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.

High-resolution images in ‘publication format’ are available on the
World Wide Web at:

http://amesnews.arc.nasa.gov/releases/2002/02images/microwaveplants/microwaveplants.html