Scientists now know that sunlight is little more than a stream of photons, or tiny particles of light. Photons don't possess mass, but they do boast linear momentum. When they bounce off a reflective surface, they push against that surface.
"The solar sail in principle is nothing more than a highly reflective sheet of metal, or plastic coated with metal, that basically reflects sunlight," says physicist and noted solar sail authority Dr. Gregory L. Matloff. "That's basically all there is to a solar sail, everything else is necessary engineering and physics complication."
The Mariner 10 probe successfully demonstrated the technology in 1974 when NASA used the probe's solar arrays as an impromptu sail. Subsequent American, Russian, Indian and Japanese missions have proven further the technology's feasibility.
Sailing to Distant Stars
With solar sail technology, it's easy to imagine manned and unmanned spacecraft traversing the solar system. Scientists have proposed the use of solar sail technology not only to propel slow-moving probes, but also to deliver supplies ahead of manned space missions and even to nudge Earth-bound asteroids off course.
But the right solar sail can take you much, much farther. In order to achieve interstellar travel, Matloff stresses that less mass and more of a solar push are key. That means deploying an ultralight solar sail really close to the sun.
"You'd have to position the craft as close to the sun as you can, probably within the orbit of Mercury," Matloff says. "And in a case like that, it would reach maybe 200 astronomical units (200 times the distance between Earth and the sun) in something like 10 or 15 years."
Of course, that also means the craft would require 7,000 years to reach the nearest star - but that's if the sails depended on current technology. Matloff believes a 50-nanometer beryllium sail, built in space, could potentially make an interstellar voyage in as little as 2,000 years. Go lighter than that, via perforated sails or lighter metamaterials, and potential speeds increase.
"There are materials coming online like carbon nanotubes and graphenes, and these may allow you to cut the mass of the sail even more," Matloff says. "So, I think we'll be able to do to a lot better than 2,000 years to the nearest star. Will we get below 1,000 years? Maybe. Will we get down to a couple hundred? Well then I have my doubts, but that's my own personal feeling."
Ride the Cosmic Laser
Sailing on the sun's solar wind certainly comes with its share of limitations. After all, a solar sail craft would have to start off very close to the sun to achieve its momentum and would experience decreased push the farther it traveled from the star.
What if you could generate your own solar wind with laser and particle beam technology? Future generations may develop a kind of laser highway to distant systems by emitting a concentrated photon beam in the right direction. A solar sail vessel would merely enter the stream and hitch a ride, like a paper airplane drifting into the wind made by an electric fan.
"With such a higher velocity, we'd be able to send an expedition to the nearest star within a human lifetime," Matloff says, "provided you built a large enough laser, a large enough sail and a light enough spacecraft."
Still, such an impressive laser highway wouldn't come cheap. An interstellar energy bream projector would require vast amounts of energy - far more than our current world power utilization. Will it be more than we're willing to pay?
"Certainly in few hundred years, as we build a civilization that's more at home in the solar system, there is no reason why we can't play games with beaming energy," Matloff says.
Image credit: NASA