The moon, a luminous disk in the inky sky, appears suddenly above the broad crescent of Earth’s horizon. The four astronauts in the Orion crew exploration vehicle have witnessed several such spectacular moonrises since their spacecraft reached orbit some 300 kilometers above the vast expanse of our home planet. But now, with a well-timed rocket boost, the pilot is ready to accelerate their vessel toward the distant target ahead. “Translunar injection burn in 10 seconds ... ” comes the call over the headset. “Five, four, three, two, one, mark ... ignition....” White-hot flames erupt from a rocket nozzle far astern, and the entire ship—a stack of functional modules—vibrates as the crew starts the voyage to our nearest celestial neighbor, a still mysterious place that humans have not visited in nearly half a century. The year is 2020, and Americans are returning to the moon. This time, however, the goal is not just to come and go but to establish an outpost for a new generation of space explorers.