Solar Sailing

 
Solor sails are a way of moving things around in space, from one orbit to another. After a year's work, they are beginning to look like the best means of space transportation for a wide range of uses: they may be both cheap and fast. Before discussing clipper ships vs. canoes, however, we should first discuss boats and the ocean.

Space is big.  It would take as many Earths to fill the solar system (500,000,000,000,000,000) as elephants to fill the sea (an unpleasant prospect). The Earth's orbit around the Sun is 23,000 times the Earth's circumference. Driving to the Moon (1/400 of the distance to the Sun) would take six months, at 55 mph of course. Driving to the nearest star would take 50,000,000 years, and so on. Space is Big. To get anywhere you have to go fast.

But, you say, since there is no air resistance in space, perhaps a patient traveler (or load of freight) could start out slowly and simply take whatever time was needed, drifting along. But, alas, gravity is in control. Objects in space don't really go anywhere, if left to themselves; they simply go around in orbits. Unless you kick something so hard that it stops completely (in which case it falls into whatever it was orbiting) or kick it so hard in the other direction that it can fly away despite gravity, never to return, the object will simply grunt at the kick, and shift its orbit somewhat. To get from one orbit to another generally takes at least two pushes: the first to put the object onto an orbit that crosses the orbit you're trying to reach, and another at the crossing point, to make the object start following the orbit you want, instead of the transfer orbit that the first push put it on. Another way to do the same thing is to push gently for a long time, and slowly twist and stretch the first orbit until it matches the second. Either way, you can add up the change in velocity that all the pushing would produce, in the absence of gravity, play around with different directions and times of push, and find that the total velocity change needed has a minimum that can't be beaten for a given trip. This requirement is usually measured in kilometers per second (1 km/sec is about 2,200 mph). One of the lowest requirements of any interest is 2.4 km/sec: the velocity needed to get off the moon.

Rockets have limits, because they must carry mass to throw away. A rocket can reach the same velocity as its exhaust fairly easily; not much fuel is needed to reach a few kilometers per second. The problem is that fuel has mass, just like the payload. Let's say you have a rocket with enough fuel to reach 1 km/sec, and to take a ton of payload with it. How much fuel would you need to reach 2 km/sec? Enough fuel to take the ton of payload to 1 km/sec, and enough fuel to take the fuel needed for the second km/sec to 1 km/sec. The total fuel mass needed turns out to increase exponentially with the velocity reached, just as population has been increasing exponentially with time. Both increases can gobble up more resources than you can afford to provide. Using the Saturn V moon rocket as a first stage, and piling up rockets from there, we could have reached 30 km/sec with enough payload to drop one haunch off an elephant into the Sun (an unpleasant prospect).

Rockets burning chemical fuels run out of ability fast when measured against the solar system, although they were decent for getting us as far as the Moon. The exponential curve that gets rockets into trouble can be made less steep, however, if more energy can be put in the exhaust. This is the principle of the electric rocket; by soaking up solar energy in space and using it to throw small amounts of mass away fast (a mass driver is particularly efficient and versatile at this job), payloads may be pushed around the solar system in a reasonable way. The main problem is the cost and mass of the solar power plant. To use it efficiently accelerations must be low and trips long. Costs are also low: freight rates from Earth orbit to Mars orbit might be as little as $.20 per pound.

Asteroid mining facility with moored sail. Top, right: solar sail(10 km diameter). Top, left; Bernal sphere colony (1/2 km diameter). Bottom, left; asteroid (1 km diameter). Bottom, center; industrial complex. Behind asteroid; mooring tower with shroud lines extending to sail in distance. The pit on the right side of the asteroid has supplied enough material to build this colony, the industrial complex, 50 power satellites and many, many sails like the one shown. The solar system contains thousands of similar asteroids. The sail shown is one of a fleet used for asteroid mining; when loaded (2,000 tons). It will depart for a two year trip to Earth.

Solar sails don't work on the rocket principle, but on light pressure. Like stage magic, this trick is all done with mirrors. Because E = mc², energy, including light, has mass. For light in particular, that little bit of mass moves very fast through space; when it is bounced off a mirror it exerts a force, just like fast ping-pong balls bouncing off a wall. If you wanted that wall to move quickly, even without friction, you'd want it to have little mass and be hit by many ping-pong balls. Similarly, the mirror that makes up a solar sail should be very thin and lightweight, and have a large area - a square mile of reflected sunlight exerts enough force to support the weight, not of a building, not of a car, a person, or a large dog, but of a medium-sized cat. The name of the game, then, is to maximize acceleration by minimizing the mass per unit area of the mirror.

People have looked at this problem, off and on, for about 20 years. They set themselves the problem of stuffing about a square mile of folded reflecting surface into the nose of a rocket, of launching it, and of making it unfold and stretch into a reasonably flat surface in space. A design for a kite-like sail, with thin, aluminized plastic film for the reflecting surface, has finally reached an advanced planning stage at the Jet Propulsion Laboratory in Pasadena, California. (See illustration on inside back cover.) Their design can accelerate at about 1/7,000 of a gravity, which is actually fairly good: the sail can reach 1 km/sec in about eight days. This lets you get around, and because it needs no fuel, and no fuel to help carry fuel, and so on, it doesn't peter out at high velocities like a rocket does. They want to use it to reach Halley's comet (an object which is going around the sun the wrong way compared to the Earth; a huge velocity is needed): the flight would take four years. They may not get to do it, because solar electric rockets, mentioned above, still look good by comparison (1/7000 of Earth's gravity isn't spectacular) and because these rockets have been sitting in everybody's "come on, let's do it" file for many years. They have seniority.

Can solar sails be made better? The answer seems to be yes, if you forget about folding them up and launching them from the ground. I came to suspect this in the summer of 1976, and now, a year later, it looks as if it may be true: solar sails can be made in space, not as aluminized plastic sheet, but as aluminized nothing, which weighs far less. Designs now worked out on paper use aluminum foil as the reflecting surface, but foil 1/1000 the thickness of the kitchen kind. These sails are over 40 times as light, and therefore over 40 times as fast, as previous designs. This is spectacular.

If I had to draw a sail today, it would be a hexagon about six miles across, and weighing 20 tons. This is somewhere between the size of Manhattan and San Francisco, but the metal of the sail could be wadded up to the size of a Volkswagen bug. They could be made both much larger and much smaller. The sail itself would be a spinning (to keep it taut) metal mesh with long, parallel strips of very thin metal foil glued to it. At regular intervals across the front, wires would come up, and be bundled to form groups, with each group having a wire coming from it, with these wires, in turn, bundled to form groups still farther in front of the sail. After this bundling and re-bundling has concentrated the load of light pressure on the sail enough (that's what the wires are for), shroud lines take the concentrated force to the payload (see drawing).

The sail would be made on a large, lightweight framework, like a loom. Wires would be laid down, and fastened to each other where they crossed. As the wires go down, a device would travel back and forth, producing thin metal foil by vapor deposition on wax, vaporizing the wax for recycling, and laying the foil on the wire mesh. The whole process would take about six months; building the "loom" would require several flights of the Space Shuttle. Additional sails require about one flight apiece to provide needed raw materials. Eventually, sails would be built from extraterrestrial materials.

What can you do with a solar sail? First, how can you "tack"? Boats can go in any direction by using both wind and water; solar sailing vessels can go in any direction by using both light pressure and the Sun's gravity. Light pressure on a mirror is always at right angles to the mirror's surface, even when the mirror tilts and bounces the light at an angle. As the mirror tilts towards being edge-on to the light, the force becomes smaller and approaches zero. This means that the mirror can collect some force in any direction that would take it farther from the light source, in this case the Sun. So how can a solar sail reach, say, Venus, which is closer to the Sun than Earth? By using light pressure to slow down in its orbit, then letting the Sun's gravity pull it in. Solar sails can go anywhere in the solar system, and, in the inner solar system (where we are), they can get there faster than almost anything proposed.

The 20 ton solar sail mentioned above could take 180 tons of payload to any place in the solar system, stop (not orbit, but stop) and hang there on light pressure. With 800 tons of payload to slow it down, it would finally have the same acceleration as a plastic film sail with no load at all. With 6 tons of payload, it could fly to Pluto in one and a half years. Pioneer 11, launched over four years ago, won't reach Saturn until two years from now, and Saturn is only 1/3 the distance of Pluto.

Rough cost estimates suggest that solar sails will cost between $.03 and 1/3 cent a square foot. Kitchen wrap costs about $ .01 a square foot. If nobody throws them out of the solar system, toasts them too close to the sun, or crashes them into something, they should last for thirty to three hundred years. Maintenance costs should be about nil (you don't fix the sail at all, and there are only about two dozen reels for the shroud lines to keep track of). Each sail, without fuel expenses, can cruise around the solar system almost indefinitely. While a rocket must be built differently for almost every mission, the same sail that flies from low Earth orbit to geosynchronous orbit and back can do a perfectly good job of flying twenty times the freight to the asteroid belt, out beyond Mars. Not only that, but the sail costs above, with 10% real rate of interest on capital, can give costs like $.10/lb for transportation around the solar system. And sailboats have always had little environmental impact....

How do (apparently) good ideas like this arise? Well, they never seem to be new. Solar sails are an old idea The literature even contains references to metal film solar sails, although not of the high performance discussed here. It even contains a reference to the idea of making the material in space. When I first had the idea, my reasons were not to seek high performance, but to try to make a sail out of metals, which are readily available in space. My background was oriented towards manufacture in space, towards materials properties, and towards vapor deposition.

Previous workers concentrated on hauling sails up from the ground, but metal film sails are too delicate for that, so they were never studied. The few who considered making the films in space considered inappropriate manufacturing techniques, which either didn't work or produced films about 500 times as thick as optimum.

Many questions come up about the new design:

• Can films be made that thin (thinner than a soap bubble )? Yes.
• Do they have decent strength? Yes, I've made some, and taken them from coast to coast in. my luggage.
• Are the films still reflective when so thin? Yes.
• Will fast moving atoms from the sun knock enough atoms from the film to destroy them quickly? No.
• Will this stream of atoms exert destructive forces on the sail during solar storms? No.
• Will electric charging or magnetic fields cause problems? No.
• Will the films be able to stand the temperatures they reach in the inner solar system? Yes.
• Will meteoroids destroy the films quickly enough to matter? No.
• Will they destroy the wire mesh that is the structure of the sail? No.
• Can the films be mass-produced in space at low cost? Yes
• How much does the device for making the films weigh ? It's manageable.
• What trajectories can the sails fly on? Name it.
• Could the sails possibly find use for near Earth transportation, as well as deep space? Maybe.
• Will the sails ripple under light pressure and gradually tear themselves apart? No.
• How well can the sails be turned to point where you want them to? Well enough.
• Will stresses from changing temperatures cause problems? No.

 How interesting? As interesting as cheap space transportation, free of fuels and complex maintenance. As interesting as moving Earth's industry into deep space and scattering her life to the Sun's light. Arthur Clarke said: "If man survives for as long as the least successful of the dinosaurs - those creatures whom we often deride as nature's failures - then we may be certain of this: For all but a vanishing instant near the dawn of history, the word 'ship' will mean - 'spaceship.' " And, those ships may yet have sails.

Heliogyro solar sail

The Jet Propulsion Laboratory in Pasadena, California, has proposed a spinning solar sail where centrifugal force keeps the twelve 4-mile-long sail blades flat. The blades would be 1 mil thick aluminized plastic 25 feet wide for a total area of 600,000 square yards and a payload capability of 1100 pounds. The blades would be variable pitch so that sunlight would spin the vehicle and allow you to control attitude and acceleration and deceleration. Such a vessel could rendezvous with Halley's Comet in 1986.

 -SB

 (sent by Mike Liebold)