Solar Sail
Scientists are looking for ways to propel spacecraft with less fuel than a rocket uses. The solar sail intrigues many because it requires no fuel at all.
In the late 1860s the theory of electromagnetism, first described by James Clerk Maxwell, defined light as a packet of energy called a photon. These photons have characteristics both of a wave and a particle, meaning at times they can behave like either, and they have energy and momentum when they move. The Russian space pioneer Konstantin Tsiolkovsky imagined that some day large thin sheets could be spread out in space to capture photons, which would push the reflective sail along as they bounced off it.
The National Aeronautics and Space Administration (NASA) has studied solar sailing. They launched small metal needles into the atmosphere high above the earth and found that sunlight altered their orbit. Using this discovery, they built the Mariner 10 spacecraft, which traveled to Venus and Mercury. It had small panels that turned the craft when they were tilted at different angles to the sun. NASA also explored the possibility of using a sail to propel a craft to rendezvous with Halley's comet when it swept through the inner solar system in 1985-86. Scientists were unable to devise such a technique by the time of the comet's arrival, so the plan was dropped.
The principles of solar sailing are very simple: The photons hit a reflective surface and propel the craft. To turn, the sail is slanted, much like a sail on a sailboat when a person tacks with the wind. The craft can increase up to 13 miles (21 km) per hour and continue this acceleration as long the sail is extended to catch photons; in just one day the vessel could gain a speed of 225 miles (362 km) per hour; in 12 days it could cover 1/2 mile (.8 km) per second. At that rate of acceleration, it could reach Mars in 400 days. A traditional rocket could beat the time to Mars, but the solar sail could haul far greater payloads.
The design and construction of solar sails is also relatively simple. The material used to reflect photons would probably be a thin layer of aluminum sprayed onto plastic, which would give it strength and form. There are only three basic configurations for a solar sail: square, disk, or heliogyro. The square design is similar to that of a kite, with stiff support pieces to give it shape. Both the disk and heliogyro spin to keep the material stable and rigid. The only difference between the two is that the disk is solid, while the heliogyro is made of rotating strips, much like the blades on a helicopter.
The size of these projected configurations is extraordinary. A square sail, to achieve the speed indicated above, would require an area of at least one square mile (2.6 square km). The heliogyro would need blades 20 miles (32 km) long and 30 feet (9 m) wide. Such a craft would be able to carry a payload about the size of the early Apollo command module.
Other variations in solar-sail design are being considered, such as lasers to provide propulsion, or beamed microwaves to provide great acceleration and speed.
In the late 1990s, solar sails became the subject of much research. The World Space Federation in the United States has built a prototype square sail with 90-foot booms. The French are also pursuing this idea: hoping to encourage more research and development in solar sailing, they are offering a prize for the winner of a solar sail race to the moon.
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