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Sailing with Solar Propulsion

Physics Continued

Cosmos 1
More on Sailing
The Physics of Solar Sailing
Physics Continued
Pictures and info
Some Controversy
Solar Sail Designs

What Pushes a Solar Sail
  • Light is composed of electromagnetic radiation that exerts force on objects it comes in contact with.
  • James Clerk Maxwell developed the laws describing electromagnetism and concluded that light is an electromagnetic wave.
  • Maxwell predicted that when light hits an object and is absorbed or reflected, the light wave pushes on the electrons in the surface of the object, which in turn push on the rest of the object.
  • When Einstein developed his theories of relativity, and gave us the equation E=mc2, it allowed us to calculate light pressure a lot easier.  E=mc2 compares energy which can easily be measured in light, to mass and movement which can easily be used to find forces.
  • E=mc2 is the power of the sunlight divided by the speed of light.
  • You get twice as much force from an object that reflects all the light as you do from an object that absorbs all the light.
  • Force equals power divided by speed of light, the steps taken by Maxwell and others had to be taken first.      


Solar sails will set new speed records for spacecraft and will enable us to travel beyond our solar system.

The Concept of Solar Sails

Nearly 400 years ago, as much of Europe was still involved in naval exploration of the world, Johannes Kepler proposed the idea of exploring the galaxy using sails. Through his observation that comet tails were blown around by some kind of solar breeze, he believed sails could capture that wind to propel spacecraft the way winds moved ships on the oceans. While Kepler's idea of a solar wind has been disproven, scientists have since discovered that sunlight does exert enough force to move objects. To take advantage of this force, NASA has been experimenting with giant solar sails that could be pushed through the cosmos by light. There are three components to a solar sail-powered spacecraft:

  • Continuous force exerted by sunlight
  • A large, ultrathin mirror
  • A separate launch vehicle


The Concept of Solar Sails - Continued

A solar sail-powered spacecraft wouldn't need traditional propellant for power, because its propellant would be sunlight and the sun would be its engine. Light is composed of electromagnetic radiation that exerts force on objects it comes in contact with. NASA researchers have found that at 1 astronomical unit (AU), which is the distance from the sun to Earth, equal to 93 million miles (150 million km), sunlight can produce about 1.4 kilowatts (kw) of power. If you take 1.4 kw and divide it by the speed of light, you would find that the force exerted by the sun is about 9 newtons (N)/square mile (i.e., 2 lb/km2 or .78 lb/mi2). In comparison, a space shuttle main engine can produce 1.67 million N of force during liftoff and 2.1 million N of thrust in a vacuum. Eventually, however, the continuous force of the sunlight on a solar sail could propel a spacecraft to speeds five times faster than traditional rockets.

With just sunlight as power, a solar sail would never be launched from the ground. More likely, a second spacecraft would launch the solar sail, which would then be deployed in space. However, another possible way to launch a solar sail would be with microwave or laser beams provided by a satellite or other spacecraft. These energy beams could be directed at the sail to launch it into space and provide a secondary power source during its journey. In a recent experiment at NASA's Jet Propulsion Laboratory, sails were driven to liftoff using microwave beams, while laser beams were used to push the sail forward.

As we found out in the last section, solar sails would not initially be driven by the amount of force that is used to launch the space shuttle. NASA believes that the exploration of space is similar to the tale of the "Tortoise and the Hare," with rocket-propelled spacecraft being the hare. In this race, the rocket-propelled spacecraft will quickly jump out, moving quickly toward its destination. On the other hand, a rocketless spacecraft powered by a solar sail would begin its journey at a slow but steady pace, gradually picking up speed as the sun continues to exert force upon it. Sooner or later, no matter how fast it goes, the rocket ship will run out of power. In contrast, the solar sail craft has an endless supply of power from the sun. Additionally, the solar sail could potentially return to Earth, whereas the rocket powered vehicle would not have any propellant to bring it back.

As it continues to be pushed by sunlight, the solar sail-propelled vehicle will build up speeds that rocket powered vehicles would never be able to achieve. Such a vehicle would eventually travel at about 56 mi/sec (90 km/sec), which would be more than 200,000 mph (324,000 kph). That speed is about 10 times faster than the space shuttle's orbital speed of 5 mi/sec (8 km/sec). To give you an idea how fast that is, you could travel from New York to Los Angeles in less than a minute with a solar sail vehicle traveling at top speed.

If NASA were to launch an interstellar probe powered by solar sails, it would take only eight years for it to catch the Voyager 1 spacecraft (the most distant spacecraft from Earth), which has been traveling for more than 20 years. By adding a laser or magnetic beam transmitter, NASA said it could push speeds to 18,600 mi/sec (30,000 km/sec), which is one-tenth the speed of light. At those speeds, interstellar travel would be an almost certainty.