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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 concept was first proposed by Friedrich Zander in the late 1920s and gradually refined over the decades. 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 absorbing (for black sail) or reflecting (for reflective sail) photons.
The radiation pressure from the Sun against an absorbing sheet with a mass of 0.8 g/m² is equal to its weight with respect to the Sun's gravity, independent of its distance to the Sun. This mass/area ratio is doubled in the case of perfect reflection. The reason why these forces balance is because both gravity and radiation pressure fall off proportional to the inverse of the square of the distance to the source. Note that a sail can be heavier than this and still be quite practical, it would simply be unable to "hover" on radiation pressure alone.
Tilting the reflective sail at an angle from the Sun produces thrust at an angle that bisects the angle between the Sun and the spacecraft (steering can be done with auxiliary vanes). For interstellar missions the thrust vector will be away from the Sun and to the target. For missions within the Solar System the thrust will be in the direction of the orbital movement (to move to a higher orbit) or against it (to move to a lower orbit closer to the Sun).
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.
Radiation pressure is small and decreases by the square of the distance from the sun, but the advantage compared to many propulsion engines is that no fuel need to be carried onboard.
One common misconception is that solar sails work by capturing energy from the solar wind. The solar wind, composed of charged particles, would indeed apply a small amount of pressure to a solar sail, but this is small compared to the pressure exerted by light being reflected from the sail. Another misconception is that the radiation pressure is an unproven effect that may violate the thermodynamical Carnot cycle. This criticism was raised by Thomas Gold of Cornell, leading to a public debate in the spring of 2004. [1] http://www.newscientist.com/news/news.jsp?id=ns99993895 Other scientists have been quick to debunk this position, pointing out that there is no contradiction as the energy gain seen by the solar sail from the radiation pressure is offset by energy lost by the reflected photons in the form of a frequency shift.
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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 were developed by Eric Drexler, in an MIT master's thesis, He designed a sail using reflective panels of thin aluminum film (30 to 100 nanometers thick) supported by a purely tensile structure. It rotated and would have to be continually under slight thrust. He made and handled samples of the film in the laboratory, but the material is too delicate to survive folding, launch, and deployment, hence the design relied on space-based production of the film panels, joining them to a deployable tension structure. Sails in this class would offer accelerations an order of magnitude higher than designs based on deployable plastic films.
The highest-thrust to mass designs for ground-assembled deployable structures 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 intention 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. Weaker sails would ripple or oscillate when the sail's attitude changed, and the oscillations would add and cause structural failure. So the difference in the thrust-to-mass ratio was almost nil, and the static designs were much easier to control.
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 simpler than a strut-based design.
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.
A solar sail can serve a dual function as a high-gain antenna. Designs differ, but most modify the metallization pattern to create a holographic monochromatic lens or mirror in the radio frequencies of interest.
Current progress
No solar sails have been successfully deployed as primary propulsion systems, but research in the area is continuing. On August 9 2004 Japanese ISAS successfully deployed two prototype solar sails in low Earth orbit. A clover type sail was deployed at 122 km altitude and a fan type sail was deployed at 169 km altitude. Both sails used 7.5 micrometer thick film.
A joint private project between Planetary Society, Cosmos Studios and Russian Acedemy of Science is planning to launch Cosmos 1 the first solar sail spaceship in March 2005. The solar sail will be used to push the 100 kg spacecraft to 800 km orbit where it will unfurl the 15 meter sails. A suborbital prototype test by the group did not succeed in 2001 because of the rocket failure.
Sail materials
The best known material is thought to 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. Although samples have been created, it is too fragile to unfold or unroll with known technology.
The most common material in current designs is aluminized 2 μm 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.
Applications
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.
To further focus the energy on a distant solar sail, designs have considered the use of a large zone plate. This would be placed at a location between the laser or maser and the spacecraft. The plate could then be propelled outward using the same energy source, thus maintaining its position so as to focus the energy on the solar sail.
Spacecraft fitted with solar sails can also be placed in close orbits about the Sun that are stationary with respect to either the Sun or the Earth, a type of satellite called a statite. This is possible because the propulsion provided by the sail offsets the gravitational potential of the Sun. Such an orbit could be useful for studying the properties of the Sun over long durations.
Such a spacecraft could even conceivably be place directly over a pole of the Sun, and remain at that station for lengthy durations. Likewise a solar sail-equipped spacecraft could also remain on station nearly above the polar terminator of a planet such as the Earth by tilting the sail at the appropriate angle needed to just counteract the planet's gravity.
See also
References
- Space Sailing by Jerome L. Wright, who was involved with JPL's effort to use a solar sail for a rendezvous with Halley's comet.
Solars sails in fiction
- A Wind from the Sun by Arthur C. Clarke, a short story (in an anthology of the same name) describing a solar sail craft.
- The Mote in God's Eye (1975) by Larry Niven and Jerry Pournelle depicts an alien spacecraft driven by laser-powered light sails.
- Rocheworld by Robert L. Forward, a novel about an interstellar mission driven by laser-powered light sails.
- Solar sails first appeared in film in Star Wars Episode II: Attack of the Clones, in which Count Dooku has a starsail spacecraft dubbed ' Sunsailor '.
External links
- How Sails Work http://solarsail.jpl.nasa.gov/introduction/how-sails-work.html from NASA
- Far-out Pathways to Space: Solar Sails http://www-spof.gsfc.nasa.gov/stargaze/Solsail.htm from NASA
- The Wind from the Sun http://www.ec-lille.fr/~u3p/bd/bda.html in graphic novel form
- ISAS Deployed Solar Sail Film in Space http://www.isas.jaxa.jp/e/snews/2004/0809.shtml