Delory is tantalized by the notion of solar sailing, and who wouldn't be? You start with an ancient technology that was central to opening up the globe--sailing--and then, as astronomer Carl Sagan's widow Ann Druyan puts it, you "learn to ride the light the way we've learned to ride wind." A solar-sail craft is driven forward by sunlight--photons--which strike the sail and bounce back. That change in the momentum of the photons is what transfers energy to the sail craft and gives it its "push." (Because the intensity of sunlight decreases with distance from the Sun, some scientists have proposed, for long journeys, a solar-powered laser moored in orbit and pointed at the receding sail.) A solar-sail craft will, theoretically, gain speed the way a savings account accrues compound interest--making it faster, in the long run, than even the nuclear-powered Voyager 1, which since its launch 27 years ago has traveled eight billion miles, beyond the orbits of all the planets in our solar system, and is now the most distant human-made object in the universe. Some astronomers think solar-sail power has the most potential for deep-space missions, such as ferrying people and cargo to Mars, or even interstellar journeys.