MadSci Network: Astronomy

Re: Please explain the physics of how solar sails work.

Date: Thu Nov 4 07:04:49 1999
Posted By: Michael Martin-Smith, Other (pls. specify below), Family Physician, Fellow,BIS, amateur astronomer( BAA), British Interplanetary Society
Area of science: Astronomy
ID: 941573618.As

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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