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Flying with Nature's Own Fuel
Jet Propulsion Laboratory
February 24, 2003

"Following the light of the Sun, we left the Old World."
- Christopher Columbus 

{at}Hundreds of years ago, early discoverers used the Sun as a compass. 
Turns out the light of the Sun can do more than just guide us; it can 
actually propel us farther and faster into the vast realm of space 
than we've ever been able to go.  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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