A NASA scientist has discovered that future solar-power satellite
systems designed to harvest sunlight, convert solar electric energy
into weak microwaves and beam them down to Earth to make electricity,
are not harmful to green plants.

During the simple experiment, the scientist bathed a tray of alfalfa
plants with weak, 2.45 GHz microwaves for seven weeks with no ill
effects. The microwaves were about 1 million times weaker than those
an average kitchen microwave oven makes. The test took place in a
laboratory at NASA Ames Research Center in California’s Silicon
Valley, and is the first of many experiments scientists plan to
conduct to see if an array of solar-power satellites designed to send
microwave power to Earth could affect plant life.

“A tray of growing plants was illuminated with microwaves while
control plants were grown behind a microwave-opaque shield. Test
plants and the control plants were subjected to the same environment
otherwise,” said NASA Ames scientist Jay Skiles, who designed the
experiment and recently presented its results at the 54th
International Astronautical Congress in Bremen, Germany. “In all
measured variables, there was no difference between the control and
the microwave treatment plants,” Skiles added. A ‘control’ is a
parallel experiment in which the factor being tested in the main
experiment is left out in order to provide a way for scientists to
judge that factor.

In 1968, scientists proposed putting solar-power satellites into
orbit about 22,000 miles above the ground, where these spacecraft
could harvest sunlight for its energy. While the satellites would
collect sunshine to make direct current (DC), they also would be
converting the DC to some form of radiation, most likely microwaves.
The satellites then would broadcast the microwave energy to the
Earth’s surface, where power plants would reconvert it into
electricity for distribution.

“Over the ensuing decades, the space-power satellite concept has been
studied from the view of engineering feasibility and cost per
kilowatt, with only little attention paid to the biological
consequences to organisms exposed to continuous microwave radiation,”
Skiles said. “The hypothesis of my experiment was that plants exposed
to microwaves would be no different from those plants not exposed to
microwaves,” he said.

Skiles used off-the-shelf equipment to conduct the experiment. He
used the same nutrients and watering techniques on two sets of
plants, only one of which was exposed to microwaves.

A microwave generator with an antenna and a parabolic reflector
beamed microwaves onto the test plants from the side so as not to
block lights placed above the plants. A sheet metal microwave shield
protected the ‘control’ plants from the microwaves so Skiles could
compare the non-microwaved plants with the microwaved plants.

Skiles measured the chlorophyll concentration of the alfalfa leaves
in the microwaved and non-microwaved plants. He measured the plants’
stem lengths, and also harvested, dried and weighed the plants. He
found there were no significant differences in the microwave-treated
plants and the untreated control plants. Skiles chose to test alfalfa
because it is an important crop that animals and people eat. Alfalfa
also represents a broad class of economically important plants, he

Unlike radioactive materials, microwaves cannot burn living things,
but microwaves do generate heat. However, Skiles reported, “Even
though I tested microwaves on alfalfa, I didn’t see any increase in
plant or soil temperatures.”

Skiles plans to conduct additional experiments to test plants
outdoors, as well as under other conditions. “I want to test plants
growing in a glasshouse to determine the effects of microwaves on the
plants during daily changes of light and temperature,” he said.
“Another experiment will be to grow cereal plants, including wheat
and oats, to determine the effect of microwaves on the kinds of
plants that humankind depend on for food,” Skiles continued.

He also plans to test whether or not microwaves provide a competitive
advantage for some kinds of plants when several different species are
growing in the same area. In another experiment, he is planning to
examine the genes of one plant species to learn the effects of weak
microwaves on that plant. Additional experiments to test effects of
climate change, watering and other conditions also may be conducted,
according to Skiles.

NASA’s Office of Space Flight Advanced Concepts funded the microwave study.

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