Until recently, astronauts used to have very limited food choices in space. They had to eat packages of freeze-dried food that didn't require refrigeration. They would add hot water to a dry mixture, and get something like a "stew" that would be squeezed from a tube.

Now, on the International Space Station, some astronauts spend months at a time in space. Would you want to eat freeze-dried stew for months at a time? Most astronauts don't want to either. Through special systems called Veg-01 and Veg-03, they are now growing lettuce, and a few kinds of cabbage on the International Space Station. In a third, more advanced system called Plant Habitat-02, they are growing radishes.

Scientists who developed the Veg-01 and Veg-03 have studied not only the best way to grow food in space, but have studied the effects that the food has on the astronauts. They found that for astronauts spending months in the limited environment of the space station, eating a few fresh vegetables is not only healthy for their bodies, but taking care of the plants actually makes them happier.

Plant Habitat-02 is an even bigger experiment. In this experiment they are growing about 20 radishes. They gave each plant a precise amount of soil, nutrients, water and light. Then, on Earth, they have the exact same setup with 20 other radishes. By comparing the "space radishes" and the "Earth radishes" they are able to understand in more detail how being in space affects plant growth and health.

There are a lot of challenges to overcome to get a plant to grow in outer space. Besides having enough soil, water, and sunlight, the biggest challenge is actually gravity! Even though astronauts can bring air, water, soil from Earth, and use real and artificial sunlight in space, they can't really recreate gravity.

Sam Anas is a scientist who studies plants and has helped astronauts conduct experiments on plants in space. In an interview he explained that on Earth, roots grow down into the ground because of gravity. When a seed produces its first root, gravity will help it grow down. As a seed produces its first shoots - what will become the stem - they will naturally grow "up" (where they will eventually find sunlight).

Without gravity, the plant does not "know" which is up or down. The roots end up growing in all kinds of directions and not necessarily toward the soil, and the shoots grow in different directions until they find the light. Without help, plants in space end up growing roots and stems in many changing directions. This makes it harder for the plant to thrive.

Lignins are structures in plants that give rigidity to the plant, and allow them to stand upright. We already know that being in space causes bone and muscle loss in humans because the physical demands are lower in space. Scientists are wondering if being in space might have the same effect on plants' lignins.

Another problem without gravity is that water won't naturally flow down toward the roots. Instead scientists have to devise machines and systems that will send the water to the roots. In one system, they put seeds inside a "pillow" of soil-like material that allows them to direct water to the plants' roots.

Even though these experiments have proven that it is difficult to grow plants in space, NASA and other scientists who study space travel have big hopes for the future. Kevin Cannon is a scientist who has been studying how humans might live on Mars. He explained that it turns out that on Mars, water is actually available. The water isn't flowing like it is on Earth. Instead it is locked up as ice underground, or absorbed into minerals. With enough digging and heat, we could build factories that would change it to liquid water. Cannon explains that once you have the liquid water, you can keep "recycling" it.

Lewis and his team also want to know if plants genetically engineered to have less lignin can survive and function normally in space. This could give space-grown plants several advantages, including being better for nutrient absorption when humans eat them and in making plant waste easier for composting. Lewis and his team believe this fundamental science information will guide our strategies for deep space exploration and colonization.

Another hopeful experiment was conducted at Beihang University in China. Prof. Hong Liu and her team built a self-sustaining ecosystem that included plants and microbes - where three people (scientists) were able to stay alive and healthy for over 100 days using only the natural materials within the "Palace." They were able to reproduce 55% of the food that they consumed.

While these are amazing ideas, they likely won't happen for decades to come. However, even if plants supporting people living on Mars or the Moon is a long way off, scientists have found that the research is also useful on Earth, as it helps them understand how we can build more automated growing systems, and how we can create environments that allow plants to retain as much energy as possible.