We are a curious species with amazing capacities to imagine and dream. We wonder about what we cannot see, are fascinated by what we do not know and are driven to explore. In keeping with our continuous quest for knowledge, President George W. Bush announced a new plan for NASA in Jan. 2004. A renewed focus on space exploration, he explained, would strengthen our leadership in the world, improve our economy and enhance the quality of our lives. The Vision for Space Exploration calls for human and robotic missions to the Moon, Mars and beyond. To realize these ambitious goals, we will need more powerful and efficient propulsion and power-generation systems — systems that can thrust a spacecraft out of Earth’s orbit to the far reaches of the Universe.