In two breakthrough developments, NASA scientists have beamed microwaves and laser energy to ‘fill’ lightweight sails in laboratory demonstrations of how these technologies might provide propulsion for interstellar exploration.
The sails used in the microwave experiment were actually driven to liftoff and flight, while the laser-driven sails achieved horizontal movement.
‘These are really two giant steps forward,’ said Henry Harris, task manager for the microwave levitation and laser experiments at NASA’s Jet Propulsion Laboratory, Pasadena, California. ‘These results would not have been possible without newly developed ultralight, high-temperature sail materials and beamed- energy propulsion methods.’
Future spacecraft that explore the depths of space will need to be very lightweight and be propelled by a reliable source of energy. Solar sails and microwave- and laser-beamed sails meet this requirement, with minimal weight since in the first case the ‘engine’ is the Sun, while in the latter two the engine is left at the point of origin.
The sails are driven by photons, particles of energy in which sunlight and other forms of electromagnetic radiation are emitted. By use of a remote laser or microwave source, beamed energy can be directed toward a space sail. In space, the laser or microwave source may be provided by a satellite or other type of spacecraft.
The microwave-beamed sail experiment was conducted in a vacuum chamber at JPL, while the laser-driven experiment took place in another vacuum chamber at Wright-Patterson Air Force Base in Ohio. Both of these experiments appear to be firsts.
‘Accelerations of several times the force of gravity were observed during the microwave tests,’ said Dr. James Benford, project director and president, Microwave Sciences, Inc., Lafayette, California. ‘In one case, the sail flew two feet in response to the high acceleration.’
About 10 kilowatts of microwave power were beamed to the sails. Analysis of data is underway to isolate the photon pressure effect from other possible causes of sail movement.
In the other tests, laser powers from 7.9 to 13.9 kilowatts were directed to the sails. Photon thrust was calculated from movements of the sails, which were mounted on pendulums. Future research will fine-tune the scientific understanding of flight using photon pressure.
Sails for both experiments were made of carbon-carbon microtruss fabric and were provided by Dr. Timothy Knowles, Energy Science Laboratory, Inc., San Diego, Calif. This very light but stiff fabric can withstand high temperatures that are typical of flight-level power densities.
‘These experiments are the first known measurements of laser photon thrust performance using lightweight sails that are candidates for spaceflight,’ said Dr. Leik Myrabo, associate professor at Rensselaer Polytechnic Institute, Troy, N.Y.
Both Benford and Myrabo are lead authors of papers describing the experiments. ‘Experimental Investigation of Laser- Pushed Light Sails in A Vacuum,’ by Myrabo, was presented June 2 during the Advanced Propulsion Conference at JPL. Benford’s paper, ‘Microwave Beam-Driven Propulsion Experiments for High- Speed Space Exploration,’ was presented at EuroEM 2000, held in Edinburgh, Scotland, May 30-June 2 and also at the JPL conference.
Knowles and Harris are among the co-authors on both papers. Harris is also co-investigator on the microwave experiment.
Energy Science Laboratories Inc. holds the patent on the sail materials used in these tests. JPL has overall responsibility for NASA’s interstellar missions, while NASA’s Marshall Space Flight Center, Huntsville, Ala., is responsible for developing transportation systems for the missions. The laser experiment was conducted in the Laser Hardened Materials Evaluation Laboratory II at Wright-Patterson Air Force Base.