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November 3, 2006
SPACE
SUNSHADE MIGHT BE FEASIBLE IN GLOBAL
WARMING EMERGENCY
The possibility
that global warming will trigger abrupt climate change is something
people
might not want to think about.
But University
of Arizona
astronomer Roger
Angel thinks about it.
Angel, a University
of Arizona Regents'
Professor and one of the world's foremost minds in modern optics,
directs the
Steward Observatory Mirror Laboratory and the Center for Astronomical
Adaptive
Optics. He has won top honors for his many extraordinary conceptual
ideas that
have become practical engineering solutions for astronomy.
For the past year, Angel has been looking at ways to cool the Earth in
an
emergency. He's been studying the practicality of deploying a space
sunshade in
a global warming crisis, a crisis where it becomes clear that Earth is
unmistakably headed for disastrous climate change within a decade or
two.
Angel
presented
the idea at the National Academy of Sciences in April and won a NASA
Institute
for Advanced Concepts grant for further research in July. His
collaborators on
the grant are David Miller of the Massachusetts Institute of
Technology, Nick
Woolf of UA's Steward Observatory, and NASA Ames Research Center
Director S.
Pete Worden.
Angel is now publishing a first detailed, scholarly paper, "Feasibility
of
cooling the Earth with a cloud of small spacecraft near L1," in the Proceedings of the National Academy of
Sciences. The plan would be to launch a constellation of
trillions of small
free-flying spacecraft a million miles above Earth into an orbit
aligned with
the sun, called the L-1 orbit.
The spacecraft would form a long, cylindrical cloud with a diameter
about half
that of Earth, and about 10 times longer. About 10 percent of the
sunlight
passing through the 60,000-mile length of the cloud, pointing
lengthwise
between the Earth and the sun, would be diverted away from our planet.
The
effect would be to uniformly reduce sunlight by about 2 percent over
the entire
planet, enough to balance the heating of a doubling of atmospheric
carbon
dioxide in Earth's atmosphere.
Researchers have proposed various alternatives for cooling the planet,
including aerosol scatterers in the Earth's atmosphere. The idea for a
space
shade at L1 to deflect sunlight from Earth was first proposed by James
Early of
the Lawrence Livermore National Laboratory in 1989.
"The earlier ideas were for bigger, heavier structures that would have
needed manufacture and launch from the moon, which is pretty
futuristic,"
Angel said. "I wanted to make the sunshade from small 'flyers,' small,
light and extremely thin spacecraft that could be completely assembled
and
launched from Earth, in stacks of a million at a time. When they
reached L1,
they would be dealt off the stack into a cloud. There's nothing to
assemble in
space."
The lightweight flyers designed by Angel would be made of a transparent
film
pierced with small holes. Each flyer would be two feet in diameter,
1/5000 of
an inch thick and weigh about a gram, the same as a large butterfly. It
would
use "MEMS" technology mirrors as tiny sails that tilt to hold the
flyers position in the orbiting constellation. The flyer's transparency
and
steering mechanism prevent it from being blown away by radiation
pressure.
Radiation pressure is the pressure from the sun's light itself.
The total mass of all the fliers making up the space sunshade structure
would
be 20 million tons. At $10,000 a pound, conventional chemical rocket
launch is
prohibitively expensive. Angel proposes using a cheaper way developed
by Sandia
National Laboratories for electromagnetic space launchers, which could
bring
cost down to as little as $20 a pound.
The sunshade could be deployed by a total 20 electromagnetic launchers
launching a stack of flyers every 5 minutes for 10 years. The
electromagnetic
launchers would ideally run on hydroelectric power, but even in the
worst-case
environmental scenario with coal-generated electricity, each ton of
carbon used
to make electricity would mitigate the effect of 1000 tons of
atmospheric
carbon.
Once propelled beyond Earth's atmosphere and gravity with an
electromagnetic
launcher, the flyer stacks would be steered to L-1 orbit by
solar-powered ion
propulsion, a new method proven in space by the European Space Agency's
SMART-1
moon orbiter and NASA's Deep Space 1 probe.
"The concept builds on existing technologies," Angel said. "It
seems feasible that it could be developed and deployed in about 25
years at a
cost of a few trillion dollars. With care, the solar shade should last
about 50
years. So the average cost is about $100 billion a year, or about
two-tenths of
one percent of the global domestic product."
He added, "The sunshade is no substitute developing renewable energy,
the
only permanent solution. A similar massive level of technological
innovation
and financial investment could ensure that.
"But if the planet gets into an abrupt climate crisis that can only be
fixed by cooling, it would be good to be ready with some shading
solutions that
have been worked out."
##
Contact:
Lori
Stiles
University of Arizona
520-626-4402
lstiles@u.arizona.edu
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