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SCIENCE OBSERVER

Solar Windjamming

David Schneider

In 1958, physicist Richard Garwin of IBM Watson Scientific Laboratory wrote a short paper in the journal Jet Propulsion explaining the possibility of "sailing" through space by capturing the pressure of the sun's rays. All that is needed is a thin sheet of reflective material. Solar photons bounce off and transfer momentum to the sail, allowing the spacecraft to accelerate without expending fuel. Garwin's was the first technical publication to describe solar sailing in an English-language journal. But some of his readers had probably learned about the concept seven years earlier from an article by engineer Carl Wiley, published under a pen name in the magazine Astounding Science Fiction. And many who missed both Wiley and Garwin's papers were awakened to the idea by Arthur C. Clarke's 1963 short story "The Wind from the Sun," which describes a solar-sailing regatta.

In the decades since elapsed, this elegant means of space propulsion has remained squarely in the realm of science fiction. Solar sailcraft have not been built, largely because formidable technical difficulties arise when one tries to design a probe that can unfurl a gossamer sail that is many tens of meters—or perhaps even kilometers—across, without using heavy booms or guy lines.

But a design for a huge yet easily deployed solar sail seems finally to have appeared on the horizon. Curiously, the key innovation comes not from some recent progress in aerospace engineering or thin-film technology. Rather, the advance results from a new way to think about solar sailing—while remembering some basic science about the interaction of the Earth and Sun.

Geophysicist Robert Winglee of the University of Washington has studied solar and terrestrial magnetospheres for the past 15 years. Like many physicists who examine the interplay of natural magnetic fields in space, Winglee was aware that solar-wind protons racing outward from the sun at 500 or more kilometers per second transfer some of their momentum to the Earth when they are deflected by our planet's magnetosphere. About five years ago, he started to consider the possibility of harnessing this effect for propulsion. He imagined that a space sail could capture the momentum of solar-wind particles rather than solar photons, as Wiley, Garwin, Clarke and many others had always envisioned.

Winglee realized that a simple coil would not do the trick, because its magnetic field, which diminishes with the cube of the distance, would not extend far enough to catch much of the solar wind. Reviewing the literature, he discovered that some visionaries had considered building a "magsail" using an immense superconducting coil, perhaps hundreds of kilometers in diameter. This scheme seemed far too impractical to merit much attention. But Winglee's background in geophysics led him to a key insight. Instead of trying to construct a huge superconducting loop in space, he envisioned a way to inflate the magnetic field from a compact coil into a billowing "mini magnetosphere" by spewing out ionized gas—that is, a plasma.

Following the well-known principles of electromagnetism, the magnetic field lines from the coil would, in a sense, be frozen into the conductive plasma. Then, as the plasma cloud expanded around the spacecraft, so too would the magnetic field. This mechanism would thus allow the craft to unfurl a huge magnetic sail—tens of kilometers wide—with no rigging other than the field lines.

"It's a very simple concept," notes George Parks, a space-plasma physicist who collaborates with Winglee at the University of Washington. When Winglee first suggested this strategy, Parks doubted that enough force could be attained. But Winglee now has him firmly convinced. Dennis Gallagher, a magnetospheric plasma physicist at NASA Marshall Space Flight Center, is also enthusiastic: "The idea of using an artificial magnetosphere for a spacecraft for propulsion—that's really cool."

Others at the space agency feel similarly: The NASA Institute for Advanced Concepts funded Winglee to mount a pilot study last year and has recently awarded him a grant to continue the work for another two years. Informal research groups at Marshall Space Flight Center and at the NASA Jet Propulsion laboratory are now gearing up to evaluate whether this novel method could prove as useful as early projections promise.

According to Winglee's estimates, a 200-kilogram probe could deploy a magnetic sail of perhaps 20 kilometers' breadth and attain a velocity of nearly 100 kilometers per second using 50 kilograms of gas and about 1,000 watts of power to keep the plasma envelope filled. Making way at that clip, a craft could reach from Earth to Saturn in less than six months. The Cassini probe now on route to the ringed planet, by comparison, will take seven years.

A round trip would also be possible, perhaps with a larger robotic probe that could collect samples. "The device, if it does scale, would be terrific for return trips to Mars or return trips to Jupiter," notes Winglee. His sail, just like the thin-film varieties that have been discussed for decades, could be canted at an angle to the solar wind, so as to slow the orbital speed of the vessel and allow it to drop toward the Sun. In the language frequently used by advocates of solar sailing, the craft can be "tacked" to travel upwind.

Indeed, using sailing terminology to discuss this sort of propulsion has proven irresistible. In some respects, such metaphors are even more apt for Winglee's sail, which would truly catch the solar wind (that is, the charged particles emanating from the Sun) rather than rely on the momentum of reflected photons. But Winglee's concept may also need to borrow some of the vocabulary of ballooning, because unlike traditional solar sails, it requires an expendable gas (helium is under consideration) to fill its magnetic envelope. And like a weather balloon rising in the atmosphere, the magnetic bubble of Winglee's sailcraft would grow in size as it moves away from the Sun. This expansion would compensate for the ebbing of the solar wind as the craft traveled farther out, thus providing a constant propulsive force. In this respect, Winglee's version is superior to all previous proposals for solar sails, which would become less effective with increasing distance from the Sun.

Whether this promising design will ever be built will depend, in part, on how successful Winglee is in crossing between disciplines, from geophysics to aerospace engineering. Appropriately he has impressed John Slough, a professor of astronautics and aeronautics at the University of Washington, James Burch, of the Southwest Research Institute's Instrumentation and Space Research Division, and Anthony Goodson, an aerospace engineer at Boeing, to join his research crew. One can't help but wish them smooth sailing.—David Schneider



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