New dawn for electric rockets.

Februar y 20 09 PA T RA W LI N G S SA IC A lone amid the cosmic blackness, NASA’s Dawn space probe speeds beyond the orbit of Mars toward the asteroid belt. Launched to search for insights into the birth of the solar system, the robotic spacecraft is on its way to study the asteroids Vesta and Ceres, two of the largest remnants of the planetary embryos that collided and combined some 4.57 billion years ago to form today’s planets. But the goals of the mission are not all that make this !ight notable. Dawn, which took off in September 2007, is powered by a kind of space propulsion technology that is starting to take center stage for long-distance missions—a plasma rocket engine. Such engines, now being developed in several advanced forms, generate thrust by electrically producing and manipulating ionized gas propellants rather than by burning liquid or solid chemical fuels, as conventional rockets do. Dawn’s mission designers at the NASA Jet Propulsion Laboratory selected a plasma engine as the probe’s rocket system because it is highly ef"cient, requiring only one tenth of the fuel that a chemical rocket motor would have needed to reach the asteroid belt. If project planners had chosen to install a traditional engine, the vehicle would have been able to reach either Vesta or Ceres, but not both. Indeed, electric rockets, as the engines are also known, are quickly becoming the best option for sending probes to far-off targets. Recent successes made possible by electric propulsion include a visit by NASA’s Deep Space 1 vehicle to a comet, a bonus journey that was made feasible by propellant that was left over after the spacecraft had acKEY CONCEPTS