In a landmark 1984 paper, the late Robert Forward, then a physicist at Hughes Aircraft's research laboratories, proposed a twist on the ancient technology of sailing. Just as wind can drive a canvas sail across the ocean, a powerful laser can push a huge sail through space. When photons in the laser beam strike the sail, they transfer their momentum and push the sail onward. The spaceship gradually but steadily builds up speed and races off to distant worlds while the laser that propels it stays put in our solar system. Frisbee regards this as the most likely technology to ferry the first spacecraft to another star.
Engineers have already built a simple space sail, one that rides on light from the sun rather than a beam of photons from a laser. Within the next few months, the Planetary Society, a private space-enthusiast organization, plans to launch its pioneering solar sail. Cosmos 1, a 50-pound, 100-foot-wide pinwheel of aluminized Mylar-like sheets, will be launched from a Russian submarine in the Barents Sea. Once in space, the sail will be pushed by sunlight to reach a higher orbit. Hoppy Price, the lead engineer for solar sails at NASA's Jet Propulsion Laboratory, thinks this type of fuel-free propulsion will allow entirely new types of planetary missions. But the strength of sunlight falls off exponentially with distance, so solar sails do not work far away from the sun.
A focused beam of laser light, in contrast, could push a craft to Alpha Centauri and beyond because a laser beam does not spread and weaken over distance nearly as much as sunlight does. Frisbee has sketched a design for a trip to 55 Cancri based on Forward's concept. His spacecraft would be powered by a 600-mile-wide aluminum-foil sail with a crew cabin at its center. A powerful laser in Earth orbit or on the lunar surface would bounce off a 600-mile-wide flexible mirror, which would focus the beam so it could drive the sail. The laser would have to pump out light for several years until the spacecraft reached its cruising speed; another couple of years of beaming would allow the craft to slow down (see below).
Frisbee's sail needs to be large in part to dissipate heat from the enormous energy of the incoming laser beam. Aluminum melts at a modest 1,220 degrees Fahrenheit. If the sail were manufactured in space, engineers could switch to lighter, more resilient materials. Geoffrey Landis at NASA's Glenn Research Center in Ohio is investigating thin films made of niobium (which melts at 4,490°F) or diamond (which breaks down into graphite at 3,270°F)—"like a soap bubble in thickness," he says. High-temperature substances could withstand a smaller, more intense laser beam. A diamond-film sail with the same capabilities as Frisbee's aluminum sail could allow faster acceleration and decrease the total trip time.
More daunting, perhaps, is the laser power needed to drive that sail to 55 Cancri: By Frisbee's estimation, it would take a steady flow of about 17,000 terawatts, or 1,200 times all the energy consumed on Earth at any moment. To meet such an enormous demand, he suggests using a solar-pumped laser, a device that gathers sunlight and transforms it into a focused, coherent beam. Physicists Roland Winston and Joseph O'Gallagher of the University of Chicago have demonstrated a system that can concentrate light to 84,000 times its normal intensity.
"Just working with solar sails, we'll wind up solving the major problems of building the laser-sail system," Frisbee says. If we can master the technology, we will no longer need to worry about how far our fuel supply can carry us. A simple design twist would even allow the laser beam to slow the ship to a halt at the other end. And the top speed of a laser sail is limited only by the velocity of light itself. In Frisbee's study, a laser sail accelerates to half light speed in less than a decade. He calculates that by using a sail 200 miles in diameter we could reach Alpha Centauri in just 121/2 years. A 600-mile-wide sail could rendezvous with a planet around 55 Cancri in 86 years.
credit to www.discover.com
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