MATTER | TECHNOLOGY Road to Alpha Centaur1 Pick your favorite travel mode—big, small, light, dark, or twisted BY GEORGE MUSSER VER SINCE THE DAWN of the space age, a and magnetic fields of the sun give way to those of quixotic subculture of physicists, engineers, interstellar space—finding, among other things, what and science-fiction writers have devoted their Ralph McNutt, a Voyager team member and planetary lunch hours and weekends to drawing up plans scientist, describes as “weird plasma structures” beg- for starships, propelled by the imperative for humans ging to be explored. The mysteries encountered by to crawl out of our Earthly cradle. For most of that the Voyagers compel scientists to embark on follow- time, they focused on the physics. Can we really fly to up missions that venture even deeper into the cosmic the stars? Many initially didn’t think so, but now we woods—out to 200 AU and beyond. But what kind of knowit’s possible. Today, the question is: Will we? spacecraft can get us there? Truth is, we already are flying to the stars, with- out really meaning to. The twin Voyager space probes Going Small: Ion Drives launched in 1977 have endured long past their original +NASA’s Dawn probe to the asteroid belt has demon- goal of touring the outer planets and have reached strated one leading propulsion system: the ion drive. the boundaries of the sun’s realm. Voyager 1 1s 124 An ion drive is like a gun that fires atoms rather than astronomical units (AU) away from the sun—that bullets; the ship moves forward on the recoil. The sys- is, 124 times farther out than Earth—and clocking — tem includes a tank of propellant, typically xenon, and 3.6 AU per year. Whether it has already exited the a power source, such as solar panels or plutonium bat- solar system depends on your definition of “solar sys- _ teries. The engine first strips propellant atoms of their tem,” but it 1s certainly way beyond the planets. Its outermost electrons, giving them a positive e