Space umbrellas to shield earth.

On a hot, clear day, an umbrella can provide cooling relief from the sun's scorching rays. The same concept might one day help protect Earth from the accumulating heat of global warming.

Building a single umbrella big enough to shade the entire planet will never be possible. Instead, astronomer Roger Angel of the University of Arizona wants to launch a trillion tiny sunshields into outer space.

Each mini-umbrella would be a small, light spacecraft, weighing about a gram (0.04 ounces) and carrying a sunshade measuring half a meter (1.6 feet) across. Working together, hordes of the devices could act as a sunscreen for the globe.

Such a sunscreen would stretch across about 100,000 kilometers (62,000 miles) of space. And the pressure of sunlight on attached solar reflectors would keep the little flyers in orbit around Earth at a distance of about 1.5 million kilometers (932,000 miles).

The sunshade would be mostly transparent, or see-through, but it would cut the amount of sunlight reaching Earth by 1.8 percent. This would be enough to reduce the impact of global warming, Angel says.

In Angel's scheme, people on Earth could make the lightweight sunscreens out of transparent film. Each shade would be full of holes like Swiss cheese.

Angel says his plan is a big improvement over previous ideas for shading Earth from the sun. Those scenarios have typically involved large, heavy spacecraft that would need to be built in outer space out of pieces of asteroids or other space rocks.

To reduce the environmental impact of launch, Angel says, astronomers could use magnetic fields instead of rocket fuel for acceleration. He envisions launching 800,000 flyers at a time for a total of 20 million launches over a decade.

Angel thinks that the first launch could happen in just 25 years. And, once the flyers are up there, he says, they would shade the planet for 50 years.

The idea is certainly creative, but critics say there are easier and much cheaper ways to tackle global warming. Covering large areas of ground with white paint, for example, could reflect light back into space and reduce heating. Better yet, by reducing the pollution we create, we can all help keep the planet from heating up.