How Things Work: Solar sails
The Starship Enterprise of Star Trek fame has been sailing through the Final Frontier for years. Today it is hard to imagine a source of enough energy to keep a ship operating at warp drive so effortlessly. Living in the age of fossil fuels severely restricts our attempts at boldly going out into space. There have been numerous proposals of alternate propulsion schemes over the years, but the most notable of the various ideas is that of the solar sail.
Solar sails use light to propel spacecraft. Light is made up of a number of particles called photons. Each of these photons has an associated momentum. Just as two colliding marbles exchange momentum, a photon is capable of transferring some of its momentum to any object it may collide with. This transfer of momentum is maximized when photons bounce off the body, in a well-known phenomenon called reflection.
A single photon, or even a large stream of photons, produces no perceptible acceleration to a body of a macroscopic size on Earth. This is why we do not feel the constant bombardment of photons from every light source around us. In the vacuum of space, however, there is no resistance offered by gases, and while still not perceptibly larger, there is an increase in the amount of force photons exert on a body. Naturally, a body with a large surface area for the photons to bounce off will experience a larger force. Furthermore, if the body is lighter, according to Newton’s Second Law, it will be accelerated faster. This is the basic concept behind the use of solar sails.
Solar sails are sheets of mirror-like materials that are made such that they are extremely light and have as large a surface area as is permissible by their weight constraints. These “sails” are attached to spacecraft and are allowed to rotate so as to reflect as much sunlight as possible (as sunlight is, again, just streams of photons). The ship with the solar sails is steered by orienting the sail so as to reflect the incident light. The ship will always accelerate only perpendicularly to the vanes of the sails, and this provides a convenient means of steering the craft.
Conventional rockets carry more fuel for propulsion than the intended payload. This large amount of fuel does not last long enough to propel a spacecraft to the farthest ends of our solar system. Though Voyager 1 did manage to travel out as far as the Kuiper Belt (an asteroid belt at the edge of our solar system), it did so using the gravity of the planets it passed to propel it (a technique known as swing-by) and did not rely solely on fuel.
Today, the use of the solar sail seems the most promising means to sustain a voyage in space indefinitely. Calculations by NASA have shown that solar sails, if made light and large enough, can accelerate a spacecraft many times faster than conventional rockets. While the acceleration is small, over time, these sails can cause the craft to travel at speeds that would allow it to travel to the edge of our solar system in around 5 years (compared to the 12 years it took Voyager 1). As a result of such studies, there have been a number of proposals for materials for the solar sails. These proposed materials should also be heat resistant, as they will be continually exposed to light. The materials may be complex chemical compounds such as Kapton or Mylar or might even be sheets of aluminum-coated carbon fiber. Energy Science Laboratories have created a material that, while thicker than standard sails, is manufactured so as to be porous. This reduces the weight of the sail to around the same as that of standard solar sails while making them easier to produce.
One problem encountered with solar sails is that they cannot be assembled here on Earth and then shot into space. The sails are fragile and are highly susceptible to damage in Earth’s atmosphere. Some materials are so fragile they cannot be unfolded by any method known to us today without tearing the sails. The sails therefore have to be carefully unfolded or assembled in space before they can begin operating as intended.
NASA is currently working on a number of projects related to materials for the solar sail, including some to make them sturdier. In less than a year, Louis Friedman and his team of scientists at the Planetary Society will launch LightSail 1, one of the first light-driven spacecraft ever. This ship will carry almost no payload but is meant for testing purposes, following which, over the next three years, more flights will be scheduled. The ultimate aim of the project is to send LightSail into deep space using no fuel. In three years, it should become clear how viable a solution solar sails are to the problem of interstellar propulsion. By then, we should also feel less envious of Captain James Kirk; only a little less envious, though, as we still have to work on the transporter and the phaser.