With new solar sail technology, scientists are finding a way to convert light energy from the Sun into a lightweight, propellant-free source of propulsion for spacecraft.

"The idea of solar sails has been around for about 100 years, but our structural technology has just now gotten to the point where we are ready to perform some missions using this type of propulsion," said Hoppy Price, solar sail technology lead engineer at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

"Solar sails are now being considered for interstellar travel because they don't have to carry any fuel to operate and can be propelled by sunlight and high-powered lasers."

Solar sails are energy efficient, inexpensive to make and can greatly reduce a spacecraft's weight. Typically, spacecraft use rockets that apply short powerful thrusts to the spacecraft, which then coasts to its destination. Solar sails replace these heavier rockets, yet still increase spacecraft speed.

Solar sails are composed of large, flat, smooth sheets of ultra-thin film supported by lightweight structures. A highly reflective material covers the side that faces the Sun. The result is a huge mirror, at least the size of a football field. These mirrors reflect continuous sunlight and transfer the momentum from photons to an object in order to propel it.

The inspiration for this technology came from 17th-century astronomer Johannes Kepler, who deduced that winds blew objects about in space after he observed comet tails blown by what appeared to be a solar breeze.

Kepler suggested that eventually ships might navigate through space using sails that could catch this wind. He was wrong about the winds, as they do not exist in the vacuum that is space. But his eyes did not fool him; he saw the gentle pressure of solar light particles (photons) on dust particles released by the comet as it was in orbit.

On Earth, frictional forces in the atmosphere are too large, making it hard to observe this pressure. It can, however, be observed and used to advantage in space.

To harness this natural force, physicists looked to Newton's law, which states that any object under the influence of unbalanced forces will undergo a net acceleration of its motion.

They also realized that the power exerted by the Sun, albeit a very tiny force, is persistent. Over weeks and months, the gentle acceleration provided by the Sun could eventually make a spacecraft reach speeds fast enough to cover the distance between Los Angeles and New York in less than one minute, five times faster than the Voyager and Pioneer spacecraft now zooming through the outer solar system.

Researchers figured they had to design a craft that combined the forces acting upon it in a way that produces a net acceleration in the desired direction of travel. They came up with solar sails, not to be confused with "solar cells," the technology that uses solar panels to convert sunlight into electricity.

"NASA has a technology program now underway to develop flight-ready solar sails," Price said. "We expect to test them on the ground in 2005 and fly science missions with them a few years later."

If successful, future flight missions may use solar sails to achieve a number of objectives related to this type of propulsion, including increased opportunities to change or correct the path of any given mission.

Small kite-sized sails several meters in diameter might adjust the orbit and stability of spacecraft, while super-sized sails (1,000,000 square meters or 250 acres), with possible assistance of laser or microwave transmitters, might help achieve interstellar travel.

Solar sails have been proposed for use on an interstellar probe that might launch as early as 2015. In that case, a sail nearly half-a-kilometer (1640 feet) wide would allow the probe to travel fast enough to surpass the distance covered by Voyager 1, currently the most distant spacecraft from Earth, by 2023. In only eight years, the probe would cover the same distance Voyager 1 traveled in more than four decades.

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