Microwaves derived from solar power and transmitted by orbiting satellites to electric power stations on Earth may someday enable U.S. energy self-sufficiency, but is this method safe for local plant life?
NASA scientists are about to test that hypothesis by evaluating the effects of continuously beaming weak microwaves on alfalfa plants during laboratory tests.
"One of our main questions is how organisms will respond to sustained microwave exposure because the beam from space will be on all the time," said scientist Jay Skiles of NASA Ames Research Center in California's Silicon Valley.
"Skiles has designed a series of experiments to test the effects of weak microwave illumination on plants at 2.45 GHz frequency. "We expect that the microwave intensity at ground level will be about a million times less than that in a typical microwave oven."
"Our hypothesis is that plants exposed to microwaves will be no different from those plants that are not exposed to microwaves," Skiles said. He now is conducting a prototype experiment in which he illuminates alfalfa plants with low-power microwaves. "The experiment is designed so that the only variable to which the plants are subjected is microwave exposure," he explained.
The space solar power concept envisioned by some engineers requires putting satellites into a geosynchronous orbit at an altitude of 22,300 miles (40,140 kilometers) over the equator.
In such an orbit, satellites revolve around the Earth at the same speed as the planet rotates, causing the satellites to appear to 'hover' over the same point on the ground below. And at that altitude, they are continuously in sunlight.
Solar cells on the satellites would change energy from sunshine into electricity. A satellite system would convert the electricity into microwaves and beam them to receiving antennae on the Earth's surface. There, systems would convert the microwave energy back into electricity and feed it into the nation's power grid.
Microwaves are a small part of the electromagnetic spectrum that includes energy frequencies from x-rays to visible light and radio waves. "These microwaves are in the radio frequency range, at the same frequencies at which many cell phone services operate," according to Skiles.
During his current prototype experiment, Skiles broadcasts microwaves over a tray of alfalfa plants in a laboratory. The microwaves reflect onto the test plants. At the same time, nearby 'control plants' are not subjected to microwaves. A 'control' in a scientific experiment is something used as a standard for comparison.
Both the test plants and the control plants are subjected to the same temperature and lighting regime, and they are grown in the same size pots in the same kind of potting mix.
Skiles is measuring plant gas exchange and leaf chlorophyll concentration. "Also measured are gross plant variables, such as stem length and overall vigor," he said. "This prototype experiment will provide preliminary results based on a 14-hour artificial day and constant temperature." Skiles also is preparing to conduct a longer, 6-month experiment in a rooftop greenhouse at NASA Ames starting in late spring.
"We are going to duplicate the prototype experiment in natural sunlight, and we will have night and day temperature changes that will give us more realistic environmental values," he said. "Alfalfa begins to flower in late September, when we begin to get shorter days in the Northern Hemisphere, and we will end the experiment then when the plants are getting ready to winter."
"Nobody has accomplished 2.4 GHz sustained microwave plant-illumination experiments before, to the best of our knowledge," he said. Skiles chose to test alfalfa because it is an important crop that animals and people eat. Alfalfa also is representative of a broad class of economically important plants, he added.
Skiles said he is planning additional, longer experiments to test a variety of plants under various conditions. "Long-duration mixes of plant species experiments as well as testing single plant species for response to microwaves under stressful conditions, including plants from a desert ecosystem, will be future tests," he said.
In 1968, Peter Glaser first described the concept of space solar power in an article, 'Power from the Sun: Its Future,' which appeared in the journal Science. "In the late 60's, the science, engineering and technology available made the implementation of space solar power infeasible," Skiles said.
"Recently, the National Research Council, an arm of the National Academy of Sciences, in a publication called, 'Laying the Foundation for Space Solar Power: an Assessment of NASA's Space Solar Power Investment Strategy,' gave a qualified go-ahead for initial space solar power technology research and development. In prior assessments, the NRC had indicated that it was premature to pursue this field of R&D," Skiles said.
The National Academy of Sciences qualified its recommendation that a space solar power project is feasible by stating that NASA should partner with other government agencies such as the Department of Energy, Skiles said. "This effort could be important for national security because space solar power provides one more option that might enable energy independence in the future," he said.
The NASA Glenn Research Center, Cleveland, Ohio, manages the space power project. NASA's Human Exploration and Development of Space strategic enterprise, Washington, D.C., directs the space solar power investigations for the agency.
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