In 2008, when my novel The Solar Sea was released, I gave a presentation about solar sails at several science fiction conventions. I summarized that information in an article called “Sailing the Winds of Space” which appeared in Strange Weird and Wonderful Magazine online. The issue no longer appears to be available and I thought it was worth updating some of the information. I’ll be presenting the article over the next three weeks here at the Web Journal. I hope you enjoy this look at solar sailing!
Pottery depicting masted ships.
Photo by Einsamer Schütze. License: CC BY-SA
Sailing is one of the oldest transportation technologies. The simple use of wind to propel a craft across the water goes back to at least 3500 BC when the first representation of a ship under sail appeared on an Egyptian vase. Scientists around the world have explored ways to adapt this ancient technology for use in space flight. Sail technology is a way to make space flight more cost effective. Not only that, it’s possible that ancient sailing technology could propel a craft to near the speed of light. Sails for spacecraft would utilize light instead of wind. Such a sail is known as a light sail—or a solar sail if the primary light source is the Sun. This simple but powerful technology has also been an inspiration to numerous science fiction writers over the last fifty years.
Photons—individual particles of light—have momentum. When something with momentum strikes another object, it imparts some of its momentum to that object. Think of what happens when you play pool and the cue ball strikes another ball. The cue ball bounces off and the other ball moves in some direction. If you shine light at a mirror, the light bounces off, but it also imparts some momentum to the mirror. The reason you don’t see mirrors moving every time you shine light on them is that here on Earth, air pressure and gravity overwhelm light pressure. Light pressure from the Sun at Earth only produces about two pounds of force for every square kilometer. However, in space, where gravity is significantly less than it is on the ground and air pressure is no longer a factor, even a gentle force such as light pressure becomes significant.
Photo by No-w-ay in collaboration with H. Caps. License: CC BY-SA
Even though gravity and air pressure are no longer factors in space, solar sails must be built out of very lightweight material. Think about the billiard balls, but imagine replacing the cue ball with a marble. If you shoot a marble at a billiard ball, the marble will still impart momentum to the billiard ball, but it will take a lot more effort for the marble to move the billiard ball. By the same token, solar sails must be built out of the lightest possible material so that photon pressure will have the greatest effect when propelling the spacecraft. The materials currently being investigated for solar sails are somewhere between 40 and 100 times thinner than a piece of writing paper.
Fuel is one of the greatest costs in contemporary space flight and spacecraft must be designed to start their journey with all of the fuel they will need for the duration of the voyage. This is a significant engineering challenge. However, in the solar system, the sun produces an abundant, steady stream of photons that could be harnessed by a spacecraft. As such, light sails become a very attractive means of space propulsion.
Because light pressure is a very gentle force, solar sails would accelerate very slowly. However, as long as there is a supply of light, there is nothing that will stop the acceleration of a solar sail. Theoretically, a solar sail will continue to accelerate until it reaches the same speed as the particles striking it—the speed of light. According to Newton’s first law, an object in motion will remain in motion unless some force acts upon it. In space, where there is no friction, once a solar sail reaches the speed of light, it could continue at the speed of light. As such, light sails could theoretically be used for interstellar travel.
A common misconception is that solar sails are propelled by the solar wind. The solar wind is a stream of charged particles emitted from the sun. This stream of charged particles will transfer some momentum to any object it strikes, just as photons will. However, photon pressure from the sun is about 5000 times greater than the force from the solar wind.
In the next installment, I’ll discuss some actual solar sail projects.
My novel The Solar Sea, which imagines a voyage aboard a solar sail spacecraft, is available at: