Made in orbit: Fresh food

Food and drink are crucial for sustaining human presence in space. We’ve all seen the dehydrated food packets sent up with astronauts to keep them fed, and how unappetising they look. This is done to keep resources light for easy flight, and to extend its shelf life. But what if we could make fresh food in space? The ISS is already making efforts to grow vegetables in orbit, so what other produce may be possible. Jennifer Bromley is a researcher in plant sciences at Churchill College, Cambridge. She studies the impact of space on the way plants grow, and whether it is possible to grow fresh produce in orbit…

Jen - Getting food into space—getting anything into space—costs approximately 20,000 US dollars per kilo. When you consider that the average astronaut eats about 1.2 kilos of packaged freeze-dried food per day, that’s around 34,000–35,000 US dollars a day to feed them. By contrast, you can send a thousand lettuce seeds, which weigh about a gram, for the same upmass cost. A thousand lettuces can be grown for that single gram’s worth of launch cost. The water is generally already up there, because astronauts use water daily. The ISS and other commercial space stations currently being built have very high water recycling efficiencies, so once the water is up there it generally stays there. You can rely on the system to be self-sustaining once the equipment is in place.

Chris - How do plants take to being in space though?

Jen - Surprisingly well. Plants respond to gravity: normally shoots grow upwards away from gravity and roots grow downwards towards it. In microgravity that stimulus is almost absent. But gravity is not the only stimulus. Light, nutrients, and water are also crucial. Roots will grow towards nutrients and water, while shoots grow towards light, just as in a commercial greenhouse with supplemental lighting. That’s the sort of system used on space stations where plants are grown. What do you grow them in? We use small closed pods for the roots. Nutrient solution—water and nutrients combined—is delivered using simple pump systems. In recent projects we tested these on zero-gravity parabolic flights and they worked just as expected, exactly as on Earth. It’s a simple hydroponic system using sealed units: the seed is placed inside, the shoot bursts out, and the root stays enclosed. You don’t want something too hygroscopic, which soaks up water without releasing it; you want something that holds water but releases it to the roots on contact.

Chris - How do plants breathe in space though? They have little holes in their leaves, stomata, through which carbon dioxide goes in and oxygen comes out. If there’s no up and down, then it’s harder to move gases around. Do plants cope with that?

Jen - In space, one big challenge is convection—the natural movement of air doesn’t occur unless you mechanically create it. We install fans around the plants to create turbulence around the leaves, just as wind does on Earth. On Earth, some leaves also have small hairs to create boundary layers. These break up the static air around the leaf, allowing for gas exchange between the inside and outside of the leaf.

Chris - And how about timing? One amazing thing about plants is that they have body clocks, just like we do. You can jet lag a plant. How do they cope? On the ISS, astronauts say it’s a challenge seeing the Sun rise and set every 90 minutes. Do plants have to be kept isolated from that with artificial light, or are they okay with lots of sunrises and sunsets?

Jen - Lots of sunrises and sunsets would cause problems, but because the ISS has only small windows, the natural light is insufficient for plants to grow effectively. So we provide artificial light. Above the growing space there is a panel that emits the full visible spectrum plus parts of the invisible spectrum important for growth, from UV through to far red. These wavelengths dictate how the plant grows—photomorphogenesis. The balance of light can make them more compact, darker in colour, or change other growth habits. Darker leaves often mean more antioxidants, so you can make the plants accumulate more beneficial biochemistry for astronauts.

Chris - And what’s the productivity like? Is it comparable to a greenhouse in ideal conditions on Earth?

Jen - It’s not as productive as a greenhouse on Earth. We can’t provide the same space, and the conditions are more challenging in space than in a greenhouse. CO₂ is higher on the ISS than on Earth, and we also have strict power limits. Power means heat, and we must consider the heat from the lights as well as the heat produced by the plants through respiration. We sometimes have to curtail plant growth to ensure the environment remains safe for humans.

Chris - And the taste? Even if productivity is lower, do they taste as good?

Jen - They can taste better, frankly. Providing more blue light produces darker leaves with stronger flavours than you’d normally get from greenhouse-grown plants. For astronauts, one big thing lacking in freeze-dried food is texture. Providing fresh plants gives them crunch, which is incredibly important—not just flavour, but texture too.