Solar sail
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Solar sails (also called light sails, especially when they use light sources other than the Sun) are a proposed form of spacecraft propulsion utilizing large membrane mirrors. Radiation pressure is small and decreases by the square of the distance from the sun, but unlike rockets, solar sails require no fuel. Although the thrust is small, it continues as long as the sun shines and the sail exists.
Solar collectors, temperature-control panels and sun shades are occasionally used as expedient solar sails, to help ordinary spacecraft and satellites make minor corrections to their attitude and orbit without using fuel. This conserves fuel that would otherwise be used for maneuvering and attitude control. A few have even had small purpose-built solar sails for this use. Some unmanned spacecraft (such as Pioneer 10) have substantially extended their service lives with this practice.
The science of solar sails is well-proven, but the technology to manage large solar sails is still undeveloped. Mission planners are not yet willing to risk multimillion dollar missions on unproven solar sail unfolding and steering mechanisms. This neglect has inspired some enthusiasts to attempt private development of the technology (http://www.space.com/businesstechnology/technology/cosmos_sail_030806.html).
The concept was first proposed by German astronomer Johannes Kepler in the seventeenth century.[1] (http://www.rednova.com/news/space/151474/measuring_up_to_a_solar_sail/) It was again proposed by Friedrich Zander in the late 1920s and gradually refined over the decades.
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How they work
The spacecraft deploys a large membrane mirror which reflects light from the Sun or some other source. The radiation pressure on the mirror provides a miniscule amount of thrust by reflecting photons. 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. In most designs, steering would be done with auxiliary vanes, acting as small solar sails to change the attitude of the large solar sail (see the vanes on the illustration). The vanes would be adjusted by electric motors.
Sails orbit, and therefore do not need to hover or move directly toward or away from the sun. Almost all missions would use the sail to change orbit, rather than thrusting directly away from a planet or the sun. The sail is rotated slowly as the sail orbits around a planet so the thrust is in the direction of the orbital movement to move to a higher orbit or against it to move to a lower orbit. When an orbit is far enough away from a planet, the sail then begins similar maneuvers in orbit around the sun.
The best sort of missions for a solar sail involves a dive near the sun, where the light is intense, and sail efficiencies are high. For this reason, most sails are designed to tolerate much higher temperatures than one might expect.
Most theoretical studies of interstellar missions with a solar sail plan to push the sail with a very large laser. The thrust vector would therefore be away from the Sun and toward the target.
Misunderstandings
Critics of the solar sail argue that solar sails are impractical for orbital and interplanetary missions because they move on an indirect course. However, when they are in Earth orbit, most of the mass of most interplanetary missions' is fuel. A robotic solar sail could therefore multiply an interplanetary payload by several times, and create a reusable, multimission spacecraft. Most near-term planetary missions involve robotic exploration craft, in which the directness of the course is simply unimportant compared to the small fuel mass and fast transit times of a solar sail. For example, most existing missions use multiple gravitational slingshots to reduce fuel mass, because even though these are terribly indirect they save years of transit time.
Another false claim is that solar sails capture 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 that would be reflected from the sail.
Another common claim 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 2003. [2] (http://www.newscientist.com/news/news.jsp?id=ns99993895)
Investigated sail designs
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"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 guide 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, including visible light.
Current progress
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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 Academy of Science launched Cosmos 1 on June 21, 2005, from a submarine in the Barents Sea, but the Volna rocket failed, and the spacecraft failed to reach orbit. A solar sail would have been used to gradually raise the spacecraft to a higher earth orbit. The mission would have lasted for one month. A suborbital prototype test by the group failed in 2001 as well, also because of rocket failure.
Sail materials
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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. The sails of Cosmos 1 were made of Mylar.
In 2000, Energy Science Laboratories developed a new carbon fiber material (http://www.space.com/businesstechnology/technology/carbonsail_000302.html) which might be useful for solar sails. The material is over 200 times thicker than conventional solar sail designs, but it is so porous that it has the same weight. The ridigidty and durability of this material could make solar sails that are significantly better than plastic films. The material could self-deploy and should withstand higher temperatures.
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.
Microwave lasers could also be used to power a painted solar sail (http://www.space.com/businesstechnology/technology/technovel_sail_050211.html), a conventional sail coated with a layer of chemicals designed to evaporate when struck by microwave radiation. The momentum generated by this evaporation could significantly increase the thrust generated by solar sails.
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 conceivably be placed 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.
Solar sails in fiction
- The 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.
- In the movie Tron, a solar sailer is used.
- 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'.
- A solar sail appears in the Star Trek: Deep Space Nine episode "Explorers", as primary propulsion system of the "Bajoran solar-sail vessel".
- The Lady Who Sailed The Soul by Cordwainer Smith, a short story (part of The Rediscovery Of Man collection) describing journeys on solar sail craft.
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.
- Solar Sailing, Technology, Dynamics and Mission Applications - Colin R. McInnes presents the state of the art in his book.
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
- U3P (http://www.u3p.net/) Multilingual site with news and flight simulators!
- 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)
- How Solar Sail Technology Works (http://science.howstuffworks.com/solarsail.htm) from How Stuff Works
- Cosmos 1 official site (http://www.planetary.org/solarsail/)de:Sonnensegel