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2 Solar Sailing: Applications and Technology Advancement Malcolm Macdonald Advanced Space Concepts Laboratory University of Strathclyde, Glasgow Scotland, E.U. 1. Introduction Harnessing the power of the Sun to propel a spacecraft may appear somewhat ambitious and the observation that light exerts a force contradicts everyday experiences. However, it is an accepted phenomenon that the quantum packets of energy which compose Sunlight, that is to say photons, perturb the orbit attitude of spacecraft through conservation of momentum; this perturbation is known as solar radiation pressure (SRP). To be exact, the momentum of the electromagnetic energy from the Sun pushes the spacecraft and from Newton’s second law momentum is transferred when the energy strikes and when it is reflected. The concept of solar sailing is thus the use of these quantum packets of energy, i.e. SRP, to propel a spacecraft, potentially providing a continuous acceleration limited only by the lifetime of the sail materials in the space environment. The momentum carried by individual photons is extremely small; at best a solar sail will experience 9 N of force per square kilometre of sail located in Earth orbit (McInnes, 1999), thus to provide a suitably large momentum transfer the sail is required to have a large surface area while maintaining as low a mass as possible. Adding the impulse due to incident and reflected photons it is found that the idealised thrust vector is directed normal to the surface of the sail, hence by controlling the orientation of the sail relative to the Sun orbital angular momentum can be gained or reduced. Using momentum change through reflecting such quantum packets of energy the sail slowly but continuously accelerates to accomplish a wide-range of potential missions. 1.1 An historical perspective In 1873 James Clerk Maxwell predicted the existence of radiation pressure as a consequence of his unified theory of electromagnetic radiation (Maxwell, 1873). Apparently independent of Maxwell, in 1876 Bartoli demonstrated the existence of radiation pressure as a consequence of the second law of thermodynamics. The first experimental verification of the existence of radiation pressure and the verification of Maxwell's results came in 1900. At the University of Moscow, Peter Lebedew succeeded in isolating radiation pressure using a series of torsion balance experiments (Lebedew, 1902). Nichols and Hull at Dartmouth College, New Hampshire, obtained independent verification in 1901 (Nichols & Hull, 1901, 1903). Around this period a number of science fiction authors wrote of spaceships propelled by mirrors, notably the French authors Faure and Graffigny in 1889. However, it was not until the early 20 th century that the idea of a
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Advances in Spacecraft Technologies 36 solar sail was accurately articulated. Solar sailing as an engineering principle can be traced back to the Father of Astronautics, Cio ł kowski (translated as Tsiolkovsky) and Canders (translated as Zander or Tsander) (Cio ł kowski, 1936; Tsander, 1924). There is some
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