To work the soil in such a way that it produces a nourishing vegetable is to experience firsthand the power of life. To do so on another planet, far from the rich soil, thick atmosphere, and warm sunlight of the Earth, would be nothing short of magic.
Yet scientists have long looked toward the possibility of harnessing the regenerative powers of plants as a means of making a hostile alien environment more livable. And as interest in long-term missions to Mars and the Moon grows, the idea of using plants as part of a cyclical life-support system is gaining prominence as a practical and cost-effective solution.
"For years, there has been a small group of people looking at using plants in long-duration missions," said Keith Henderson, a plant physiologist with NASA's Advanced Life Support Program. "The thinking is that if we had a really long mission we could use plants to clean up the air, produce food, and clean up the water."
Although many associate using plants on other worlds with the visionary notion of terraforming actually converting a hostile atmosphere and surface into a lush, Earth-like environment over many years plants have a more immediate role to play in the creation of closed-loop biosystems. Most scientists agree that plants would be an economic and efficient cornerstone of any long-term stay on Mars or the Moon.
Although most of the experiments thus far have been directed toward proving the ability of plants to generate sufficient oxygen to support humans in a closed environment, scientists are also examining the possibility of using plants to produce food (either as a main food source or to add variety to a staid processed diet) and purify water. In addition, plants can provide a much-needed psychological boost to a long-term stay on a hostile planet, creating a small garden oasis in the middle of a cold desert world.
"If you're in a closed environment for a long time, growing plants is a wonderful diversion from the monotony," said Henderson, while adding that mechanical-chemical processors would form an integral part of a closed-loop system by enabling astronauts to recycle air and water while waiting for the plants to reach maturity. "Most people would prefer plants
to some chemical process because psychologically it's more appealing."
A system that performs all these functions in a closed environment has been dubbed a Controlled Ecological Life Support System, or CELSS. Although experiments using plants to regenerate oxygen began in the 1950s, NASA stepped into the effort in the 1970s as a way of supporting long-term manned missions. Under the direction of the Johnson Space Center in Houston, the Advanced Life Support Program pulls together the research and resources of botanists, engineers, and plant researchers dedicated to finding ways of making the closed-loop system as efficient and automated as possible.
The program's main directive has been to examine the construction and operation of surface-based systems that would recycle the carbon dioxide produced by humans to grow plants that would in turn provide oxygen and food. While some plants could conceivably be grown onboard spacecraft to provide food variety and psychological benefit, the ALSP focuses
its efforts on building full-scale life-support systems for extended surface missions.
"The kinds of systems we now envision are complex systems that require a great deal of power and need `real estate' you can't cram all this equipment onto a spacecraft," Henderson said.
So far, ALSP has conducted three manned experiments in sealed chambers to test the efficiency of regenerative plant-based and chemical systems. During Phase I of the program, conducted in July 1995, scientist Nigel Packham lived in a 7.2- meter-long chamber in JSC's Building 7, which also houses the Plant Growth Laboratory. The chamber was divided into two compartments, a 4.2-meter plant-growth compartment and a 3.0-meter airlock chamber used for Packham's living quarters. By separating the chambers, scientists were able to monitor and maintain optimal oxygen and carbon dioxide levels in the plant and human compartments.
For Phase I, a productive variety of wheat was selected as the support crop, both for its ability to produce abundant oxygen and its ability to tolerate a 24-hour light cycle (enabling maximum photosynthesis). The wheat was grown hydroponically under high-pressure sodium lamps, which provided the equivalent of continuous three-fourths full sunlight. Humidity and temperature levels were maintained at constant levels.
Although the experiment provided a wealth of usable data, the basic conclusion was elegantly simple.
"We produced exceeded the oxygen required to support one person for the 15-day stay," Henderson said.
The test also revealed a significant relation between the metabolic activity of the human subject who exercised daily on a stationary bicycle and the rate at which the plants produced oxygen, suggesting that the relationship between humans and plants is a complex and synergistic one.
"When he was exercising on the bike, and required more oxygen, the plants would respond to that," Henderson said. "We could get the system to `speed up' during that time. The system could also be `slowed down' during times of low activity."
While Phase II of the program used chemical and physical means to recycle oxygen and water, the 30-day test provided further data on closing the loop on air and water use. During the crew's 30-day stay in a three-story refurbished hyperbaric chamber (originally built in the 1960s for Skylab simulations), 90% of the water and air was recycled.
During Phase III of the program, conducted in late 1997, a crew of four sealed themselves in the same three-story chamber at JSC. Carbon dioxide waste was piped to the Plant Growth Facility, while oxygen produced by the plants was recycled back to the chamber. The 90-day test also saw the beginnings of experiments in recycling inedible plant biomass and incinerated human waste, which was sent to Kennedy Space Center and converted to a nutrient solution.
The culmination of the program, known as the Advanced Life Support Systems Integration Test Bed (or BIO-Plex), will incorporate five sealed chambers interconnected by tunnels in Building 29 at the Johnson Space Center. Two of the chambers will be dedicated to plant growth, while other chambers will be used for habitation and life-support equipment. The first test of the BIO-Plex system, using only three of the chambers, is slated for 2001. Scientists hope to produce enough oxygen and 50% of the food required to support a set number of crew members for 120 days. The full run of the BIO-Plex project, a 420-day test, is planned for the year 2005 and may include some astronauts as part of the crew.
"If we're going to send astronauts to Mars or the Moon, they're going to have to know how to grow plants, in addition to having the usual engineers, medical personnel, and other specialties that are required on a space mission," said Henderson, who added that some of the farming activities will have to be automated to allow scientists to concentrate on other surface activities. "You can't spend 95% of your time trying to keep the equipment running."
The Earth-bound experiments have given scientists insight into how humans and plants interact in a closed-loop environment, and how such systems could be made more efficient and less costly. In turn, the need to grow highly productive plants in a small space has led to the development of several special plant varieties and soils.
The most prominent of these is a space-age wheat known as USU-Apogee, developed at the Crop Physiology Laboratory at Utah State University. The dwarf red spring wheat produces the equivalent of nearly 600 bushels of grain per acre, which is three times that of normal wheat varieties. Apogee was developed to thrive under "space farm" conditions, including constant artificial light and high carbon dioxide levels. The plant produces heads 23 days after germination, a full week sooner than other varieties, and stands only 18 inches tall when mature.
Botanists have also developed special productive varieties of sweet potatoes, rice, peanuts, and beans.
"Our list of plants for crop production is a short list based on nutrition, productivity, and ease of growth," said Henderson. "For a closed-loop system, we need dwarf plants that are highly productive."
Developing plants for food production in space has proved to be more of a challenge than growing plants for oxygen revitalization. In particular, botanists working in the program have yet to develop productive and space-saving varieties of such plants as sweet potatoes and rice. Nonetheless, Henderson and others remain optimistic that a CELSS system will incorporate both aspects of human life support.
"I think we can keep a crew of four to eight alive and produce food and oxygen for these people," he said. "It takes more to grow food. If I can grow enough crop to feed one person, I can grow enough to produce oxygen for four."
While most of the plant-growth experiments have used hydroponics to supply nutrients, researchers are also looking into the possibility of using recycled wastewater, composted plant biomass, and even processed martian soil to supply the nutrients and root support needed for crop growth.
Researchers working on the project have even developed an artificial soil from zeolite, a naturally occurring volcanic clay that is especially suited to absorbing chemicals and nutrients. By loading the zeolite with the right mix of chemicals necessary for plant growth, including phosphorous, potassium, nitrogen, carbon, magnesium, iron, and zinc, scientists are able to feed the plants directly through the artificial soil without the waste and runoff associated with traditional fertilizers and hydroponics systems. The NASA-created zeoponic plant growth media is marketed commercially by Zeoponix Inc. in Boulder, Colorado, and has been used on some golf courses.
"By using this material, you are able to eliminate some of the runoff of fertilizer," said Henderson.
Of course, NASA's closed life-support and plant-growth experiments have already yielded some benefits beyond adding to the world's golf courses. NASA has been growing plants for years in space, onboard the MIR space station and space shuttle. These experiments have been conducted to study the way microgravity affects basic biological functions, but they have also provided some valuable lessons about controlling the growth environment, lessons that would be well heeded by even casual gardeners on Earth.
"The lesson we have learned is that you really do need good environmental control," Henderson said. "You can't just grow plants in a spacecraft without careful consideration of lighting conditions, trace contaminants, and carbon dioxide concentrations."
The program has also caught the attention of officials in remote areas of Alaska seeking solutions to growing crops and recycling waste in hostile Arctic conditions. Some architects have also paid close attention to the experiments, in hopes of developing closed-loop, environmentally conscious workplaces.
Visions of such modern buildings paradoxically filled with natural plants and gardens attests to the inexorable connection between plants and humans. The lessons learned on Earth about the importance of plants in maintaining the human population in the last 30 years have in some ways formed the vision of the way humans will one day survive on other worlds. The vision of humans living and working alongside plants is not just about cost savings or efficiently recycling waste products. It's an aesthetic vision that appeals to one's desire to bring a small piece of Earth to a hostile, distant planet, and in the process, bring that planet home to Earth .
A revised version of this article will appear in Texas Garden Yearbook 1999, distributed by Book Marketing Plus.