The problem is power. For example, Dawn runs on three ion engines, each of which puts out a steady, but measly, one third of an ounce of thrust. (Each engine on a Boeing 777 churns out about 100,000 pounds of thrust.) This is great for long, unmanned missions—it took 16 months to propel the probe to Mars—but it's not ideal for humans looking to spend as little time in transit as possible. NASA is retooling its engine for triple the thrust, which could get a probe to Mars faster, but it's still too slow for a large spaceship heavy with crew and gear. If a little more thrust is good, a lot more is better. The Texas-based aerospace company Ad Astra's VASIMR engine creates a thicker ion stream by shooting radio waves, rather than electrons, at argon gas. Then, the engine's superconducting magnets fling the ions to generate 50 times as much thrust.