana, a spindly plant with white flowers, in 1982, aboard Salyut, a now defunct Russian space station. The inaugural plant species was chosen for practical reasons; scientists call Arabidopsis thaliana the fruit fly of plant science, thanks to a fairly quick life cycle that allows for many analyses in a short time.
Now, plants grow on the International Space Station, humankind’s sole laboratory above Earth. They are cultivated inside special chambers equipped with artificial lights pretending to be the sun. Seeds are planted in nutrient-rich substance resembling cat litter and strewn with fertilizer pellets. Water, unable to flow on its own, is administered carefully and precisely to roots. In microgravity, gases sometimes coalesce into bubbles, and overhead, fans push the air around to keep the carbon dioxide and oxygen flowing.
The most advanced chamber on the station, about the size of a mini fridge, has precise sensors monitoring the conditions inside, and all astronauts need to do is add water and change filters. Scientists back on the ground can control everything, from the temperature and humidity to levels of oxygen and carbon dioxide.
Plants did not evolve to exist in this unusual setup. But astronauts have grown several varieties of lettuce, radishes, peas, zinnias, and sunflowers, and they do just fine. “Plants are very adaptive, and they have to be—they can’t run away,” says Gioia Massa, a scientist at NASA’s Kennedy Space Center who studies plants in microgravity.
Scientists were surprised to learn that the lack of gravity, the force that has shaped our biological processes, doesn’t derail plants’ development. On Earth, plants produce a filigree-like pattern of roots, as they grow away from the spot they’re sown in search of nutrients. Scientists had long assumed the movements were influenced, in part, by the force of gravity. On the International Space Station, roots exhibited the same pattern, without gravity as a guide.
“Plants don’t really care about the gravity so much if you can get the environment right,” Massa says.
For NASA, the growth chambers on the space station are the predecessors of extraterrestrial farms beyond Earth. If human beings ever travel to another planet, they will need enough food for the journey. NASA has spent years perfecting thermo-stabilized or freeze-dried entrées and snacks for astronauts on the International Space Station, from scrambled eggs to chicken teriyaki. The meals are meant to last, but they wouldn’t survive the long journey to Mars, says Julie Robinson, the chief scientist for the International Space Station.
“We don’t have a system today that would preserve all the nutrients in food for all that time, even if it was frozen,” Robinson says.
Future Mars astronauts will likely bring with them an assortment of seeds, a Svalbard-like vault to kickstart the first generations of crops. None will be able to grow in Martian soil, which resembles volcanic ash; it’s devoid of the organic matter—formed on Earth by generations of decomposed plants—that supports life. It also contains chemical compounds that are toxic to humans. Astronauts could flush out the toxins with their own chemical solutions and convert the soil into something workable, but it may be easier to replicate the growth chambers on the International Space Station instead.
On Mars, plants will likely grow in climate-controlled greenhouses, from nutrient-rich gels and under bright lighting, with water delivered through liquid solutions at their roots or by a fine mist released from the ceiling. And anyone living on Mars will need many of these alien gardens; you can’t grow a salad from a petri dish.
Astronauts have already made a space salad. In 2015, astronauts on the space station were allowed to try the leaves of a red romaine lettuce that was cultivated in NASA’s first fresh-food growth chamber. They added a little balsamic dressing and took a bite. “That’s awesome,” NASA astronaut Kjell Lindgren said then. “Tastes good.”
No one on Earth has sampled a space vegetable yet, according to Massa. Some plants grown on the station are sent down to the ground for study in the lab, but they usually come back frozen or preserved in a chemical solution. “Frozen would be better, but I don’t think lettuce popsicles will be very popular any time soon,” she says.
NASA scientists are thinking about more than nutrition in these experiments. Growing plants, just for the sake of growing plants, is quite nice. Research has shown gardening is soothing and can be beneficial for good mental health. Future deep-space astronauts, cooped up in a small spaceship for years with the same people, will need all the soothing activities they can find. Plants, especially flowers, grown not for consumption but for decoration, may help far-flung astronauts feel connected to the comforts of Earth.
“There’s a great deal of joy in growing and watering the plants and producing a flower,” Robinson, the ISS scientist, says. “There can also be some real sadness if plants you’ve been cultivating are not successful and are dying on you.”
Anyone who has enthusiastically purchased a succulent and witnessed it inexplicably wilt days later might relate. Imagine the magnitude of that disappointment on Mars, where the closest store is all the way across the solar system, and the only option is to grow another one.