(Excerpted from Wikipedia http://en.wikipedia.org/wiki/Solar_sail)
Solar sails (also called light sails or photon sails) are a form of spacecraft propulsion using the radiation pressure (also called solar pressure) of a combination of light and high speed ejected gasses from a star to push large ultra-thin mirrors to high speeds. Light sails could also be driven by energy beams to extend their range of operations, which is strictly beam sailing rather than solar sailing.
Solar sail craft offer the possibility of low-cost operations combined with long operating lifetimes. Since they have few moving parts and use no propellant, they can potentially be used numerous times for delivery of payloads.
Solar sails use a phenomenon that has a proven, measured effect on spacecraft. Solar pressure affects all spacecraft, whether in interplanetary space or in orbit around a planet or small body. A typical spacecraft going to Mars, for example, will be displaced by more than 1,000 km by solar pressure, so the effects must be accounted for in trajectory planning, which has been done since the time of the earliest interplanetary spacecraft of the 1960s. Solar pressure also affects the attitude of a craft, a factor that must be included in spacecraft design.
The total force exerted on a solar sail may be around 1 newton or less, making it a low-thrust spacecraft, along with spacecraft propelled by electric engines.
History of concept
Johannes Kepler observed that comet tails point away from the Sun and suggested that the sun caused the effect. In a letter to Galileo in 1610, he wrote, “Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.” He might have had the comet tail phenomenon in mind when he wrote those words, although his publications on comet tails came several years later.
James Clerk Maxwell, in 1861-64, published his theory of electromagnetic fields and radiation, which shows that light has momentum and thus can exert pressure on objects. Maxwell’s equations provide the theoretical foundation for sailing with light pressure. So by 1864, the physics community and beyond knew sunlight carried momentum that would exert a pressure on objects.
Jules Verne, in From the Earth to the Moon, published in 1865, wrote “there will some day appear velocities far greater than these [of the planets and the projectile], of which light or electricity will probably be the mechanical agent…we shall one day travel to the moon, the planets, and the stars.” This is possibly the first published recognition that light could move ships through space. Given the date of his publication and the widespread, permanent distribution of his work, it appears that he should be regarded as the originator of the concept of space sailing by light pressure, although he did not develop the concept further. Verne probably got the idea directly and immediately from Maxwell’s 1864 theory (although it cannot be ruled out that Maxwell or an intermediary recognized the sailing potential and became the source for Verne).
Pyotr Lebedev was first to successfully demonstrate light pressure, which he did in 1899 with a torsional balance; Ernest Nichols and Gordon Hull conducted a similar independent experiment in 1901 using a Nichols radiometer.
Albert Einstein provided a different formalism by his recognizing the equivalence of mass and energy. We can now write simply p = E/c as the relationship between momentum, energy, and speed of light.
Svante Arrhenius predicted in 1908 the possibility of solar radiation pressure distributing life spores across interstellar distances, the concept of panspermia. He apparently was the first scientist to state that light could move objects between stars.
Friedrich Zander (Tsander) published a technical paper that included technical analysis of solar sailing. Zander wrote of “using tremendous mirrors of very thin sheets” and “using the pressure of sunlight to attain cosmic velocities”.
J.D. Bernal wrote in 1929, “A form of space sailing might be developed which used the repulsive effect of the sun’s rays instead of wind. A space vessel spreading its large, metallic wings, acres in extent, to the full, might be blown to the limit of Neptune’s orbit. Then, to increase its speed, it would tack, close-hauled, down the gravitational field, spreading full sail again as it rushed past the sun.”
Sailing operations are simplest in interplanetary orbits, where attitude changes are done at low rates. For outward bound trajectories, the sail force vector is oriented forward of the sun line, which increases orbital energy and angular momentum, resulting in the craft moving farther from the sun. For inward trajectories, the sail force vector is oriented behind the sun line, which decreases orbital energy and angular momentum, resulting in the craft moving in toward the sun. To change orbital inclination, the force vector is turned out of the plane of the velocity vector.
In orbits around planets or other bodies, the sail is oriented so that its force vector has a component along the velocity vector, either in the direction of motion for an outward spiral, or against the direction of motion for an inward spiral.
An active attitude control system (ACS) is essential for a sail craft to achieve and maintain a desired orientation. The required sail orientation changes slowly, often less than 1 degree per day, in interplanetary space, but much more rapidly in a planetary orbit. The ACS must be capable of meeting these orientation requirements.
Control is achieved by a relative shift between the craft’s center of pressure and its center of mass. This can be achieved with control vanes, movement of individual sails, movement of a control mass, or altering reflectivity.
Holding a constant attitude requires that the ACS maintain a net torque of zero on the craft. The total force and torque on a sail, or set of sails, is not constant along a trajectory. The force changes with solar distance and sail angle, which changes the billow in the sail and deflects some elements of the supporting structure, resulting in changes in the sail force and torque.
Sail temperature also changes with solar distance and sail angle, which changes sail dimensions. The radiant heat from the sail changes the temperature of the supporting structure. Both factors affect total force and torque.
The ACS must compensate for all of these changes for it to hold the desired attitude.
In Earth orbit, solar pressure and drag pressure are typically equal at an altitude of about 800 km, which means that a sail craft would have to operate above that altitude. Sail craft must operate in orbits where their turn rates are compatible with the orbits, which is generally a concern only for spinning disk configurations.
Sail operating temperatures are a function of solar distance, sail angle, reflectivity, and front and back emissivities. A sail can be used only where its temperature is kept within its material limits. Generally, a sail can be used rather close to the sun, around 0.25 AU, or even closer if carefully designed for those conditions.
Parachutes have very low mass, but a parachute is not a workable configuration for a solar sail. Analysis shows that a parachute configuration would collapse from the forces exerted by shroud lines, since radiation pressure does not behave like aerodynamic pressure, and would not act to keep the parachute open.
Eric Drexler proposed very high thrust-to-mass solar sails, and made prototypes of the sail material. His sail would use panels of thin aluminium film (30 to 100 nanometres thick) supported by a tensile structure. The sail would rotate and would have to be continually under thrust. He made and handled samples of the film in the laboratory, but the material was too delicate to survive folding, launch, and deployment. The design planned to rely on space-based production of the film panels, joining them to a deployable tension structure. Sails in this class would offer high area per unit mass and hence accelerations up to “fifty times 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 close to the Sun for maximum thrust. Most designs steer with small sails on the ends of the spars.
In the 1970s JPL studied many rotating blade and ring sails for a mission to rendezvous with Halley’s Comet. The intention was to stiffen the structures using angular momentum, eliminating the need for struts, and saving mass. In all cases, surprisingly large amounts of tensile strength 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. The difference in the thrust-to-mass ratio between practical designs was almost nil, and the static designs were easier to control.
JPL’s reference design was called the “heliogyro.” It had plastic-film blades deployed from rollers and held out by centrifugal 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 cyclic 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” (Spinning Disk Sail in the above diagram), 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. Masses 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 the gravity on the surface of 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.
Pekka Janhunen from FMI has invented a type of solar sail called the electric solar wind sail. Mechanically it has little in common with the traditional solar sail design. The sails are replaced with straightened conducting tethers (wires) placed radially around the host ship. The wires are electrically charged to create an electric field around the wires. The electric field extends a few tens of metres into the plasma of the surrounding solar wind. The solar electrons are reflected by the electric field (like the photons on a traditional solar sail). The radius of the sail is from the electric field rather than the actual wire itself, making the sail lighter. The craft can also be steered by regulating the electric charge of the wires. A practical electric sail would have 50-100 straightened wires with a length of about 20 km each.
A magnetic sail would also employ the solar wind. However, the magnetic field deflects the electrically charged particles in the wind. It uses wire loops, and runs a static current through them instead of applying a static voltage.
All these designs maneuver, though the mechanisms are different. Magnetic sails bend the path of the charged protons that are in the solar wind. By changing the sails’ attitudes, and the size of the magnetic fields, they can change the amount and direction of the thrust. Electric solar wind sails can adjust their electrostatic fields and sail attitudes.
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 aluminized PET film (Mylar).
Research by Dr. Geoffrey Landis in 1998-9, funded by the NASA Institute for Advanced Concepts, showed that various materials such as alumina for laser lightsails and carbon fiber for microwave pushed lightsails were superior sail materials to the previously standard aluminium or Kapton films.
In 2000, Energy Science Laboratories developed a new carbon fiber material 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 mass. The rigidity and durability of this material could make solar sails that are significantly sturdier than plastic films. The material could self-deploy and should withstand higher temperatures.
There has been some theoretical speculation about using molecular manufacturing techniques to create advanced, strong, hyper-light sail material, based on nanotube mesh weaves, where the weave “spaces” are less than half the wavelength of light impinging on the sail. While such materials have so far only been produced in laboratory conditions, and the means for manufacturing such material on an industrial scale are not yet available, such materials could mass less than 0.1 g/m², making them lighter than any current sail material by a factor of at least 30. For comparison, 5 micrometre thick Mylar sail material mass 7 g/m², aluminized Kapton films have a mass as much as 12 g/m², and Energy Science Laboratories’ new carbon fiber material masses 3 g/m².
- ^ R.M. Georgevic (1973) “The Solar Radiation Pressure Forces and Torques Model”, The Journal of the Astronautical Sciences, Vol. 27, No. 1, Jan-Feb. First known publication describing how solar radiation pressure creates forces and torques that affect spacecraft.
- ^ a b c d Jerome Wright (1992), Space Sailing, Gordon and Breach Science Publishers
- ^ Johannes Kepler (1604) Ad vitellionem parali pomena , Frankfort; ( 1619) De cometis liballi tres , Augsburg
- ^ Jules Verne (1865) De la Terre à la Lune (From the Earth to the Moon)
- ^ P. Lebedev, 1901, “Untersuchungen über die Druckkräfte des Lichtes”, Annalen der Physik, 1901
- ^ Lee, Dillon (2008). “A Celebration of the Legacy of Physics at Dartmouth”. Dartmouth Undergraduate Journal of Science. Dartmouth College. Retrieved 2009-06-11.
- ^ Svante Arrhenius (1908) Worlds in the Making
- ^ Friedrich Zander‘s 1925 paper, “Problems of flight by jet propulsion: interplanetary flights,” was translated by NASA. See NASA Technical Translation F-147 (1964)
- ^ J.D. Bernal (1929) The World, the Flesh & the Devil: An Enquiry into the Future of the Three Enemies of the Rational Soul
- ^  http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/relmom.html
- ^ Wright, ibid, Appendix A
- ^ Wright, ibid., Appendix A
- ^ McInnes, C.R. and Brown, J.C. (1989) Solar Sail Dynamics with an Extended Source of Radiation Pressure, International Astronautical Federation, IAF-89-350, October.
- ^ Wright, ibid, Appendix B.
- ^  http://www.swpc.noaa.gov/SWN/index.html
- ^ Wright, ibid., Ch 6 and Appendix B.
- ^ “MESSENGER Sails on Sun’s Fire for Second Flyby of Mercury”. 2008-09-05. “On September 4, the MESSENGER team announced that it would not need to implement a scheduled maneuver to adjust the probe’s trajectory. This is the fourth time this year that such a maneuver has been called off. The reason? A recently implemented navigational technique that makes use of solar-radiation pressure (SRP) to guide the probe has been extremely successful at maintaining MESSENGER on a trajectory that will carry it over the cratered surface of Mercury for a second time on October 6.”
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- ^ Mautner, Michael N. (1995). “Directed panspermia. 2. Technological advances toward seeding other solar systems, and the foundations of panbiotic ethics”. Journal of the British Interplanetary Society 48: 435–440.
- ^ Wright, ibid., p71, last para
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- ^ Zubrin & Andrew’s presentation in a pdf.
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- ^ “Cosmos 1 – Solar Sail (2004) Japanese Researchers Successfully Test Unfurling of Solar Sail on Rocket Flight”. 2004.
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- ^ a b “LightSail Mission FAQ”. The Planetary Society. Retrieved 18 May 2012.
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- ^ “NASA Chat: First Solar Sail Deploys in Low-Earth Orbit”. NASA. 2011-01-27. Retrieved 18 May 2012. “Sometimes the satellite is called NanoSail-D and sometimes NanoSail-D2. … Dean: The project is just NanoSail-D. NanoSail-D2 is the serial #2 version.”
- ^ Nasa report on mission
- ^ Nasa report on mission
- ^ “Nasa Solar Sail Demonstration”. www.nasa.gov.
- ^ “IKAROS Project|JAXA Space Exploration Center”. Jspec.jaxa.jp. 2010-05-21. Retrieved 2011-01-18.
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- ^ http://240plan.ovh.net/~upngmmxw/projets/doc/HoustonU3P.pdf
- ^Solar sail at Memory Alpha (a Star Trek wiki)