For future spacefarers gardening will be a matter of survival. Not only will plants provide food when deliveries from Earth aren't possible, but plants will also work to make air breathable and water drinkable.

Plants and people -- two very different kinds of astronauts -- will eventually live together in balanced, sustainable habitats where contact with Earth is a luxury, not a necessity.

This vision of self-contained colonies in space -- or even on other planets -- has existed for decades in the pages of countless science-fiction novels. The growing International Space Station (ISS) brings that vision closer to reality, but the ISS isn't self-sufficient. Its life support systems are strictly mechanical so food must be ferried from Earth.

"In order to have affordable -- and even doable -- long-term exploration (of space), you need to incorporate biology into the life support system," said Chris Brown, director of space programs at the Kenan Institute for Engineering, Technology & Science at North Carolina State University.

NASA researchers at the Kennedy Space Center (KSC) and the Johnson Space Center (JSC) are figuring out how to do just that.

They're exploring technologies that could wed people, plants, microbes, and machines into a miniature "ecosystem" capable of supporting space travelers indefinitely.

This type of life support system -- called "bioregenerative" -- would be fully self-contained, creating an ecologically sound microcosm where each element supports and is supported by each of the others.

"If we really want to leave (the Earth) on a permanent basis, we need to figure out how this blue ball in space supports all of us, and somehow replicate the parts that are necessary so that we can move on," said Jay Garland, principal scientist for the Bioregenerative Life Support Project at Dynamac, Inc., at KSC.

Humans and planets are ideal space traveling companions. Humans consume oxygen and release carbon dioxide. Plants return the favor by consuming carbon dioxide and releasing oxygen.

Humans can use edible parts of plants for nourishment, while human waste and inedible plant matter can -- after being broken down by microbes in tanks called "bioreactors" -- provide nutrients for plant growth.

Plants and microbes can also work to purify water, possibly with help from machines. The only input needed to keep such a system going is energy in the form of light.

This is a simplified portrayal, of course. For scientists and engineers who are trying to design a real system, the devil is in the details.

For example, finding just the right plant varieties for the "space garden" is a painstaking process.

"Plants are going to be central linchpins of the life support system -- or at least the biological part of the life support system," Brown said.

The ideal space-plant would have short stalks to save room, would have few inedible parts, would grow well in low light, and would be resistant to microbial diseases.

Research is underway at KSC to choose varieties of wheat, rice, lettuce, potatoes and other plants that meet these criteria.

Researchers are also working to develop a "greenhouse" that will function properly in space.

In an orbiting greenhouse, freely-falling plants don't feel the constant downward pull of gravity. As a result, water spreads out evenly in the soil-like material around their roots, which makes it harder for both air and water to reach the roots.

Researchers had to choose the size of the granules in the "soil" very carefully. If the grains are too big, the roots won't get enough water; if they're too small, not enough air. (The right size turns out to be 1 to 2 millimeters; for more information, see the editor's note at the end of this article.)

Also, there is less natural air circulation in an orbiting outpost -- plants can therefore suffocate on their own "exhaled" oxygen! Designers have to provide fans to keep the air moving.

Researchers caution that ironing out these sorts of details won't guarantee a working system when all the pieces are assembled.

"There's a question of how the complete system will develop with time," Garland said. "On top of the ecological concerns of how the various microbe species will undergo succession (i.e., a sequence of replacements of one species by another species), you've got evolutionary effects. For microbes, with their short generation times, you're talking about real evolutionary time scales for prolonged missions."

To test how the humans, plants, and microbes fare when sealed together for extended periods of time, JSC is building a test chamber called BIO-Plex. This facility will incorporate all of the elements of a bioregenerative life support system -- including the people.

And just in case self-contained bubbles of life outside Earth's atmosphere aren't "sci-fi" enough for you, NASA researchers are also considering how biotechnology and nanotechnology could be used to improve such "bubbles" in mind-boggling ways.

For example, foreseeable advances in biotech and nanotech could make it possible to alter plants' genes so that their cells produce little molecular sensors, transmitters, and receivers.

These would monitor the plants internally and report on their health to ensure a good crop, and could even make the plants controllable, sprouting and flowering on cue.

Another idea is to engineer plants to produce chemicals that protect them from the increased radiation in space and on planets with thin atmospheres, such as Mars. Brown also suggested that nanotech devices in the plants' cells could deliver light directly to the cell parts that perform photosynthesis, making the plants more efficient.

"There are feasibility issues, but ... none of them should stop us completely," said Brown, who wrote a study on the potential uses of nanotechnology for these life support systems.

"Maybe we can't quite do it now, but nothing we are considering is against the laws of physics or chemistry or nature," he said.

A bioregenerative life support system will probably never fully replace the mechanical one on the International Space Station, Garland added. At most, a small crop might be grown there to provide fresh food.

But eventually, with the help of plants and microbes, future space stations -- or outposts on the Moon or Mars -- will truly become worlds unto their own.

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