Carl Wiley, in his 1951 factual article on solar sailing, “Clipper Ships of Space,” in the May 1951 issue of Astounding Science Fiction, looked to a day when sails would be built in space itself. However, the technical literature since Sputnik has followed an understandable direction. Since the only way to get into space is in the nose of a rocket, most sail designers aimed to stuff a sail into a small, rugged package, and to have it spring unaided to full size in space.
Since the force of sunlight on a mirror the size of a football field amounts to the weight of a small marble, a sail must be both big and light to give a useful cargo a reasonable acceleration. To survive packaging, launch, and deployment, the reflector for such a sail must be flexible and tough. Designers settled on aluminized plastic as the best choice. Workable designs emerged. Their projected performance improved with time. They were no use, however, in a crash program to reach the moon, and available rockets could always launch the next robot toward the next planet. Interest waxed and waned, but no sails were built.
JPL studies in the mid-1970s brought positive indications that the sail could not only satisfy many of the energy requirements for inner solar system exploration, but that a sail 640,000 square meters (a square 800 meters or one-half mile, on a side) could take a 1,500-kilogram scientific payload to a rendezvous with the comet named for Edmund Halley. The propellant to add that much speed to a conventional rocket would be over ten billion tons per ton of payload. Yet a single solar sail launched aboard a shuttle and weighing about five tons could develop a small but steady acceleration to accomplish the rendezvous in less than four years.
Meanwhile, others interested in the Halley mission designed a competing system using electrically powered rockets. The two systems (both to be launched by conventional boosters) had comparable performance, but the National Aeronautics and Space Administration finally had to scrap the entire mission for lack of funds.
This time, however, interest did not fade—it shifted. After years of trying to get NASA and Congress to support a program to develop a sail, designers and their supporters have decided to go private and build one themselves.
The Solar Sail Project of the nonprofit World Space Foundation has begun construction of a kitelike plastic-film sail. Project members, mostly space engineers donating their free time, have built spars and sail material on special machines and have tested deployment procedures. Their first prototype, unveiled in a demonstration in 1981, was about 14 meters on a side, the second is planned at 28 meters.
In addition to the World Space Foundation, other groups have expressed interest in building and launching prototype sails. This may really take off if plans for a Columbus Quincenery Regatta are realized (see the chapter on “Rebel Technology”).
Another approach, which I have proposed, is to build a kind of solar sail, called a lightsail, directly in space. If you imagine a network of carbon-fiber strings, a spinning spiderweb kilometers across with gaps the size of football fields between the strands, you will be well on your way to imagining the structure of a lightsail.
Imagine the gaps bridged by reflecting panels built of aluminum foil thinner than a soap bubble, tied close together to make a vast, rippled mosaic of mirror. Now picture a load of cargo hanging from the web like a dangling parachutist, while centrifugal force holds the spinning, web-slung mirror taut and flat in the void.
Lightsails are what solar sails seem likely to become when we build them in space. They differ considerably from the deployable, plastic-film sails designed for launch by rocket from the ground. Not needing the toughness to survive folding, launch and deployment, lightsail reflectors need no plastic backing tens of thousands of atoms thick: they can be unbacked aluminum films just a few hundred atoms thick. Such thin foil cannot be made by smashing a bar of aluminum between rollers, as kitchen foil is made. Instead, thin films are made by piling up atoms on a smooth surface.
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A simple process starting with a little detergent . . .
To make a thin metal film, first take a glass microscope slide and smear it with detergent. Let it dry, then wipe off all that is visible, leaving behind an invisible film. Then place the slide in a vacuum chamber and pump out the air. Next, run current through a tantalum strip in the chamber, until it glows white, to vaporize a bead of aluminum from its surface. This turns the slide and the vacuum-chamber window into instant mirrors. Finally, remove the slide and dip it slowly in water, which will creep under the metal film, following the detergent, floating the film off to turn a rectangular area of water into a smooth, rippling reflector.
Such films have less mass than a slice of air a third of a millimeter thick. A piece of film rubbed between fingers coats them with sparkling dust.
Lightsails have little mass because they use unbacked reflectors and a structure of tension members. Tension members are lighter and slimmer than compression members of similar strength: compare a tower twenty stories high with a rope twenty stories long, if each supports a ton. This combination of reflectors and structure makes lightsails twenty to eighty times lighter than deployable plastic-film sails. This low mass increases the sail’s top acceleration by the same factor. One shuttle payload could supply materials enough to build a hundred square kilometers of reflector and structure, divided among a few dozen sails.
Concerns such as heating and micrometeroid damage have been examined and pose no threat to a properly designed sail. The manufacturing facility is small and simple compared to existing factories on Earth. If we plan to use space, we must learn to build in space; building lightsails may be a good way to start because lightsails can help open up the solar system.
Compare a lightsail with an ordinary rocket, operating in deep space. A one-ton rocket could push a cargo pod weighing more than a ton to a speed of one kilometer per second in a few minutes. A one-ton sail (more than a mile wide) would take all day to reach the same speed with the same cargo. But the next day the rocket would coast, drained of fuel, while the sail would add another kilometer per second to its speed. Accelerating just over one-thousandth as fast as a falling brick, it would pass twenty times the speed of sound in less than a week.
Worse yet for the rockets, a two-ton rocket cannot double the speed of the one-ton rocket because the extra fuel would burden it. Indeed, every 3 kilometers per second added to the final speed of the cargo roughly doubles the mass of the required rocket, but adds just three days to the acceleration time using a sail.
Since solar system journeys, by rocket or sail, will generally take months to years, lightsails have a strong advantage for long voyages: not only is the sail reusable for decades, and never in need of refueling, but also on any given flight around the solar system it will generally be both lighter and faster than the competing rocket. Rockets eat sails at giving swift kicks, but in the long haul they run out of gas and then can only coast along.
Sails in space also have advantages over their relatives below. At sea, the winds shift, stop and storm unpredictably. In space, the “wind” is sunlight, and holds quite steady. At sea, winds can drive a ship only to a modest speed. In space, the “wind” blows at the speed of light and the “hull” has no drag; thus, speed can increase enormously. High velocities are essential, circling the sun along Earth’s orbit would take more than a century at the speed of a jet liner.
At sea, mere sails are not enough; seagoing ships have keels so they can tack to go upwind. Lightsails have no keels and do not tack, yet they can manage to navigate in any direction. Light pressure can never pull a sail toward the sun, but the direction it pushes can be steered from side to side by tilting the sail, while gravity provides a steady pull inward. This steerable push and steady pull combine to move the sail in any direction chosen, from any orbit to any other, so long as it remains beyond reach of an atmosphere or the roasting of a close solar approach. At sea, ships have wind and water; in space, sails have light and gravity.
This type of lightsail (unlike some others you’ll read about) spins
like a gyroscope, and therefore, it requires a steady twisting force to turn it to a new orientation. The sail can turn itself in two ways: it can shift the cargo off axis by reeling and unreeling the connecting lines, or it can tilt its reflecting panels to shift the distribution of light pressure. The first turns the sail swiftly, for maneuvers near planets; the second turns it slowly, for maneuvers in orbit around the sun. In solar orbit, the sail can spin in one orientation for weeks at a time without moving a motor. Any panel can jam in any position with little effect, since there are so many of them; thus sails should run for decades without repair.
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Lightsails can change the way humanity uses space because they open new opportunities there. They can carry robots to the planets in record time or haul satellites from low to high orbits. They can do far more, however, because, for a given propulsion capability, they should cost far less to use than any rocket on the drawing boards.
Eventually, even the sophisticated techniques described above will be outstripped by our needs and our ambitions. But now we can see, on the horizon, ways to build even lighter, more powerful lightsails. Before we launch vast sails to the stars, for instance, we may learn a more subtle trick—arranging atoms one by one using “nanotechnology.” Nature shows how atom-arranging can be done using molecular machines like those in cells. Novel molecular machines can convert raw materials into better sails.
Freeman Dyson points out that the best sails aren’t films at all, but meshes fine enough to catch photons. Mesh sails would be lighter and faster. And because they would catch less air, they would have less drag, letting them dip lower into the fringes of planetary atmospheres. Adding thin spines to the back of a mesh sail would help it radiate heat, letting it tolerate brighter light, hence greater force, hence greater acceleration. This could help laser-driven sails on their way to the stars. The molecular machines of nanotechnology, building atom by atom, could build such sails by the mile. (They will build much, much more than that, but that is a different story.)
Sailing the Solar System
Today, the solar system seems vast and inaccessible. But our sense of distance reflects, in part, our experience and the cost of going places with rockets. From this viewpoint, developing lightsails would be somewhat like moving everything in the inner solar system to a 2,000-kilometer orbit above Earth, just outside the fringe of the atmosphere.
Among the most important changes would be access to asteroids. The planets themselves might remain deadly deserts, their gravity blocking access by lightsails and handicapping industry. Planetary atmospheres block solar energy, spread dust, corrode metals, warm refrigerators, cool ovens and blow things down. Even the airless moon rotates, blocking sunlight half the time, and has gravity enough to ground a lightsail beyond hope of escape.
The asteroids, however, are different. They are tiny by planetary standards, and their orbits crisscross the solar system. Some cross the orbit of Earth, and some have hit it. Since the asteroids are small, their gravity is easy to overcome or ignore. They hold no atmosphere to interfere with industry (or to be polluted by it), and the steady sunlight of free space lies near at hand to power equipment and heat furnaces. There, too, lightsails can stop to retrieve precious cargos, for many asteroids hold more than mere rock.
While this notion is controversial at present, many scientists believe asteroids contain abundant supplies of metals of kinds made scarce here on Earth ages ago, elements that sank in the formation of Earth’s metal core. Meteoritic steel, for example, is a strong, tough alloy containing nickel, cobalt, platinum-group metals, and gold. Raw materials from the asteroids can provide most of the needs of an industrial civilization in space.
Lightsails could bring robots and refining equipment to an asteroid and return treasure beyond the dreams of Cortez. Since it makes scarcely more sense to bring back a whole asteroid than to bring back the moon, the chunks should be small.
Most of this great wealth need never be returned to the Earth’s surface at all. It can be kept in space and used in orbit, aiding us in the creation of industries out where they can do no harm to the environment and where solar power is free, twenty-four hours a day. With proper planning, this wealth can benefit all mankind.
Ironically, the first lightsail mining expeditions may ignore steel and platinum and be sent instead to other asteroids, those rich in volatiles such as water, the key to life support in space, and to making rocket fuel. Brought to Earth, water would be useless, of course. But kept in space, water would be precious. Water from asteroids could make living and working in space economical.
A lightsail program could build on itself. With asteroidal metal, the fabrication facility could build cheap freighter sails using little material hauled up from Earth. Such sails might cut the cost of space transportation so low that eventually asteroids could be mined for ordinary steel.
So far, we have been discussing projects that have already received serious study. Scientists argue over when such things might happen, but not whether they are possible. What next, though? In time, principles used for hauling a satellite could serve in a sail large enough to haul an “ocean liner” to Mars in less than a year. But that’s only the beginning of the extravagant possibilities.
Eventually, we may be able to pack human-level intelligence into a chip the size of a postage stamp. If so, our first interstellar probes may be sent to neighboring stars by the thousands, tiny ambassadors blown out on miniature sails, like grains of pollen before the wind.
In time, lightsails could be used to drive real starships. A sail pushed by sunlight can leave the solar system in less than two years, its speed climbing past 100 kilometers per second before the sun fades astern. But there are ways to improve on this. As hinted by Isaac Asimov in his introduction, a large laser orbiting the sun could absorb solar energy and convert it into a narrow beam of light. Directed at a sail, such a mega-laser could drive a ship far out into the interstellar darkness, pushing it toward the speed of light.
The only problem then is stopping when you get to your destination. Freeman Dyson of the Institute for Advanced Study in Princeton proposes drag brakes using the interstellar plasma; Robert Forward at the Hughes Research Laboratory in California suggests using light from the laser bounced off the sail as a “brake” to decelerate a smaller mirror. (Many of these ideas will be covered later in the course of this book, especially in the essay by Forward and Davis.) One way or another the stars themselves lie within reach of sails.
The Prospect
Today, despite some recent moves toward peace, earth-bound governments still growl at each other, threatening war over the limited resources of a single planet. Perhaps the greatest promise of space is the perspective it provides: that to fight over the resources of Earth is to fight over crumbs.
For a nation interested in prosperity and in peace, the challenge of space cannot be ignored. European, Soviet, and Japanese efforts in space have grown while the U.S. space effort has floundered, and even the leaders of these countries still act as if the Earth were the whole world. Perhaps, after five hundred years, it is time the Copernican revolution came to global politics. Perhaps, five hundred years after Columbus’s ships opened a new frontier, it is time to spread our sails and to once again challenge the stars.
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K. Eric Drexler is a researcher concerned with emerging technologies and their consequences for the future. This interest led him to study space technology and to design (and patent) structures and fabrication techniques for a class of high-performance, thin metal film solar sails. He later performed ground-breaking studies in the field of nanotechnology—based on molecular machines able to build objects to complex specifications. Now a Visiting Scholar at Stanford University, he discussed the future of both space technology and nanotechnology in his book Engines of Creation.
And now our modest, unassuming (and hardworking) young managing editor comes on stage with a story illustrating some intriguing possibilities for the pract
ical use of solar sails.
Ice Pilot
by David Brin
June 12, 2092
FROM: Jeminalte Smythe
North Intellectual Commune
Semi-Anarchy of Vesta
Middle Belt
TO: Akiro Hsien-Fu
Sovereign Federation of Asiatic Historians
Offshore Shanghai
Coastal China Post Zone
Dear Akiro,
Thanks so much for that bit-burst of old records you transmitted up to me last month. Six gigabytes of old U.S. Congress records. (Kwak, but they could talk back then, even more than now!) It was an intimidating morass. Even with the help of an expensive software librarian-persona, it took some time to track down what I was looking for.
Found it at last, though. Secret, closed-door committee testimony that was declassified decades ago, but by then, who cared anymore, you know?
You asked what it was I was looking for. Well, scan this excerpt for yourself.
TESTIMONY of QUENTIN R. LEWIS
BEFORE THE SELECT COMMITTEE ON SPACE RESOURCES
UNITED STATES SENATE
AUGUST 25, 2013
SENATOR MURCHISON: Please state your name and occupation.
Project Solar Sail Page 5
