It may sound far-fetched, but the day is coming when lasers will beam power to spacecraft, lunar/Martian rovers, and sherpalike robots that carry payloads up a thin tether into the upper atmosphere and beyond. Although researchers worldwide concur that power-beaming capabilities are decades away, a range of experimental efforts give promise that lasers may, in the not too distant future, provide cheap, safe, and reliable access to space.

LASERS AS CONDUCTORS

The notion of beaming power is not recent. It was the brainchild of Nikola Tesla, one of the most prolific inventors of the 20th century. Although little is known about the mechanism behind his so-called Death Ray, Tesla said the device could focus tremendous amounts of energy into a thin beam so concentrated it would not scatter, even over huge distances. It is uncertain if Tesla ever successfully built a full-scale version but recent advances in lasers, optics, and solar-panel technologies may soon turn his power-beaming vision into more than just a tool of destruction.

Engineers at NASA and the Space Transportation arm of EADS (European Aeronautic Defense and Space Co.), for example, were among the first to successfully power a miniature airplane and rover, respectively, with lasers.

NASA engineers from the Marshall Space Flight Center in Huntsville, Ala., and Dryden Flight Research Center, Edwards, Calif., beamed propulsive power to a propeller-driven model plane from the ground. Dryden engineers built the Mylar film-covered plane from balsa wood and carbon-fiber tubing.

The radio-controlled craft has a 5-ft wingspan and weighs about 10 oz. It is outfitted with a panel of photovoltaic cells optimized to the laser wavelength by a team of participants from the University of Alabama. The photovoltaic cells sit on the fuselage underbelly and convert the laser energy into electricity for the tiny 6-W motor that spins the propeller.

Likewise, engineers at the EADS Space Transportation facility in Bremen, Germany, collaborated with Swiss engineering firm FiveCo, to power a minirover via laser. The 20-cm (8-in.) long vehicle reached speeds of 1.6 cm/sec (0.63 ips) and maneuvered with ease. The laser — 250 m (820 ft) away — power beamed energy to photovoltaic cells at the center of a large silvery disc fixed atop the rover.

The experiment serves EADS's ultimate goal of harnessing the sun as a giant, inexhaustible electric powerplant. Under the Solar Power Initiative (SPI) they propose (within the next 50 yr) putting a generator in geostationary orbit to collect solar energy and direct it in concentrated form at the earth where fields of photovoltaic cells would pick it up.

According to EADS SPI team leader Frank Steinsiek, there are two obvious wireless-transmission options: Laser (top choice at EADS) or microwave (the preferred option of Japanese researchers). According to Steinsiek, EADS prefers laser technology because it requires much smaller orbital structures. Second, laser beams concentrate energy more effectively and better control lateral dispersion over long distances. There are also no negative effects on electronic communications or navigation systems in the vicinity.

One of the main obstacles to address with the minirover was in pointing the laser beam at its target. EADS spent two years tackling the problem in collaboration with the University of Kaiserslautern in Germany. The result is a technology that lets the laser beam transmit position recognition data in addition to energy — similar to carrier frequencies of radio waves. The receiver is correspondingly equipped with sensors that continually determine its position in relation to the transmitter. This helps ensure the photovoltaic cell panel is always positioned at right angles to the laser beam.

B.Y.O.B. —BRING YOUR OWN BEAM

Another option for power beaming is the so-called Space Elevator. Yuri Artsutanov, a Russian engineer, first proposed the Space Elevator in the 1960s. Since then a number of space elevator concepts have been presented. A common one uses a nanocarbon-reinforced composite tether (cable) that will originate from a fixed platform near the equator and end at a counterweight 35,786-km (22,241 miles) above geosynchronous orbit (GEO).

As the earth rotates, inertia from the counterbalance (possibly an asteroid) will work against centripetal force and keep the tether taut. Climbing machines will scale the tether using electricity generated by solar panels and a ground-based booster light beam. The climbing robots will travel at speeds of about 200 km/hr (120 mph), do not undergo accelerations and vibrations, can carry fragile payloads, and have no propellant stored on board. They will be able to release payload in GEO or send it into outer space.