How the Solar Sail Could Fuel Interplanetary Travel

Stephen Baxter's new novel Proxima includes a sentient solar sail, but this unique type of spacecraft has a long history in science fiction — and an even longer one in science. Though the idea is 400 years old, it might still become the kind of vessel that we eventually ride to the stars.

A solar sail is simply a spacecraft propelled by sunlight in exactly the same way a sailing ship is propelled by the wind. This is possible because light is made up of energetic particles called photons. When a beam of light strikes a reflective surface, its photons bounce off, in the process transmitting their momentum to the surface. Twice, in fact: once by the initial impact and again when they are reflected. Although this energy is almost immeasurably slight, a steady beam of light will exert a push on the reflective surface. If the beam of light is steady, the constant application of energy will eventually add up to considerable velocities.

The German physicist, Johannes Kepler, hinted at the idea in a letter he wrote to Galileo in 1610. After observing that the tails of comets always point away from the sun, he suggested that one might someday "Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void."

Although Kepler thought that there might be some sort of repulsive force emitted by the sun, this was not proven until James Clerk Maxwell in the early 1860s showed that light has momentum and thus can exert pressure on objects, though this was not demonstrated experimentally until 1899.

Jules Verne, being the smart cookie he was, picked up on this immediately. In From the Earth to the Moon, published the year after Maxwell's paper, he suggested that "there will some day appear velocities far greater than [those of the planets and his hero's projectile], of which light or electricity will probably be the mechanical agent ... we shall one day travel to the moon, the planets, and the stars." This, so far as I've ever been able to discover, was the first time anyone had proposed the use of light pressure as a method of spacecraft propulsion.

Verne, though, was not the only nineteenth century author to capitalize on the idea. While he had made proposed a vague idea without suggesting how it could be implemented, French science fiction authors G. Le Faure and Henri de Graffigny were much more explicit in their multi-volume novel, Aventures extraordinaires d'un savant russe (1889). In describing more than half a dozen different methods of traveling through space, they include solar sailing.

How the Solar Sail Could Fuel Interplanetary Travel

A flight from the moon to Venus by exploiting the pressure of sunlight. The ship itself is a large sphere surrounded by a broad disk made up of separate, movable vanes. The ship is launched by placing it in the center of a huge reflector, 750 feet in diameter, made up of individual, movable mirrors. Using this to focus sunlight onto the vanes of the ship, the spacecraft is propelled away from the moon. This is uncannily similar to the present-day idea of using enormous lasers to propel solar sails out of the solar system.

The next fictional solar sail was described in a Russian novel published in 1909, the year after Svante Arrhenius suggested the possibility that the pressure of sunlight might distribute the spores of life forms across interstellar distances. In On the Waves of the Ether, B. Krasnogorskii described a solar sail that resembled that in the earlier French novel: a small, central gondola surrounded by an enormous circular mirror. It flies through space by the force of the sun's radiation impinging on the latter. Steering is accomplished by changing the angle of the mirror relative to the direction of the sun. The planned journey to Venus is abandoned when the mirror is irreparably damaged by meteors.

Krasnogorskii may have picked up his idea from the work of fellow Russian, Friedrich Tsander, who had recently published a paper in which he described solar sailing by means of "tremendous mirrors of very thin sheets."

Shortly afterward, in 1929, J.D. Bernal wrote in The World, the Flesh & the Devil that "A form of space sailing might be developed which used the repulsive effect of the sun's rays instead of wind. A space vessel spreading its large, metallic wings, acres in extent, to the full, might be blown to the limit of Neptune's orbit. Then, to increase its speed, it would tack, close-hauled, down the gravitational field, spreading full sail again as it rushed past the sun."

The concept was first introduced to American readers by Carl A. Wiley, a mathematican at Goodyear Aircraft. Writing as "Russell Saunders" in an article published in the May 1951 issue of Astounding, "Clipper Ships of Space," Wiley described in detail the idea of exploring space by means of "sunjammers." These would be, he said, magnesium hemispheres 50 miles in diameter but just 0.15 micron thick. The rim of the sail would contain a wire to distribute stress, with rigging wires converging—like the lines of a parachute—to the ship proper. In the article, which was one of the first—if not the first—to describe solar sails in serious detail—Wiley considered the problems of both construction and navigation. Ironically, when he sent the article to space expert Willy Ley, Ley dismissed the idea of solar sailing, saying that "the utilization of solar light pressure just wouldn't pay."

How the Solar Sail Could Fuel Interplanetary Travel

Wiley's article re-introduced the concept to science fiction and within a few years several stories and novels appeared featuring solar sails. Among the first were Cordwainer Smith, who included one in his 1960 story, "The Lady Who Sailed the Soul." Jack Vance, Pierre Boulle and Larry Niven were also among the first to exploit the solar sail in fiction. In his 1964 short story, "Sunjammer," Arthur C. Clarke memorably described a race between solar sail spacecraft. It's entirely possible that Clarke got not only the idea but the title of his story from Wiley's article.

How the Solar Sail Could Fuel Interplanetary Travel

The first real effort to work out the technology and design for a solar sail did not occur until 1976 when JPL proposed a solar sail mission to rendezvous with Halley's Comet.

Since then, the idea has been taken much more seriously, driven largely by advances in developing strong, lightweight materials.

Japan was the first to successfully concept-test a solar sail when in 2010 it launched the IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) spacecraft. This consisted of 200 m2 plastic sail only 0.0075 mm thick. Using light pressure alone as its primary propulsion, IKAROS took six months to travel to Venus, after which it began three-year journey around the sun.

How the Solar Sail Could Fuel Interplanetary Travel

Since then, any number of solar sail projects have been on the drawing boards, such as NASA's Sunjammer, scheduled for 2015, the Planetary Society's LightSail-1 (following several earlier, abortive attempts by the Society to launch an experimental solar sail) and the European Space Agency's Gossamer project.

How the Solar Sail Could Fuel Interplanetary Travel