As an extra bonus, Gregory says, “After you’ve blown this stuff off, you’ve made the sail even lighter.” And the lighter the sail, the greater the acceleration for a given force. Scaled-up versions of the experimental sails would weigh about 5 tons per square kilometer. But James believes that graphene, an ultrathin form of carbon just a single atom thick, might eventually be used in diaphanous sails weighing just 500 pounds per square kilometer.
In other experiments, the researchers found that a microwave beam can provide a restorative force that stabilizes the sail and keeps it billowed, assuming the sail has the right shape. Previous solar sails had been flat, designed that way to catch more sunlight, but it was difficult to keep them properly aligned with the sunlight. A beam-driven sailship should be concave — resembling an umbrella — to keep it aligned with the beam, so that when light waves hit the sail, they naturally tend to center it on the beam.
“What needs to happen next will be to carry out real sail experiments in space,” says Gregory. He and James almost got their chance in June 2005, when a Russian Volna rocket was scheduled to launch Cosmos 1 — a 6,415-square-foot solar sail — into space. Unfortunately, the rocket failed 83 seconds after takeoff, and Cosmos 1 never made it into orbit.
The Planetary Society has seen some success with its LightSail designs, but the Benfords’ plans are currently shelved. So, James decided to assess the economic feasibility of beamed propulsion in order to “put some numbers on this idea.” His 2013 study concluded that it would cost about $30 billion to build a beamer that can send a probe out of the solar system, but once built, the system could be operated cheaply. With four and a half hours of acceleration, a sailship could reach Pluto in one year at a cost of about $40 million. By contrast, the New Horizons space probe took almost nine and a half years to reach Pluto and cost about $700 million.
Decoding the Beam
Benford’s analysis convinced James Guillochon and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics that sailships might afford the most practical means of space travel, both within and out of the solar system. They also applied the idea to another sci-fi staple: aliens.