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Solar sails (also called light sails or photon sails) are a form of spacecraft propulsion using the radiation pressure (also called solar pressure) from stars 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 thousands of kilometres 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.〔Georgevic, R. M. (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.〕 The total force exerted on an 800 by 800 meter solar sail, for example, is about at Earth's distance from the Sun, making it a low-thrust propulsion system, similar to 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.〔Johannes Kepler (1604) ''Ad vitellionem parali pomena'', Frankfort; (1619) ''De cometis liballi tres '', Augsburg〕 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 (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).〔Jules Verne (1865) ''De la Terre à la Lune'' (''From the Earth to the Moon'')〕 Pyotr Lebedev was first to successfully demonstrate light pressure, which he did in 1899 with a torsional balance;〔P. Lebedev, 1901, "Untersuchungen über die Druckkräfte des Lichtes", ''Annalen der Physik'', 1901〕 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. He simply wrote p = E/c as the relationship between the momentum, the energy, and the 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.〔Svante Arrhenius (1908) ''Worlds in the Making''〕 Friedrich Zander (Tsander) published a technical paper in 1925 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".〔Friedrich Zander's 1925 paper, "Problems of flight by jet propulsion: interplanetary flights", was translated by NASA. See NASA Technical Translation F-147 (1964)〕 JBS Haldane speculated in 1927 about the invention of tubular spaceships that would take humanity to space and how "wings of metallic foil of a square kilometre or more in area are spread out to catch the Sun's radiation pressure".〔JBS Haldane, ''The Last Judgement'', New York and London, Harper & Brothers, 1927.〕 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."〔J. D. Bernal (1929) ''The World, the Flesh & the Devil: An Enquiry into the Future of the Three Enemies of the Rational Soul''〕 The first formal technology and design effort for a solar sail began in 1976 at Jet Propulsion Laboratory for a proposed mission to rendezvous with Halley's Comet.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Solar sail」の詳細全文を読む スポンサード リンク
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