When Kimiya Komurasaki was a boy, he loved science-fiction stories about humankind being forced by a swelling sun to flee the planet.

Now he is working on a technology that could someday make that escape -- or less dramatic trips into space -- a lot easier.

The concept, called beamed-energy propulsion, sounds like something out of one of his boyhood space fantasies: using microwaves and air to power a rocket. But if it ever gets off the ground, it would mean a dramatic reduction in launch costs and an equally eye-opening increase in carrying capacity.

"The payload of a conventional 100-ton rocket could be carried by a vehicle weighing as little as 20 tons -- and be launched for one-tenth or even one-hundredth of the cost," said Komurasaki, a professor of astronautics at the University of Tokyo.

Earlier this year, a research team under his direction used the method to boost a 10-cm plastic rocket about 2 meters into the air.

It works by using microwaves beamed up from the ground to superheat oxygen to the point of combustion. Komurasaki estimates that generating the microwave beams to boost a 100-kg rocket into space would require several gigawatts of power for one minute -- for an electricity bill of 1 million yen.

The biggest expense would be building the ground station, the researcher said.

But if the technology can be scaled up and costs brought down, he believes microwave propulsion would allow a radical rethink in rocket design.

Currently 90 percent of a rocket's weight is fuel: liquid oxygen and liquid hydrogen. A microwave propulsion system would free up more than half of the rocket to carry its payload.

Komurasaki envisions microwave rockets doing some serious heavy lifting, like the parts of an orbiting solar power station or even a space colony.

"Right now those kinds of big projects aren't economically feasible because the launch costs are prohibitive," he said. In the United States, researchers are working on a similar technology that uses laser beams instead of microwaves. Komurasaki tinkered with laser propulsion himself before becoming interested in microwaves. He said microwaves are better because the cost of building a ground station would be much lower. "The big challenge is developing more powerful means of generating microwave beams," he said. "I think the technical problems could be solved in 10 years if we could get funding."

But he still has a lot of convincing to do.

"It's an attractive idea on paper -- but I think it's a long way off," said Hiroshi Maeda, a professor emeritus of aeronautical engineering at Kyoto University. "People have been kicking around plans for super-efficient rockets for some time."