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Gone with the Solar Wind

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As a human race we have come a long way from setting sail into the ocean to explore other continents to inventing machines than can fly around the world. We have come far from creating the wheel and manipulating its use to help us travel large distances without expending too much of our own physical energy. And yes, we have come a very long way from creating a machine that can fly in the sky and look as free as the birds. The next generation of technology is here… behold… the Solar Sail. This neat piece of technology will eventually keep satellites on the poles of the Earth in orbit by the use of solar sails. Well… what is a solar sail? According to Wikipedia, “Solar sails are a proposed form of spacecraft propulsion using large membrane mirrors.” One can think of a solar sail as a really big mirror which is connected to a space-craft. This is the same idea as a ship in the ocean with its big sails open being propelled by the wind. But in space the only form of wind is the solar wind; which really is the force of light acting on the mirror. The average pressure exerted by the sun at 1 AU (1 AU = 1.5E11 meters) is roughly 1E-5 Pascals. This amount is very small but as long as there is a constant light source that exerts pressure on the sail it will keep on accelerating.

Scientists at the Jet Propulsion Laboratory (JPL) are currently researching the potential for this type of technology. We would have to create our own source of radiation pressure which we need to exert on the sail to have any significant results or undertake any worthwhile interplanetary missions. JPL scientists envision conducting interstellar expeditions and a topic of hot discussion is a trip to Alpha Centauri which is roughly four and a half light years away from us. The idea is such that we have to create a laser that exerts a significantly large amount of radiation pressure on the solar sail; something of the order of magnitude of about 10,000 suns!!! This is a huge amount of energy and this will have to be used from an orbiting satellite. So, once this laser is mounted on the orbiting satellite it would focus a beam onto the solar sail in the direction of Alpha Centauri and would reach a speed of around 3E7 meters per second; this fraction is about 10% the speed of light. This is quite fascinating since it would only take the space-craft with the solar sail between 40 and 50 years to reach its destination. But this is all theoretical at the moment but definitely not fiction! On the other hand if we could create a beam that is powerful enough to propel a space-craft with a large enough solar sail; we can probe our solar system with a lot more efficiency since these crafts would reach very high velocities.

All of this sounds very intriguing but let’s get a taste of reality. Let’s forget our crazy powerful laser beams and imagine that the only constant and consistent source of energy the solar sail can get its energy from is the sun. Consider a solar sail that has an area of 10,000 m2 and the radiation pressure being received from the sun is roughly 700 watts/m2. Now it is quite easy to figure out the power that is being exerted onto the sail; we just multiply both the quantities to get a power of 7 million watts!!! But how does this power translate into the force exerted but the photons? That is easy as well, we just divide this power by the speed of light and we get an answer of 0.0233 Newton’s!!! This is a very tiny force being exerted by light at a given radius from the sun so we can see that we would need a very large solar sail to get any kind of significant force from the photons that are being emitted by the sun.

The idea of interstellar exploration definitely sounds like a science fiction movie but at the pace at which science is moving today it might just become a reality sooner than we think. On the other hand, exploring our solar system with this technology seems to be the first step for the use of the solar sail. As humans we have come a very long way from fantasying about stellar explorations and finally a new frontier has appeared before us… behold… The Solar Sail.