From December 1968 to December 1972, nine crews of human beings orbited and walked on and even drove on the face of the ancient moon. But as suddenly as the visits began, they stopped. The humans left and the quiet resumed. All of that, however, may soon change. For the first time in five decades, the U.S. - along with private-industry and international partners - has committed itself to returning to the moon, and to doing it on a defined timeline. In December 2017, President Trump signed the first of three Space Policy Directives, putting manned lunar exploration back at the top of the NASA agenda. Rather than the so-called flags-and-footprints model of lunar exploration - with short-term crews in throwaway vehicles landing on the surface ... the U.S. now hopes to establish a long-term presence on and around the moon. The centerpiece of the new system will be what NASA calls the Lunar Orbital Platform-Gateway, a mouthful of a name that hides a relatively simple idea. Gateway, as NASA sees it, will be a sort of mini space station in lunar orbit. Like the ... International Space Station, this one would be built with the help of more than a dozen other nations. Unlike the existing station, which consists of 15 habitable modules and a vast array of solar panels, Gateway will be comparatively small-a 75-ton assembly, consisting of just one or two habitable modules, each roughly the size of a school bus, plus a snap-on module for power and propulsion and two others that would serve as an air lock for spacewalking astronauts and a docking port for incoming vehicles.