SOLAR AND BEAMED ENERGY SAILS
Les Johnson
Les Johnson’s concern for the human future is readily apparent. He has written widely about the subject, suggesting high-tech methods for preserving the environment while we solve the global energy problem, especially in his collaboration with fellow physicist Gregory Matloff and one-time NASA artist C Bangs to produce Paradise Regained ( 2010). The same trio gave us a survival handbook to take along when we begin our conquest of space, Living off the Land in Space (2007). With Matloff and Giovanni Vulpetti, Johnson suggests in Solar Sails (2008) a novel method for expanding through and possibly beyond the solar system. He has also collaborated with physicist Travis Taylor to write Back to the Moon, a novel in which the United States’ return visit, engineered by a future NASA, becomes a desperate rescue effort for a stranded Chinese mission.
When we speculate about traveling to the stars, we tend to think in terms of giant nuclear-powered rocket engines. And, admittedly, if we succeed in making the journey, they may indeed prove to be the key. But not necessarily. In fact, Johnson suggests that softer power may be the ultimate answer. Sailing vessels showed the way for early exploration on the world’s oceans. Their days may not be over.
Johnson is a physicist, the Deputy Manager for the Advanced Concepts Office at the NASA George C. Marshall Space Flight Center in Huntsville, Alabama,. and one of the editors of this book.
We can’t feel it, but the light from the Sun is pushing on us. It’s a small push, less than an ounce per square football field. Whenever we are in sunlight, or any light, we are being pushed. This solar pressure is much smaller than the other forces we experience in our everyday lives. The force of the wind from the room air conditioner vent is far stronger than the force we experience in full sunlight. It is so small that very sensitive instruments are required to measure it. And it can only be measured in a vacuum because the various forces around us will otherwise swamp the effect. But solar pressure is real, it is constant, and it can be used to propel a spacecraft to incredible speeds.
About four hundred years ago, Johannes Kepler observed that the tail of a comet appeared to be created by some sort of cosmic breeze and postulated that this breeze could be used to move ships in space in a manner similar to which the sailing ships of his day were propelled by wind. While Kepler was wrong about the nature of these cosmic winds, he was correct in his observation that something coming from the Sun, which we now know is sunlight itself, can be used to move a spacecraft.
An earthly sail moves a ship by transferring the momentum of the wind to the ship by reflecting it from a sail. The force exerted on the sail pushes the ship, causing it to move. In physics, momentum is defined as the product of mass times velocity. Lots and lots of air molecules, each having mass and some velocity, reflect from a sail and transfer their momentum to it. The ship then begins to move, its momentum coming from the wind.
In 1923, the physicist Arthur Compton observed that photons (particles of light) have momentum even though they have no rest mass. In other words, these massless particles that we call light have momentum even though they would have no mass if we could catch one and slow it down to weigh it. This is yet another weird property of light—but one that will be very useful for taking us to the stars.
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!