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.



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."



Janhunen is leading a three-year, $3.25-million project (partially funded by the European Union) to demonstrate the fundamental e-sail concept, first on Estonia's upcoming EstCube 1 nanosatellite and then on the Finland's Aalto 1 nanosat. Each tiny satellite is to simulate the basic interaction between an electric field and surrounding charged particles by unreeling a short, high-voltage wire tether into the plasma of Earth's ionosphere to measure any electrostatic forces that develop, Janhunen says. This simple setup will simultaneously demonstrate what engineers call a plasma brake—a device that could someday be attached to orbital debris and space junk such as old satellites and spent rocket stages to create drag to slow them down, thus increasing their rates of orbital decay, hastening reentry into Earth's atmosphere.



If those tests are successful, the eventual plan is to fly an e-sail craft that will unfurl a circular array of up to 100 20-kilometer-long conductive filaments only 25 microns thick, Janhunen says. The e-sail craft would then spin to deploy a giant structure of centrifugally stretched wires that resembles the spokes of a wheel. When high voltage (supplied by photovoltaic panels) is applied to the conductive filaments, overlapping electrical fields would form that serve as a barrier to the charged particles in the oncoming solar wind. The e-sail would thus catch the solar wind like some huge dandelion seed that is blown aloft by terrestrial breezes.



Tilting the array's plane to the oncoming flow will allow the novel spacecraft to tack and change trajectory while cruising either toward or away from the sun, he notes. Although payloads will probably be limited in weight so as not to overtax the virtual shrouds, the electro-sail's potential applications might also include: serving as a brake for interplanetary probes as they approach their targets; as propulsion for nine-month, inward-spiraling expeditions to study Mercury or the sun up close; round-trip sample missions to asteroids; or even journeys beyond the solar system and into interstellar space.