“For about two years, I had the coolest job title in NASA: manager of the interstellar propulsion research project.”
Johnson’s team determined that the most practical path to the stars was via solar sails, which required fewer scientific breakthroughs than fusion-powered nuclear engines or exotic propulsion methods like warp drive. Ultra-thin sails would use the faint but constant pressure of sunlight or high-powered lasers to propel them to a few percent of the speed of light. (NASA plans to launch a 124-foot solar sail, called Sunjammer after a sail in an Arthur C. Clarke novel, in 2015, although it will stay well within the bounds of the solar system.) “Sailships are the only way we know to get to velocities that are anywhere close to the speed of light,” Gregory Benford, another physicist/sci-fi author, tells the Starship Congress attendees.
Yet even with this relatively reasonable-sounding technology, the problems are so vast that we won’t be sailing to the stars anytime soon. Johnson says that to propel a craft to Alpha Centauri, the nearest star system, a solar sail would have to be as big as the state of Alabama, and would need a millennium to travel the 4.3-light-year distance. Change the power source from solar radiation to terawatt-scale lasers and you could cut the travel time to a century. The big drawback? Such a system would require power “equivalent to the total output of humanity today,” Johnson says.
Lubin acted as conference contrarian, frequently asking presenters pointed questions about their proposed technologies. But he also offered up his own sci-fi-sounding project: a planetary defense system that could double as a solar sail’s power source, using beamed energy to propel an unmanned probe to the stars.
The system would collect sunlight with miles-wide solar arrays in Earth orbit and convert it to a beam of energy, similar to a giant laser. Lubin says that over a year, such a beam could completely vaporize a threatening asteroid a third of a mile (1,760 feet) wide at a range of one astronomical unit—the distance from Earth to the sun (93 million miles)—and deflect much larger ones. “It wouldn’t require any miracles, just a lot of hard work,” he says. Such a system could start on a much smaller scale—big enough to zap space debris, perhaps—then be expanded as engineering and funding allow.
If used to propel starships, the energy beam could boost probes to substantial speeds, Lubin says. A 100-kilogram (220-pound) probe with a 100-foot reflector to catch the beam could reach Mars in three days; with a much larger reflector, such a probe could hit three percent of lightspeed—up to 20 million mph—by the time it reached the edge of the solar system in less than a month.