Sail E-way: Spacecraft Riding the Solar Wind on Electric-Field Sails Could Cruise at 180,000 Kph

A photo of the experimental solar sail, NanoSa...
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An effective sail area that is as much as a million times bigger

A sail formed not of material, but by electric fields reaching a diameter of 40 kilometers could tap the solar wind and propel the fastest man-made object ever

It takes large quantities of rocket fuel to power space probes through the cosmos. So much so that many long-range missions, including exploratory voyages to the outer planets and beyond, are typically impractical or too time-consuming to contemplate carrying out using conventional rocket motors. To address the problem, scientists have developed ingenious alternative propulsion systems such as ion-drive technologies that require much less propellant than standard chemical rockets but, nonetheless, travel much faster over time. But even ion thrusters have limitations.

What if spacecraft could traverse our solar system or even interstellar space at yet greater velocities using no propellants at all? Such is the allure of solar sails—large, ultrathin mirrors that harness the faint pressure of the sun’s reflected light  to move through the vacuum of space. It is no wonder then that engineers at NASA and the Japan Aerospace Exploration Agency (JAXA) are now flight-testing prototypes of these photon-propelled solar sails—dubbed, respectively, NanoSail-D and IKAROS.

Although these pioneering craft garnered headlines when they were deployed in 2010,a different solar sail concept is also currently in the works, one that replaces physical sails with mostly nonmaterial shrouds comprising electric fields emanating from long, lightweight wires that extend outward like umbrella stays. And because the electric solar wind sail, or e-sail, concept offers the opportunity to field truly enormous virtual sails as much as 40 kilometers across, it could enable the development of the fastest man-made objects ever flown—perhaps at speeds around 50 kilometers per second, says its chief inventor, Pekka Janhunen, a research manager at the Finnish Meteorological Institute in Helsinki.

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E-sails differ from photon solar sails in that they catch the solar wind rather than sunlight, Janhunen explains. The solar wind is a high-speed but extremely tenuous stream of electrically charged gases—ionized hydrogen and helium—that flow outward from the sun. And although that ion stream exerts a dynamic pressure that is some 5,000 times smaller than that produced by solar photons, each charged wire produces a cylindrical field that can be as large as 100 meters in diameter, which makes for an effective sail area that is as much as a million times bigger. “That’s the trick behind the e-sail’s efficiency,” he says. “When you switch from physical to electric sails, you lose a [pressure] factor of 5,000 but gain a[n area] factor of a million.”

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