NEIL Armstrong's first footsteps on the moon in 1969 inspired universal wonder and excitement. In that moment, it seemed the unimaginable had become reality, and a course for an enterprise in space in which anything was possible was established. Today we know much more about the difficulties of space exploration by humans or machines, and our thinking about space has evolved with our growing awareness of its costs and hazards. Remarkably, those first footsteps continue to inspire. President Bush's remarks on the Columbia tragedy capture a widespread sentiment: "Mankind is led into the darkness beyond our world by the inspiration...