Many proposed spacecraft propulsion mechanisms use power in the form of electricity or heat. Usually these schemes assume either solar-electric power, or an on-board reactor. However, both power sources are heavy. Therefore, one could instead leave the power-source stationary, and power the spacecraft with a maser or a laser beam from a fixed installation. This permits the spacecraft to leave its power-source at home, saving significant amounts of mass.
Microwave broadcast power has been practically demonstrated several times. The first time was at Goldstone California, in 1974.
The power beam can also be used to provide impulse directly, for example using a solar sail to reflect a laser beam or using a magnetic sail or MMPP sail to divert a beam of charged particles from a particle accelerator.
Recent tests by Leik Myrabo, with the US Army, have demonstrated the feasibility of using ground-based lasers to propel objects into orbit; possibly reducing orbit-flight costs by a factor of 1000. The test succeded in reaching over 100 feet, which compares to Robert Goddard's first test flight of his rocket design.
Myrabo's "lightcraft" design is a reflective funnel-shaped craft that channels heat from the laser, towards the center, causing it to literally explode the air underneath it, generating lift. This method, however is dependent entirely on the laser's power, and even the most powerful models currently can only serve for modest test puposes.
See also: spacecraft propulsion
wikipedia.org dumped 2003-03-17 with terodump