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How Do Solar Sails Work?
Solar sails would, theoretically, ride the radiation pressure of light that are being emitted by the Sun, the sails being large, ultra-thin mirrors that will efficiently reflect the photons to provide the push. Note that this is different than solar wind, which provides a slightly smaller force. While this exerts relatively little force upon the sails, over time it could accelerate the craft to huge speeds, estimated to above 200,000 kilometers per hour within three years! This set up is analogous to Earthly sailboats that rode the winds back in the Age of Discovery, this endless source of propulsion will enable easy exploration amongst our planets. Even old school navigation techniques are applicable, such as tacking into the wind. On top of that, it has even been suggested that the surface area of the sails could double as a large antenna, optimum for deep space travel. Pretty cool, right?
Sort of. While seemingly easy enough of a concept, it is difficult to execute, and not necessarily as useful as it might seem. While solar sails could potentially reach great speeds, it takes considerable time for them to do so, which is impractical for human travel and any unmanned satellites that require fast deployment. Also, for this to be at all effective, even at a small scale, the sails have to be quite large, which has some serious logistical deployment problems.
The issue of speed may be alleviated by proposed hybrid models, some of which involve both a chemically powered rocket to give the satellite an initial boost, and then the solar sails kicking in to provide even more speed in the long run. Another model yet, developed by the Japanese, involves both a solar-powered ion drive and solar sails.
The deployment issue is still one that engineers are wrestling with. There are many competing designs for solar sails, from square sails to heliogyros to circular disks to circular rings, and which one works best is still up in the air. Engineering solar sails has been largely theoretical up to this point, spotted with a few unhelpful failures.
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A History of Failure
Despite its promising premise, all attempts at a working solar sail have thus far failed.
The first test of the solar sail model was way back in 1993 from the Russian Mir station, Znamya 2 and 2.5. The first one deployed correctly, while the second failed; neither of them demonstrated propulsive potential.
In 2004, a Japanese attempt called ISAS also managed to deploy solar sails, but did not manage to demonstrate any amount of navigational control.
An attempt was in 2005 by the Planetary Society, a nonprofit group created by legendary astronomer Carl Sagan, completed jointly by Cosmos Studios and the Russian Academy of Science. This mission was known as Cosmos 1, punning considering that it is the name of Carl Sagan's television show. The reason for failure had nothing to do with the solar sail itself, but with a failure on the part of the Russian rocket, launched from a submarine. So, the verdict on the solar sail remains somewhat open.
NASA's own NanoSail-D mission in 2008 failed for similar reasons: a failed flight of SpaceX's Falcon 1 rocket. Catching a pattern here? Thus far, it doesn't seem to be an inherent fault in solar sail technology, rather, a string of bad logistical luck, or a simple lack of developmental ambition.
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The next attempt at solar sails is another by the Planetary Society, called LightSail 1. Funded by an anonymous donation of a million dollars, plus other funds, it has been set for launch sometime in 2010. The sails consist of 32 square meters of Mylar—the same stuff emergency blankets are made out of—and several modules, one for controlling the solar sail deployment, and the others for containing the solar sails.
What makes LightSail 1 different from previous missions is largely its smaller mass, which should make it more efficient. Think of Newton's 1st Law: F=ma, the force exerted on an object equals its mass multiplied by its acceleration. The force exerted by the radiation pressure of the Sun is constant, thus, with a smaller mass of the satellite and its solar sails, acceleration will be greater. Furthermore, the smaller size opens it up to a myriad of launch options, including less expensive private commercial companies.
The launch vehicle for the LightSail 1 has not yet been identified, but there are many Russian and American missions planned for 2010 that the mission could potentially hitch a ride with into orbit—and potentially into space navigation history. Hopefully one of these rockets prove to be more reliable than past choices.
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Future LightSail Missions
The LightSail 1 mission was never planned to stand alone, however, and is rather the first part of a longterm project. If it proves successful, there are already plans in place for another two missions, these ones pushing solar sails even further, each one larger and more complex than the last.
LightSail 2 is intended to be able to navigate outside of Earth's orbit. LightSail 3 will potentially be able to reach the Sun-Earth L1 Lagrange Point, and will hopefully demonstrate the ability to monitor solar activity—a boon to future solar astronomers and demonstrating a practical application of solar sails.
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LightSail 1 isn't the only solar sail mission planned for the near future. There are multiple solar sail development projects come from a variety of places, from Finland to work on NASA's spare NanoSail-D. That being said, solar sail programs have been cut back due to the recession and budget crises, so this bid by the Planteary Society remains the main push for the first functional solar sail.
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For More Information
Wikipedia has an excellent article on solar sails, covering both their historical development and the detailed physics behind them, as well as information on the LightSail 1 mission.
For more information specific to the LightSail 1 mission, check out its homepage at the Planetary Society website.
Really interested? Check out the most recent developments at the Solar Sail Wiki.