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An interstellar lightsail detaches its outer ring for its deceleration phase

Rig Stabilized Solar Sails
Tech level: 12
Spin Stabilized Solar Sails
Tech level: 13
Tech Level: 13
Magnetic Sails
Tech Level: 14
Star Wisp
Tech Level: 15
Interstellar Light Sail
Tech Level: 16

Solar sails are without a doubt the most poetic of all forms of spacecraft; gigantic mirror-like sails, 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. Solar sails have appeared in a number of science fiction sources for over a half-century now, most significantly in the short story "The Wind From The Sun" by Arthur C. Clarke, and the novels A Mote in Godís Eye by Larry Niven and Jerry Pournelle and Flight of the Dragonfly by Robert L. Forward. Tentative plans are being made to use solar sails on an experimental basis 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.


We tend to think of light as something insubstantial, and while it is true photons have no mass, they still have momentum and can therefore exert pressure. In fact, 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 out of the sunlight, a solar sail has to be optimized to reflect as much sunlight as possible, necessitating them to 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. It is also advantageous to have this material be highly reflective, on the order of over 99% reflective, as the sail will gain almost twice as much thrust from reflecting photons as opposed to 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 use as sail material.

Solar sails, even when in the inner solar system where sunlight is strongest, would experience only very gentle accelerations, far less than the weight of a paper clip on the surface of the Earth per square meter. However, solar sails never run out of sunlight to use, so while their accelerations and decelerations are extremely gentle (about 1/7000th of a g for the most common designs), they can pile on enormously over the weeks, months or even years a solar sail might be en route. A solar sail vessel may 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 high-intensity light sources (see below) can reduce this significantly, but the enormous sail-to-payload ratio will always a fact of solar sail life.

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 the sun; the other is the sunís gravity.

Everything in the Solar System is in solar orbit; i.e., it is being acted on by the sunís 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 sailís 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 it 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 of the main problems that keeps rising in RL solar sail experiments is how to both 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.

One tertiary use for solar sails is that, because they are basically gigantic space-based mirrors, they can be deployed in orbit solely to focus more sunlight on cities in extreme latitudes, such as the arctic circle. Cities in extreme northern climes can therefore be made more hospitable for human inhabitants.

Tech Level: 12

These are also called 3-axis stabilized solar sails.

One of the most straight-forward 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 keeps 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.

Tech Level: 13

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 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.

Tech Level: 13

A heliogryo uses a sail on numerous long, rectangular vanes, relatively thin but many kilometers in length. These vanes are deployed using rollers. The spacecraftís spin unrolls the sail, forcing the vane 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.

Tech Level: 14

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 shipís 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.

If the loop is made of super-conductive 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 planetís magnetic field for thrust if it passes over one or both of the worldís magnetic poles. Both the magnetic sail and the planetís 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 would generally take months to achieve escape velocity using this method by repeatedly passing over Earthís magnetic poles.

One of the more interesting ways to use a magnetic sail is as a "brake" of sorts for interstellar vehicles. As quoted from the Wikipedia article (author unknown; link given on bottom of page):

"In interstellar spaceflight, outside the heliopause, a magnetic sail could act as a parachute, to decelerate a spacecraft. This would save the deceleration half of an interstellar spacecraft's fuel, and provide an auxiliary propulsion system in the target solar system. Interstellar space contains very small amounts of hydrogen. A fast-moving sail 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 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.

Tech Level: 15

Starwisps were originally put forward by Robert L. Forward as an early 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 kilometer wide, 16-gram wire mesh probe with nanocircuits and sensors built into its infrastructure.

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 solar orbit to draw as much power as needed. However, the beam would spread out to uselessness over interstellar distances, so the beam is projected onto enormous focusing lens about 185 km in diameter beyond the orbit of Mars. These lenses tighten and focus the beam onto the starwisp.

Using a 10 gigawatt beam, the starwisp would accelerate at 115 gís for a number of days, reaching it theoretical maximum of 20% lightspeed. There is, of course, no way to slow it down, but once it approaching its target system, the microwave beam would be reactivated, providing the nanocircuitry in the wire mesh with the power its needs to run its systems. The wire mesh would act as a dish antenna, allowing it to transmit its data back to Earth.

Starwisps could of course 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 starwisps 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 using starwisps probes.

One obstacle to building a starwisp may not be so much technical as it would be political. I.e., the construction of what would amount to 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. Political entities not in direct control of the microwave array may object to both its construction and use, no matter how many assurances the building party may give them.

Tech Level: 16
Relative sizes of interstellar lightsail constructs and the Moon.

To use a light sail for interstellar travel, one needs to use a light source far more intensely focused than sunlight. The answer is an enormous laser array, often quoted as 1000 or so kilometers in diameter, in close solar orbit so that it can 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 total energy output of modern-day Earth is about 1 terawatt, this represents an enormous leap of power production and handling ability.

Like 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 are often quoted to 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 its travelling, i.e., 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 a trip to the nearest star, Proxima Centauri, a distance of about 4.3 light years, a sail of about 117 kilometers 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 able to travel 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 lightspeed is well within its capabilities, but some very optimistic estimates place its upper limit at 50% lightspeed or more.

The problem of decelerating the craft as it approaches its destination was solved rather ingeniously by Robert Forward. The sail would actually be constructed in two parts; an outer ring sail and an inner circular sail. During the initial acceleration, both parts of the sail are connected and act as a single unit. When the craft needs to decelerate, the outer ring sail detaches and moves forward of the inner sail. This outer sail is then used as a mirror to reflect and focus the driver beam onto the inner sail, slowing it down. It should be noted that the outer ring sail continues to accelerate away even as the inner sail slows down. The outer ring is ultimately considered expendable, much like the first stage of a modern chemical rocket.

A three-stage sail is also envisioned, the second stage acting like a focusing mirror to send a much smaller inner sail and a smaller payload back to the home system.


In Print:

Starsailing by Louis Friedman

Indistinguishable From Magic by Robert Forward

On the Web:

Sites dedicated to Solar Sail technology:

An article on NASAís future plans for solar sails:

A Wikipedia article on magnetic sails:

JPLís page on magnetic sails:

A very in-depth and hard-science article on designing and using magnetic sails for the GURPS rpg:

An article on lightsails, magnetic sails, and starwisps:

A site discussing both starwisps and the interstellar lightsail scheme:

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