Riding a Beam of Light

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.