The idea of leaving the engine of a craft behind and using the endless fuel supply of solar or star light seems like a great way of reducing the mass of a craft. The obvious disadvantage is the dependence on a high flux of photons to give the craft the needed acceleration. For interstellar travel, light-sail craft have to depend on extremely large-scale constructions such as huge solar-power relays around Mercury and enormous Fresnel zones in the outer Solar System.

Since light applies pressure to surfaces, the stream of photons can be used for propulsion in a near-frictionless environment. This concept is the background for light (or solar) sails. It is a method of space travel that negates the need for onboard fuel. Sails using the solar wind or only the light from stars are less efficient at larger distance from the Sun. In science fiction, solar sails are encountered from the 1920s, with early models being giant, multiple-sail craft. In other works, light sails are used to propel ramjets up to ram speeds. To increase efficiency, ground based lasers can be used to push the craft – using monochromatic light increases the reflectivity of the sail material and gives more acceleration. The efficiency decreases with distance, but much slower than if the craft was riding sunlight alone. The laser beam can be refocused by gigantic Fresnel zones. Light sails will have enormous areas, but the craft will carry no fuel or bulky engines. Alternatives to light sails include microwave sails, particle sails, magnetic sails, and laser or solar thermal or electrical propulsion.

Basically, the light sail is a use of James C. Maxwell’s discovery in 1873 that light reflected in a mirror applied pressure to the mirror. Since photons according to Einstein have mass then, by using the rather low friction coefficient of space, a craft is able to travel from A to B without having to carry bulky propulsion devices and especially without the need for onboard fuel. This is a large plus in terms of logistics. The fuel is supplied from nearby stars or by high-power lasers.

Arthur C. Clarke wrote “Sunjammer” in the early 1960s, but one of the earliest sources of light sails is a compilation of works by Cordwainer Smith, the name under which Dr. Paul M. A. Linebarger wrote science fiction in the 1950s. In this story the first manned interstellar ships are propelled by light sails. The smallest, earliest sails were only about 5000 km2. They were made of “tissue metal” – probably a fine mesh to give lightness to these enormous sails. Each ship carried many sails, and was steered by the manipulation of the sails much like sail-carrying ships today. Larry Niven is a later science-fiction writer who uses light sails extensively in his “Tales of Known Space” setting, where they are a way to propel a vessel to speeds where ram scoops can be used. These light sails are almost always pushed by giant lasers, placed either on tracks on Mercury, or on asteroids in the belt between Mars and Earth. In one story, “The Mote in God’s Eye”, written with Jerry Pournelle and set in Pournelle's own universe, the first encounter with an intelligent alien species is in the form of a meeting with a laser-pushed light sail.

There are several types of light sails – in the basic model the craft uses only light from stars to generate push. This is the model used by Cordwainer Smith. Acceleration is only provided as long as the light is intense, that is in the inner parts of solar systems. In those areas, light sails may be a cheap and efficient way of getting about. Acceleration is low, and the maximum speed is usually guessed to be about 25% of the speed of light. As the distance to the photon source increases, so does the efficiency of the light sail needed to sustain significant acceleration. There might very well be a point where a sail’s degree of push is countered equally by the friction of the incoming interstellar matter in the craft’s path.

The simplest form of light sail is a big circle or square with the craft dragged along on wires. This model is used in Robert Forward’s “Rocheworld”, where buckling of the sail occurs, but with no great concern from the crew. The danger of having the sail flap around or collapse in the fluctuating photon stream from the Sun seems to be a problem considered only in the real world, where ways of reinforcing the sail with poles seem popular.

Other forms of sail include the laser-powered light sail, where a giant laser or system of lasers bombards the sail with monochromatic light. This is favourable since it is easier to create efficient reflectors of monochromatic light compared to reflectors of ordinary sunlight composed of a spectrum of wavelengths. To be effective, the laser will have to be enormous and have its beam focused before the light reaches the sail. The laser-powered solar sail is seemingly the only way of getting a light sail to be effective on interstellar journeys. Larry Niven's early “Tales of Known Space” have giant laser batteries on Mercury, surrounded by a loose net of solar collectors.

One proposal by Robert Forward involves a lens the size of Texas (a Fresnel zone) placed between Saturn and Neptune, which focuses the lights from several thousand solar collectors in orbit round Mercury. These solar-pumped lasers will have a collective power of 65 GW. The light sail itself is a two-stage one, where the large outer sail is slightly bowl-shaped. It will be separated from (but very likely still connected to) the inner sail, upon arrival at Alpha Centauri (or Barnard’s Star?). The large outer sail then focuses the laser beam back onto the inner sail, thus braking the craft. The downside to laser-powered light sails is the tremendous laser power needed to propel the craft over interstellar distances.

Alternatives to light sails have been proposed. The microwave sail is also an idea from Robert Forward. A tiny spacecraft, the Starwisp, is propelled by microwaves transmitted from a solar-powered satellite in Earth orbit. The craft would be mostly a 1 km diameter mesh sail covered with microcircuitry, weighing only a few grams. The beam power should be 65–100 GW, pushing the 4–5 g craft to 20% of the speed of light, using a Fresnel-zone type lens to focus the microwave beam.

Particle sails are not exactly sail craft, but still use the concept of catching a beam from a ground-based emitter to gain acceleration. The beam in this case is a stream of heavier, slower particles like protons, emitted from a fusion reactor as plasma. The beam would disperse quickly, but the push gained could be up to 1000 g propelling the craft to 1/3 of the speed of light before the effect dissipates. The beam-projection limitations would make interstellar missions a one-way venture.

The magnetic sail is a use of Lenz’s Law (flux will attempt to remain constant in a electrified wire loop). The loop should be made from superconducting wires, and will expand to a circle when powered. The craft will be attached to this loop. Charged particles meeting the loop or magsail at other angles than parallel to the magnetic field will transfer some of their momentum to the field and thus push the craft. A magsail weighing 36 tons could receive accelerations of 0.0001 m/s to 0.009 m/s varying with the orientation of the sails. The mag loop is very small compared to a standard light sail, being only about 10 km in diameter.

Solar sails are very close to being non-fiction. The Russians have conducted the Znamya tests of light thin-film applications in space. The tests seemed to be for Earth-surface illuminatory purposes, but were also a test of alternative propulsion methods. In the United States, a light, bowl-shaped object was lifted 20 m using a laser beam. In Europe, ESA and DLR have developed solar-sail technology small enough to be carried into space and light enough to enable efficient sailing. A 20 m x 20 m model consisting of aluminium-coated sail segments with carbon-fibre-reinforced plastic hooks has been manufactured and tested.

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