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Solar sail

Solar sails (also called light sails, especially when they use light sources other than the Sun) are a proposed form of spacecraft propulsion. The spacecraft deploys a large, lightweight sail which reflects light from the Sun or some other source. The radiation pressure on the sail provides thrust by reflecting photons. Tilting the sail at an angle from the Sun produces thrust at an angle that bisects the angle between the Sun and the spacecraft. Steering is usually with auxiliary vanes.

NASA study of a solar sail. The sail would be half a
kilometer wide.

A number of demonstration projects have proven this method's feasibility. Some unmanned spacecraft have been constructed with reflective panels that can be used instead of small rocket motors, to conserve fuel for maneuvering and attitude control. Solar collectors or sun shades can also serve as crude solar sails, and can help a spacecraft correct its attitude and orbit without using fuel.

Investigated Sail designs

"Parachutes" would have very low mass, but theoretical studies show that they will collapse from the forces placed by shrouds. Radiation pressure does not behave like aerodynamic pressure.

The highest-thrust to mass designs known are square sails with the masts and guy lines on the dark side of the sail. Usually there are four masts that spread the corners of the sail, and a mast in the center to hold guy wires. One of the largest advantages is that there are no hot spots in the rigging from wrinkling or bagging, and the sail protects the structure from the sun. This form can therefore go quite close to the sun, where the maximum thrust is present. Control would probably use small sails on the ends of the spars.

In the 1970s JPL did extensive studies of rotating blade and rotating ring sails for a mission to rendezvous with Halley's comet. The thought was that such structures would be stiffened by centripetal forces, eliminating the need for struts, and saving mass. In all cases, surprisingly large amounts of tensile structure were needed to cope with dynamic loads caused by a need to control the sail's attitude, reducing the advantage almost to nothing.

JPL's reference design was called the "heliogyro" and had plastic-film blades deployed from rollers and held out by centripetal forces as it rotated. The spacecraft's attitude and direction were to be completely controlled by changing the angle of the blades in various ways, similar to the cycle and collective pitch of a helicopter. Although the design had no mass advantage over a square sail, it remained attractive because the method of deploying the sail was so simple.

JPL also investigated "ring sails," panels attached to the edge of a rotating spacecraft. The panels would have slight gaps, about one to five percent of the total area. Lines would connect the edge of one sail to the other. Weights in the middles of these lines would pull the sails taut against the coning caused by the radiation pressure. JPL researchers said that this might be an attractive sail design for large manned structures. The inner ring, in particular, might be made to have artificial gravity roughly equal to Mars.

Eric Drexler designed a sail called a "lattice ship" based only on tensile structures. It rotated and would have to be continually under thrust. A system of tensioning masses distributed stresses over a set of hexagonal cellular sails. It requires materials that do not exist (as of 2003), and would be operationally complex to deploy and rendezvous with destinations.

Sail Materials

The most common material is 2μm Kapton film. It resists the heat of a pass close to the Sun and still remains reasonably strong. The aluminium reflecting film is on the Sun side.

In the far future, the material may be a thin mesh of aluminium with holes less than 1/2 the wavelength of most light. Nanometer-sized "antennas" would emit heat energy as infrared.

Interstellar ships

Robert Forward proposed the use of lasers to push solar sails, providing beam-powered propulsion. Given a sufficiently powerful laser and a large enough mirror to keep the laser focused on the sail for long enough, a solar sail could be accelerated to a significant fraction of the speed of light. To do so, however, would require the engineering of massive, precisely-shaped optical mirrors or lenses (wider than the Earth for interstellar transport), incredibly powerful lasers, and more power for the lasers than humanity currently generates.

A potentially easier approach would be to use a maser to drive a "solar sail" composed of a mesh of wires with the same spacing as the wavelength of the microwaves, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light. The hypothetical "Starwisp" interstellar probe design would use a microwave laser to drive it. Microwave lasers spread out more rapidly than optical lasers thanks to their longer wavelength, and so would not have as long an effective range.

No solar sails have been successfully deployed as primary propulsion systems, but research in the area is continuing. The Russian space program has attempted to deploy at least one prototype test unit in Earth orbit.


See also spacecraft propulsion.