Astronauts living aboard the International Space Station (ISS) have successfully grown an edible, safe-to-eat crop of romaine lettuce in what could pave the way for supplying fresh food for future long-distance exploration missions through space.

Although the salad was grown under different environmental conditions than plants experience on Earth – such as lower gravity and more intense radiation – space-grown lettuce crops were found to be safe to eat and at least as nutritious as Earth-grown crops, announced researchers in the March issue of Frontiers in Plant Science. Successful cultivation of produce in space not only breaks away from the monotony of the packaged and processed food that currently makes up astronaut diets – think dried fruit, nuts, chocolate, peanut butter, chicken, and beef, all freeze-dried, heated and sterilized to last – but it could be useful for future long-term exploration missions that span great distances.

"The ability to grow food in a sustainable system that is safe for crew consumption will become critical as NASA moves toward longer missions. Salad-type, leafy greens can be grown and consumed fresh with few resources," said study co-author Dr Christina Khodadad in a statement.

Astronauts aboard the ISS collaborated with Earth-bound researchers at the Kennedy Space Center between 2014 and 2016 to test the capability of growing produce in space. Lettuce was grown from sterilized seeds placed within the Vegetable Production System, nicknamed Veggie. This self-contained growth chamber was created by NASA to grow crops in space and is equipped with LED lighting, a watering system, and a system to monitor environmental conditions like temperature, carbon dioxide, and humidity. This data was sent to scientists on Earth who then mimicked conditions within Veggie to serve as a control for the experiment. In both locations, the lettuce was grown undisturbed for a period of 33 to 56 days.

Taste tests by the astronauts proved that the lettuce is not only efficiently grown in space, but is equally as edible as Earth-grown salad. Lettuce that wasn’t eaten by ISS astronauts was deep-frozen for transportation back to Earth.

Chemical and biological analyses showed that lettuce grown in space is similar in composition to plants grown on Earth, although some trials had plant tissue that was richer in potassium, sodium, phosphorus, Sulphur, and zinc. Some lettuce also had phenolics, which are molecules proven to have antiviral, anticancer, and anti-inflammatory characteristics while both showed similar levels of antioxidants. Researchers also used DNA sequencing to identify 15 microbial genera on the leaves and 20 in the roots – similar numbers on both the lettuce grown in space and Earth.

"The International Space Station is serving as a testbed for future long-duration missions, and these types of crop growth tests are helping to expand the suite of candidates that can be effectively grown in microgravity. Future tests will study other types of leafy crops as well as small fruits like pepper and tomatoes, to help provide supplemental fresh produce for the astronaut diet," concludes co-author Dr Gioia Massa, project scientist at Kennedy Space Center.

Lettuce may not have much nutritional value, but in addition to understanding proper conditions growing food in space, the authors add that identifying which types of fruit and vegetables would be the most successful and beneficial to supplement astronauts’ diets will prove valuable for future missions.