Solar Sails

A proposed means of interplanetary or even interstellar propulsion. A large, low-mass surface is deployed in space. In addition to the flood of photons we know and bask in, the sun produces a 'solar wind', which is a stream of energetic particles flowing outwards from the sun. A large enough surface would absorb enough energy from the Sun's EM emissions and this flow to produce acceleration. Since there is no 'air resistance' in space, such a'sail' would just keep on going forever (theoretically) unless damaged, although the farther it gets from its energy source the slower the rate of acceleration. Some concepts for deep-space travel include the use of large lasers or masers on Earth or on a fixed base in the Solar System to provide those sails with an extra 'boost.'

A solar sail is an interplanetary drive.

It consists of a large area (typically hundreds of square meters) of very lightweight material that is held at an adjustable angle to the suns rays, and by reflecting the light, obtains thrust.

The thrust obtained this way is incredibly low, and solar sails would have to spend many months or years travelling, but the main advantage is that they use no propellent at all.

They are only suitable for use when already in orbit, very much away from any atmosphere.

Dr. Robert L. Forward has shown that much larger solar sails, on the order of a thousand square kilometers, might be used for effective interstellar travel. Away from a star, of course, there is no light to provide pressure against the solar sail. Engineers in the star system of origin could construct an array of lasers, a beam combiner and a giant Fresnel lens to project an incredibly powerful beam of collimated (laser) light. The light, when it hit the solar sail, would provide much more acceleration than normal sunlight at any distance. As the starship drew further away from the system of origin, the engineers back home would need to build more lasers and make the beam more powerful. Leaving your propulsion system at home like this is a really neat idea, since people on Earth have infinitely more resources than the people aboard the spacecraft and can tinker with the lasers, making use of technological improvements. Used in this way a solar sail might be better known as a lightsail, but either terminology is accepted.

Surprisingly enough, a solar sail used this way can also provide the means for deceleration. As the spacecraft approaches its destination, the habitation module located at the center of the solar sail detaches from the bulk of the lightsail and fractionally decelerates using rockets. The larger sail structure moves ahead of the habitation module and its smaller braking sail, and reflects laser light back onto the leading surface of the spacecraft's braking sail, slowing it down. Once the habitation module is in system, it uses the braking sail as a standard solar sail to navigate through the destination system.

                            main sail
             braking sail  /|
                        | / |
 huge beam of         Io|/       <--- Reflected light pushes back on
 laser light          Io|\            spacecraft to decelerate it
                        | \ |

In addition to being a consulting physicist, Dr. Forward is also a successful hard science-fiction writer. The bulk of his solar sail work is summarized in a lengthy but approachable appendix to his science-fiction novel Rocheworld, in which a solar sail starship crewed by colorful characters travels to the Barnard star system. The book is an excellent read, and is also a good illustration of the possibilities of solar sail propulsion.

For the more technically inclined, Forward has published an article on lightsail propulsion for interstellar probes in Jane's Spaceraft, the text of which is freely available at

"We have lingered long enough on the shores of the cosmic ocean.
We are ready at last to set sail for the stars."
-Carl Sagan

Inspirations for setting sail for the stars in science fiction goes back at least as far as Cordwainer Smith's The Lady who Sailed the Soul published in 1960. Arthur C. Clarke popularized the idea four years later in his short story Sunjammer, since reprinted in 1972 under the title The Wind from the Sun.

I first read about the idea of a spacecraft unfurling a huge but incredibly thin solar sail,in Larry Niven’s sci-fi novel The Mote in God's Eye . His idea was to utilize the pressure of sunlight on the sail - radiation pressure – on a craft weighing several tons that could accelerate to more than a kilometer per second within days, and then go on accelerating so long as it remained relatively close to the sun. It was one of his technological ideas I could understand and it has fascinated me ever since. Niven’s idea is similar to what Xeger discuses in the previous write up. By using giant ground based lasers that would give the craft an initial shove and it would even make it possible to tack the craft by angling the sail. By using the light of the sun which is composed of electromagnetic radiation that exerts force on objects it comes in contact with with a solar sail and lasers the combination would create the potential to send a craft anywhere within the solar system.

Related to many gossamer dreams about space travel, solar sailing is most often read about in science-fiction tales, however using the sun to glide through space has more than just a fictitious etymology; it’s now being given more serious consideration as new materials composed of lightweight carbon fibers only a few microns thick become available. Ed Gabris, a senior engineer at NASA, notes:

    "Solar sailing is more than a science fiction fantasy. NASA used solar sailing to increase the experiment time for the Mercury Mariner spaceprobe in 1974-75. The 'sail' was the spacecraft's solar panels. And by controlling the attitude of the spacecraft and the angle of the solar panels to the sun, the operations team was able to cause the spacecraft to visit Mercury several times more than would have been possible with the on-board liquid propulsion system".
The proposal of using the sun's energy to propel spacecraft across the cosmos has been around for centuries, says one expert:
    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

    To give you an idea how fast (solar sailing) 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.

Actual theories about solar sailing had their beginnings in the Russian aeronautics pioneer Konstantin Tsiolkovsky and his associate Fridrickh Tsander. In 1924 they were making notes about "using tremendous mirrors of very thin sheets" and "using the pressure of sunlight to attain cosmic velocities". It was American engineer Richard Garwin who has been attributed with coining the term in the latter part of the 1950s. Early on models included huge aluminum-coated Mylar sheets that could be aimed at the sun and "blown" toward deep space, powered by sunlight. However, such relatively heavy sails would take a very long time to go anywhere, so scientists have spent years researching and developing fresh kinds of sails and innovative techniques to thrust them into space faster and more efficiently. The promise of solar sailing in space continues, NASA has recently been in the news about awarding funds for the expansion of solar sail hardware and simulation development. The time is coming soon where we can set sail for the stars. A solar sail powered space ship is scheduled to be launched in the fall of 2002:
    The Cosmos 1 mission is a joint venture of the Planetary Society and Cosmos Studios, a group of film-makers and writers set up by the widow of scientist and writer Carl Sagan.

    The craft will be launched on a rocket fired from a submarine in Russian waters. The solar sail spacecraft will separate from the rocket, then unfurl and fly for a few weeks or months around the Earth pushed by the Sun.

For many space enthusiasts the modest sum of a four million dollars price tag for this Kitty Hawk moment embodies the future of practical, reasonable and quicker space travel exploration. Soaring through galaxies on sunbeams is magic and I for one can’t wait!


How Stuff Works: Exclusive: Breakthrough In Solar Sail Technology: carbonsail_000302.html

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