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MATTER | TECHNOLOGY

Roadmap to Alpha Centauri
Pick your favorite travel mode—big, small, light, dark, or twisted

BY GEORGE MUSSER

EVER SINCE THE DAWN of the space age, a quixotic subculture of physicists, engineers, and science-fiction writers have devoted their lunch hours and weekends to drawing up plans for starships, propelled by the imperative for humans to crawl out of our Earthly cradle. For most of that time, they focused on the physics. Can we really fly to the stars? Many initially didn’t think so, but now we know it’s possible. Today, the question is: Will we?

Truth is, we already are flying to the stars, without really meaning to. The twin Voyager space probes launched in 1977 have endured long past their original goal of touring the outer planets and have reached the boundaries of the sun’s realm. Voyager 1 is 124 astronomical units (AU) away from the sun—that is, 124 times farther out than Earth—and clocking 3.6 AU per year. Whether it has already exited the solar system depends on your definition of “solar system,” but it is certainly way beyond the planets. Its instruments have witnessed the energetic particles and magnetic fields of the sun give way to those of interstellar space—finding, among other things, what Ralph McNutt, a Voyager team member and planetary scientist, describes as “weird plasma structures” begging to be explored. The mysteries encountered by the Voyagers compel scientists to embark on follow-up missions that venture even deeper into the cosmic woods—out to 200 AU and beyond. But what kind of spacecraft can get us there?

Going Small: Ion Drives
NASA’s Dawn probe to the asteroid belt has demonstrated one leading propulsion system: the ion drive. An ion drive is like a gun that fires atoms rather than bullets; the ship moves forward on the recoil. The system includes a tank of propellant, typically xenon, and a power source, such as solar panels or plutonium batteries. The engine first strips propellant atoms of their outermost electrons, giving them a positive electric charge. Then, on the principle that opposites attract,

ILLUSTRATION BY CHAD HAGEN

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