Scientists in Japan have successfully "launched" a tiny metal rocket using an unusual source of thrust - microwaves. The test was the latest a proof of principle for a kind of propulsion that has never been the beneficiary of the levels of investment poured into traditional chemical rockets, but which its proponents say could some day be a superior way to get spacecraft into orbit.
Sending rockets into space using a combustible mixture of on-board fuel isn't an optimal solution to the problem of escaping Earth's deep gravity well. Not only is it dangerous to strap humans and satellites on top of giant bombs, it's also incredibly wasteful: 90% of the weight of a rocket sitting on the launch pad is fuel. For example, the Space Shuttle burns 50% of it's lift off weight within the first minute of flight.
In the beginning of the 20th century, it occurred to Russian rocket scientist Konstantin Tsiolkovsky that there was another way: by keeping the energy source on the ground and beaming the required power to a rocket, it could be launched with very little fuel on board.
With the invention of the maser, or microwave laser, scientists were granted a tool to realize Tsiolkovsky's dream. So in the 1970's they began to model just what it would take. Some were optimistic about its potential to decrease the cost of going to orbit by orders of magnitude, but the bottom line is that, for a lack of funding, the technology never took off.
Every few years, however, someone reminds the world that it's at least possible to get a rocket off the ground with little or no fuel. The latest demonstration used a Gyrotron - essentially a maser - at the Naka Fusion Institute of the Japan Atomic Energy Agency. (This super high-powered microwave beam emitter was originally developed as part of Japan's contribution to ITER, the international effort to create a workable fusion reactor.)
Using this beam, the scientists were able to send pulses of microwave energy into the bottom of their hollow 126 gram rocket model, heating the air within to 10,000 degrees Celsius and resulting in its rapid expansion. The result is a little boom, "like thunder," they report.