Solar Sails

by Paul Gilster

Solar sails have scientists excited, and for good reason. They’re a way around one of the most intractable problems of deep space flight, that of propellant. Everything we launch into space takes its toll — the more weight we must lift in fuel, the less scientific payload we can carry. That tradeoff was critical in the Apollo Moon landings, where the Saturn V rocket weighed 600 times more when fueled than it did when empty. Extend missions farther out and the burden of lifting still more fuel becomes crippling, unless you can find a way to leave the fuel at home.

Solar sails get around this problem by catching a ride on light itself, an abundant quantity in the inner Solar System and still useful as far out as Jupiter. The trick is this: Although photons have no mass, they do carry momentum. As long ago as the era of Johannes Kepler (1571-1630), we have speculated on the possibilities of using this force. Kepler noticed that the tails of comets always pointed away from the Sun, surely the result of some kind of pressure. He suggested creating ‘vessels and sails adjusted to the heavenly ether…’ Today we know there is no ether, but the success of Japan’s IKAROS solar sail in 2010 shows us that solar sailing works.

Like its Earthly counterpart, a solar sail takes advantage of an ambient phenomenon to gain propulsive force. And like Earthly sails as well, a solar sail gathers more of a photon push the larger it is. When an engineer named Carl Wiley wrote about solar sails in Astounding Science Fiction in 1951, he brought the idea of sail technology to a wide audience. Wiley suggested a sail some 80 kilometers in diameter, one that looked something like a parachute. In the late 1950s, IBM engineer Richard Garwin published the first scientific paper on solar sail technologies.

As you might guess, the pressure from photons is slight, but the beauty of it is that it keeps mounting up. A large, lightweight sail gets an insistent and continuous push, a phenomenon that was noticed in early satellite systems like Echo-1, which was so thoroughly and unexpectedly pummeled by solar photons that the satellite, a thin-film balloon in orbit, was orbitally disrupted and eventually destroyed by nothing more than the pressure of sunlight. Later, the Mariner 10 spacecraft would explore Mercury, but various malfunctions aboard the vehicle would leave controllers with no way to adjust its orientation except using its solar panels essentially as sails.

Mariner 10 was rescued because the closer you get to the Sun, the stronger the momentum from its light, giving NASA plenty to work with to readjust the spacecraft’s attitude. A true solar sail, designed from scratch, would be designed to optimize the effect. Various sail designs have emerged but the most common is one using giant sheets of thin, highly reflective material to harness sunlight not just for propulsion but maneuvering, all activities that would otherwise require onboard supplies of fuel. By ‘thin,’ we’re talkiing about materials like aluminized 0.5-micron mylar or the thicker 8-micron Kapton. Bear in mind that a micron is one millionth of a meter. By comparison, something as wispy as a human hair is 100 microns in diameter.

The solar sail conundrum is how to launch a sail that is tightly stowed to fit the confines of the launch vehicle but must be deployed to a shape tens to hundreds of meters to the side once in space. One NASA report proposed using silver, titanium or beryllium films connected to a extremely lightweight ‘carbon-fiber’ truss, with the sail being launched wrapped around a cylinder and later deployed using centrifugal force. But current sail materials, like the polymide resin used in the IKAROS mission, require mechanical systems to unfold the sail while protecting it from tears or wrinkles that could compromise its effectiveness. One method under study is to use inflatable booms of Kevlar webbing that become rigid once expanded to their full length.

Although both NASA and the European Space Agency (working with the DLR German Aerospace Centre in Cologne) have developed prototype sails and built them on the ground, NASA has launched only a single small demonstrator sail, NanoSail-D. Even so, sails have a long history at NASA, where they were intensively studied back in the mid-1970s as candidates for a mission to Halley’s Comet which never flew. Later experiments in Russia, which deployed an attached sail mirror in a series of experiments called Znamya, and Japan, where sail deployments occurred in 2004, have helped us gain expertise in the practical side of sailing.

But the Japanese IKAROS sail was the first successful free-flying sail mission in space. IKAROS continues to demonstrate the viability of the concept, using an ingenious system that can change the reflectivity of individual parts of the sail, a method being tested to change the sail attitude in space. We have much to learn about how solar sails work, and The Planetary Society is now moving ahead with a project called LightSail 1, which will attempt the launch of a small sail some time in 2011. Larger sails will inevitably follow, demonstrating whether the hopes of leaving the fuel on the ground pan out in fast and flexible missions within the Solar System.

Paul Gilster is a full time writer, and the Writer/Editor of Centauri Dreams and Lead Journalist at the Tau Zero Foundation

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