Sound power for deep-space travel beyond sun's reach.

For voyages beyond Mars, sunlight is weak, so solar-power cells can't supply much electricity. For such missions, spacecraft typically rely on thermoelectric generators, which contain materials that produce electricity in response to the difference in temperature between a hot mass of radioactive plutonium-238 and the chill of space. However, such generators convert only about 5 percent of heat into electricity.

Now, Scott N. Backhaus of Los Alamos (N.M.) National Laboratory and his colleagues have built a prototype power generator with much higher efficiency. It converts heat from the plutonium into acoustic energy and then into electricity.

Small enough to fit inside a microwave oven, the generator consists of a loop of metal tubing filled with high-pressure helium gas. Heat differences along the loop cause rapid expansion and contraction of the gas: sound. The oscillations drive a pair of pistons connected to wire coils inside a magnetic field, thereby generating electricity. The generator works with an efficiency of up to 18 percent, the researchers report in the Aug. 9 Applied Physics Letters.

Because the loop's stiff tubing doesn't transmit acoustic vibrations to the outside, the generator operates silently, Backhaus explains.--P.W.