|MadSci Network: Astronomy|
Sunlight has two methods of generating propulsion- the first is light itself, or photons, each of which, although massless, carries energy by virtue of being a quantum particle. Each photon carries a definite momentum related to its speed, frequency, and Planck's constant - an excessively small number relating to the quanta of energy carried by electrons as they are excited from one energy level in their atomic orbits to another. This energy is carried away by photons with a small but definite momentum which they impart to a solar sail on impact. Such sails must be very light and very extended in area to be useful but films of less than a micron thick covering hundreds of square metres are in principle capable of being manufactured and unfurled from spacecraft payloads, which could utilise solar photons (light) for propulsion. The ejection of charged particles (the solar wind) at 400-800 km per second offers another small source of imparted momentum for a light sail- although the charged particles can also, depending on material and construction, inflict some damage to the sail. The tails produced when comets approach the Sun are the result of this solar wind on the dust ejected by heated cometary material. Light sails accelerate slowly over weeks but a spacecraft could be accelerated over time to a very high speed over a spiralling course away from the sun, and the energy is essentially free! Michael Martin-Smith Ref: "Man and the Planets", Duncan Lunan, Ashgrove Press, Bath, 1983 ISBN 0906798175
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