Going Interstellar (collection)

Imagine a large, very thin, lightweight and very highly reflective sail deployed in space for the sole purpose of reflecting sunlight. We’ve just imagined a solar sail and they are far from imaginary. Solar sails reflect sunlight, transferring the tiny momentum of each reflected photon to the sail, causing the sail to move. The force is tiny. At the Earth’s distance from the Sun (ninety-three million miles), the force from sunlight is about five pounds per square mile. In other words, we’d have to have a sail area of one square mile to feel five pounds of force. For comparison, just one of the Space Shuttle’s main engines produces about five hundred thousand pounds of thrust. The primary difference is that the shuttle’s engines can only produce this thrust for a very short period of time before running out of fuel while a solar sail can produce thrust as long as it remains in sunlight. And since the distances involved in space travel are so large, the sail will remain in sunlight for a very long time no matter its destination.

In this case, the space shuttle engine is the hare and the solar sail is the tortoise. Chemical rockets will never take us to the stars, but solar sails might. It is important to note that while solar sails may one day take us to Alpha Centauri, they will never get us off the surface of the Earth. To lift from the surface of the Earth, we need a propulsion system that can produce more thrust than the rocket weighs. Chemical rockets are capable of producing these high thrust levels; solar sails cannot.

Before we start building our solar sail-propelled starship, we need to discuss a few more critical issues that will affect our design. First of all, the sail will still be subject to Newton’s Second Law, which states, “a body of mass (m) subject to a force (F) undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass.” In other words, to get a mass to accelerate, we need to apply a force. In order to get the accelerations needed to achieve very high speeds, such as those required for interstellar travel, we need a large force or a small mass, or in this case, we need both.

Newton’s Second Law requires our solar sail design to be very large so the sail can capture as much sunlight as possible in order maximize solar photon thrust. It also requires us to use very lightweight materials so that we can make our ship as low mass as possible. The sail must also be highly reflective so that we can capture as much momentum from each photon as possible.

Is there anything we can do to increase the force acting on the sail from the sunlight? Even though we have the benefit of time, five pounds of thrust per square mile is ridiculously small. We would require a sail almost one hundred thousand square miles in area to equal the thrust produced by one space shuttle engine. Such a sail would have roughly the same surface area as Alabama and Mississippi combined! Surely we can do something to increase our thrust so that we can make a smaller sail.

It turns out that another interesting fact about sunlight allows us to do just this. We can dramatically increase the force acting on the solar sail by flying closer to the Sun thanks to a property of sunlight called The Inverse Square Law. According to this law, if we move an object twice the distance from the light source, it will receive only one quarter of the illumination. Two times the distance (2) means one-fourth (¼) the illumination—two squared is four. If we move out to four times the distance from the Sun, the illumination drops to one sixteenth of the previous amount—four squared is sixteen. Less illumination translates directly into less force. Fortunately, we can use this geometric property to our benefit by moving closer to the Sun. If we reduce the distance to ½ its previous value, we get four (4) times the force. If we reduce it to one fourth, then we get sixteen times the force. And if we get sixteen times the force per square mile, then we can reduce the overall surface area of the sail by the same factor. And when we are talking about sails the size of US states, a factor of sixteen is significant.

This all sounds great, but are solar sails real? Have they been built and tested in space? Has anyone actually used one for sending a spacecraft anywhere? Yes, yes, and yes!