Sailing the Photon Sea: Spaceflight with Solar and Magnetic Sails

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Paul Lucas

Issue: 15 March 2004

Solar sails are without a doubt the most poetic of all forms of near-future spacecraft: gigantic, mirror-like, hundreds of miles wide but gossamer thin, riding on currents of unfiltered sunlight.

The idea of using sunlight directly for propulsion actually goes back centuries, as astronomers observed that some kind of solar "wind" was "blowing" comet tails away from the sun. This, along with the idea that light waves needed something material to propagate them, led to numerous theories that a mysterious "ether" permeated space as a universal background medium. Some envisioned that one day bold adventurers might sail ether currents in the sky just as ancient sailors plied the seas of old Earth. Ether theory has proven to be a false path, but the idea of sailing the spaceways continues.

The first scientific suggestion that light could be used as a propulsive force in space dates back to a 1924 paper by Russian space pioneer Konstantin Tsiolkovsky and an engineering associate, Fridrickh Arturovich Tsander, working on known principles of electromagnetism dating back to Maxwell's equations that shows light can impart momentum. In 1951 the non-fiction article "Clipper Ships of Space," by Carl Wiley, appeared, the first serious examination of light propulsion. A more detailed paper followed in 1958 by Richard Gamin, and the subject has been frequently revisited in scientific and engineering literature ever since.

In the early '80s NASA planned a rendezvous with Halley's Comet using a solar sail, but a conservative NASA administration eventually nixed the project. Today, theoretical work on solar sails both in the private sector and by NASA is still underway, including a test performed in 2000 that showed solar-sail material can be pushed using focused light sources. Tentative plans are also being made to use solar sails in space, including an ambitious plan to place a solar sail-equipped satellite at a point between Earth and the sun in order to provide advanced warning of potentially disruptive solar storms.

Solar sails have also appeared in a number of science fiction sources for over a half-century, most significantly in the short stories "The Wind From The Sun" by Arthur C. Clarke and "Sail 25" by Jack Vance, and the novels A Mote in God's Eye by Larry Niven and Jerry Pournelle and Flight of the Dragonfly by Robert L. Forward.

Basics

We tend to think of light as something insubstantial, and while it is true that photons have no mass, they still have momentum and can therefore exert pressure. In fact, it has been found that sunlight exerts about two pounds of pressure per square kilometer on the surface of the Earth. Solar sails attempt to take advantage of this natural resource, using light pressure reflecting off a sail to accelerate a spacecraft through space.

However, to get any appreciable thrust, solar sails have to be optimized to reflect as much incoming light as possible, and must be hundreds if not thousands of square miles in area. Such sails, if made of conventional materials, would prove to be prohibitively heavy, so solar sails are usually envisioned of being made of micron-thin material. The material should also be over 99% reflective, as the sail will gain almost twice as much thrust from reflecting photons as from absorbing them. The "dark" side of a sail would be optimized as a radiator, to make sure the energy it does absorb does not heat the sail enough to deform it or degrade its reflectivity. Reflective substances such as Mylar and kapton, reinforced by materials such as plastic, aluminum, and/or carbon nanotubes, are considered the most likely choices for sail material.

Solar sails, even when in the inner solar system where sunlight is strongest, would experience only very gentle accelerations, since the pressure of sunlight on each square meter of sail would be far less than the weight of a paper clip on the surface of the Earth. However, solar sails never run out of sunlight to use, so while their accelerations and decelerations are extremely slight (about 1/7000th of a G for the most common designs), this will mount over the weeks, months or even years a solar sail might be en route. A solar sail vessel might take up to two years to reach Mars, but would require no fuel to either speed up or slow down the entire way.

Payloads for solar sails would be very tiny in comparison to the sail itself. It takes a lot of square mileage of sail to move even a small amount of payload mass. A cargo of about 100 pounds would require a sail dozens of miles across. A payload of 1200 tons (the estimated required weight of a manned interplanetary mission) would need a sail well over 1,000 miles across. The use of artificial high-intensity light sources such as lasers (see below) can reduce this significantly, but the enormous sail-to-payload ratio will always be a fact of solar sail missions.

Payloads are usually envisioned as being embedded in the center of a sail or its rigging, or towed behind the sail by kilometers of super-strong wire.

It is a common misconception that a solar sail can only move away from the sun. Actually, sunlight is only part of the forces acting on the vessel from our star; the other is the sun's gravity.

Everything in the Solar System is in solar orbit; it is being pulled by the sun's gravity. The same is also true for any solar sail operating in the solar system. Solar sails will almost never move in straight lines away from the sun. Instead, they will move in broad orbital spirals as the sail is "tacked," or angled, for acceleration or deceleration.

When a solar sail tacks away from the sun, the sail's surface is angled in such a way that the sunlight it reflects pushes the sail up and out, away from the sun, so that it slowly spirals outward. Think of a wind sail tacking away from the wind, where most of the sail is angled to catch the wind blowing on it from behind and accelerate the ship. When a solar sail tacks into the sun, the sail is angled so that the sunlight hitting it slows the ship down, forcing it to slowly spiral inward in its orbit. Once again, think of a wind sail tacking into the wind, heading into it at an angle and using the wind hitting the front of the vessel to slow it down.

The sun is not necessarily the only source of propulsive light available for a solar sail. Gigantic laser projectors, constructed in orbit or deep space, can train their beams on solar sail vessels, pushing them with continuous concentrations of high-intensity light beams that can reach for many millions of miles. Using beamed light in this way, solar sails could operate efficiently in the outer system and even interstellar space.

One tertiary use for solar sails is that, because they are basically gigantic space-based mirrors, they can be deployed in orbit to focus more sunlight on cities in extreme latitudes, such as those within the Arctic Circle. Cities in extreme northern climes can therefore be made more hospitable for human inhabitants. In 1993, Russia launched the short-lived Znamya 2 project, which deployed a solar-sail like mirror 20 meters across in space, in order to test the feasibility of this technique. A follow-up experiment, dubbed Znamya 2.5 is still in development.

One of the main problems that keeps arising in actual solar sail experiments is how to fold and unfurl the ultra-thin sail material without it tearing or creasing, as well as how to keep it taut enough to operate efficiently but loose enough so that it can be easily manipulated and tacked.

Let's now consider some more specific designs for solar sails.

Rig-stabilized Solar Sails

These are also called 3-axis stabilized solar sails.

One of the most straightforward ways to deploy and stabilize a solar sail is with an array of gigantic booms, rigging wire, and stays. A number of configurations for this have been proposed, the most popular of which are the "kite" sail, a diamond-shaped, slightly concave sail with large booms stretching along the long and short axes, and rigging attaching various points around the rim in order to make it slightly concave. Light pressure and specially-constructed stays keep the sail taut.

Another rigged solar sail design was created by the Canadian Solar Sail project as part of the Canadian Space Agency. Its sail is divided into six triangular sections, which are independently controlled and can tilt like Venetian blinds for steering and tacking.

Spin-stabilized Solar Sails

This scheme spins the solar sail, using centripetal acceleration to keep the sail taut and flat. Tension lines would actually be used to bear most of the force caused by the spinning, taking the load off the ultra-thin sail material itself.

Spin-stabilized sails would need only a fraction of the heavy support needed for rigid rig-stabilized sails, and thus would be lighter and faster. However, controlling and tacking a spinning solar sail is also that much more difficult to accomplish with precision.

Heliogyro

A heliogyro uses sails mounted along numerous long, rectangular vanes, relatively thin but many kilometers in length, giving the spacecraft the appearance of a gigantic pinwheel or many-bladed windmill. These vanes are deployed using rollers. The spacecraft's spin unrolls the sail, forcing the vanes outward. The craft continues to spin once the sails are fully deployed to keep them taut.

This is considered a somewhat more practical and "safer" configuration than the previous two designs; if a major mishap occurred in deploying a single, enormous, rig- or spin-stabilized solar sail, the mission would be in serious jeopardy. But as a heliogyro has up to several dozen vanes, the failure of one or even several vanes to deploy would not necessarily threaten the success of the mission.

The craft is steered by tilting and gimballing vanes in unison to control the sunlight falling on them.

Magnetic Sails

Instead of a large, gigantic mirror-like sail, a magnetic sail deploys a conductive or superconductive loop anywhere from tens to hundreds of kilometers in diameter. In order to extend the field further, the craft billows out ionized gas that "carries" the ship's magnetic field with it as it expands, allowing the vessel to create a magnetic field much larger than itself. This field itself acts as a sail, riding not on sunlight but on the charged particles of the solar wind. Its use is very similar to that of a conventional solar sail, with tacking of the roughly lozenge-shaped field used as the main method for maneuvering. While it would have to continuously spray the ionized gas from an onboard supply, this "fuel" would still supply the magnetic sail with an engine efficiency of ten to twenty times that of the Space Shuttle.

If the loop were made of superconductive material, a magnetic sail would actually have a better thrust-to-mass ratio than a conventional solar sail, mainly because the protons of the solar wind carry much more momentum than photons. However, the solar wind is not as steady and smooth a source of propulsion as sunlight is, and a magnetic sail would have to deal with different concentrations of it from such phenomena as solar storms and high sunspot activity. Also, thermal issues arise in the need to keep the superconducting material at cryogenic operating temperatures, especially within the inner solar system.

Around a planet, the magnetic sail can use the planet's magnetic field for thrust if it passes over one or both of the world's magnetic poles. Both the magnetic sail and the planet's magnetic field can be thought of as simply bar magnets. As the magnetic sail is approaching a pole, it orients its magnetic field to attract the pole, thus accelerating itself. It shuts off this attractive force as it passes over the pole so as not to lose any of its momentum as it moves away. Alternately, it can re-orient its magnetic field to repel the magnetic pole it is passing over, thus lifting itself higher via magnetic levitation. However, both methods are a very gradual process, and a ship with a magnetic sail would generally take months to achieve escape velocity by repeatedly passing over Earth's magnetic poles.

One of the more interesting ways to use a magnetic sail is as a "drag chute" of sorts for decelerating interstellar vehicles outside the heliopause of a star system. This would save on an interstellar spacecraft's fuel, and provide an auxiliary propulsion system in the target solar system. Interstellar space contains very small amounts of hydrogen, on the order of about one atom per cubic centimeter. A sailcraft's magnetic field would ionize this hydrogen by accelerating the electrons in one direction, and the oppositely charged protons in the other direction. The energy for the ionization and cyclotron radiation would come from the spacecraft's very high velocity and kinetic energy, slowing the spacecraft.

A magnetic sail craft could take advantage of beamed power for propulsion using orbital or deep-space particle beam projectors to fire a stream of charged particles at the craft, allowing it to operate efficiently both in the outer solar system and interstellar space.

Starwisp

Starwisps were originally put forward by the late Robert L. Forward as a precursor type of interstellar probe propelled by short-wave radio energy: in essence, a microwave sail. It is most significant in the fact that it requires both nanoscale and macroscale level engineering to pull off.

The system consists of three parts: a microwave projector in close solar orbit, a Fresnel zone lens focusing element in a farther orbit, and a superlight probe.

The mesh probe would be made of micron-thin material, weighing less than an ounce, but spread over a square kilometer area as a fine wire mesh. Nanocircuitry would be embedded into the mesh, acting as controls, sensors, and cameras. The entire probe would weigh around 20 grams, and would require advanced nanoscale engineering to pull off.

The microwave beam transmitter would be in close orbit around the sun to draw as much power through solar cells as needed. However, the beam would spread out to uselessness over interstellar distances, so the beam would have to be projected onto an enormous focusing lens about 185 km in diameter beyond the orbit of Mars. The lens would tighten and focus the beam onto the starwisp.

Using a 10-gigawatt beam, the starwisp would accelerate at 115 Gs for a number of days, reaching a theoretical maximum of 20% light speed. Of course, there would be no way to slow it down, but once it approached its target system, the microwave beam would be reactivated, providing the nanocircuitry in the wire mesh with the microamperes of power it would need to run its systems. The wire mesh would act as a dish antenna, allowing it to transmit its data back to Earth.

Starwisps could only be used for quick flybys of nearby star systems, but once the microwave beam/focusing elements were in place, the cost of creating new starwisp probes would be very cheap compared to other interstellar options. Thus, the human race could check out its interstellar neighborhood in a relatively short time.

One obstacle to building a starwisp may not be so much technical as it would be political. Their deployment would require the construction of a gigantic and powerful maser gun in close solar orbit. While the beam would be too diffuse to do much physical damage beyond a few tens of thousands of kilometers, it could still seriously damage delicate electronic systems or scramble radio signals of any space-based assets as far out as Mars. Those not in direct control of the microwave array might object to both its construction and use.

Interstellar Light Sail

To use a light sail for interstellar travel, you need a light source far more intensely focused than sunlight. One answer is an enormous laser array, perhaps 1000 or so kilometers in diameter, which like the Starwisp microwave emitter would be placed in close solar orbit to draw all the energy it needs for operation. Depending on how far the destination is (the upper limit for an interstellar light sail system is estimated at about 40 light years) the power output of the laser array would have to be on the order of anywhere from 47 to 43000 terawatts. As the combined total energy output of modern-day Earth is about 1 terawatt, this represents an enormous leap of power production and transfer ability.

As with the Starwisp, an additional focusing element for the laser would have to be located in the outer solar system to focus the beam on the sail. These lenses could also be Fresnel lenses, constructed of many thousands of tight concentric plastic rings, with the outermost ring being about 300 kilometers across.

How large the sail would have to be depends on how far it is intended to travel -- how far the beam spreads in interstellar space at extreme distances will determine the diameter of the sail needed to reflect a useful amount of that light for propulsion. For a trip to the nearest star, Proxima Centauri, a distance of about 4.3 light years, a sail of about 117 kilometers in diameter would be needed. For a mission to the outer range of the light sail system, 40 light years, a sail 936 kilometers in diameter would be required. Both figures assume a 1000-ton payload.

Also, the sail would have to be over 99.95% reflective, to prevent the intense energy transfer of the beam from melting the sail material.

Exactly how fast an interstellar light sail would be is a matter for some debate. A great deal depends on technical variables, such as how reflective the sail is, the energy density and focusing ability of the driver beam, and so on. Most agree that 10% to 20% of light speed is well within its capabilities, but some very optimistic estimates place its upper limit at 50% of light speed or more.

The problem of decelerating the craft as it approaches its destination was solved rather ingeniously by the ubiquitous Robert Forward. The sail could actually be constructed in two parts, an outer ring sail and an inner circular sail. During the initial acceleration, both parts of the sail would be connected and act as a single unit. When the craft needed to decelerate, the outer ring sail would detach and move forward of the inner sail. This outer sail could then serve as a mirror to reflect and focus the driver beam onto the inner sail, slowing it down. The outer ring sail would continue to accelerate away even as the inner sail slowed down; it would be expendable, much like the first stage of a modern chemical rocket.

Conclusions

In the next century, light sails and magnetic sails both have the potential for opening up the solar system to mankind, as they use an engine that never runs out of fuel: the sun. Once the technology matures, space exploration agencies employing sails would have cheap and often reusable means of visiting ever corner of our local stellar neighborhood. And in the long term, we may even ply the seas of interstellar space on sails of the thinnest gossamer.

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Paul Lucas grew up on the shores of Lake Erie just a few snow drifts away from Buffalo in the sleepy little town of Dunkirk, NY. He currently resides in Erie, PA, where he freelances as a writer and artist. His previous publication in Strange Horizons can be found in our Archive.