Riding a Beam of Light

Abstract

The single most important characteristic of a solar sail is its power source—the Sun. The Sun supplies a continuous source of sunlight, providing the gentle push that makes a solar sail such a useful propulsion system. Unfortunately, the Sun is also the limiting factor in the overall usefulness of a solar sail. When a spacecraft gets far from the Sun, there is simply not enough light available to provide additional propulsion. Recall the “inverse square law” discussed previously. In deep space, the Sun is essentially a point source, with sunlight radiating away from it in all directions forming an ever-expanding sphere of light. Since the total amount of light from the Sun is the same when the expanding light sphere reaches the orbit of Mercury, Venus or Earth, we are not “losing” sunlight. What we are doing, however, is reducing its intensity. The amount of sunlight may be the same, but the surface area of the sphere is much larger the farther you get from the Sun. The only way that the amount of sunlight can remain constant (which we intuitively know it must), yet cover a much larger area, is for the amount of sunlight per unit area to decrease. And decrease it does; as the distance from the Sun doubles, the amount of sunlight falling on a 1 m² area on that sphere drops to one fourth of its previous value. The distance is doubled, and the amount of light is reduced by a factor of four. Since 4 = 2², this predictable decline in sunlight is governed by the inverse square law and holds true no matter how far away from the Sun the sphere of light travels. If you measure the total amount of light falling on a 1 m² area of sail and then quadruple the distance, the amount of sunlight falling on that same sail drops to 1/16 of its previous value: 4² = 16. As we move away from the Sun, the push our sailcraft receives drops rapidly.

Further Reading

Principles of beamed propulsion are reviewed in E. Mallove and G. Matloff, The Starflight Handbook, Wiley, NY, 1989. For a more up-to-date technical treatment and review, see G. L. Matloff, Deep-Space Probes, 2nd ed., Springer-Praxis, Chichester, UK, 2005.

AIP Conference Proceedings 664: First International symposium on Beamed-energy Propulsion, May 2003, American Institute of Physics.