| SWOT mission aims to fill global gaps in key water data. By foot, horse, and canoe, European explorers centuries ago undertook years-long expeditions to document the length and breadth of major rivers. Today, satellites make the first pass of discovery. Though rivers meander and melting glaciers birth new lakes annually, the world’s major drainages have largely been mapped. Yet one fundamental dimension remains largely a mystery: the rise and fall of water bodies globally. Accurately measuring, at low-cost, the weekly changes in rivers, lakes, and wetlands would allow scientists to observe how much water moves through them. Land-based gauges do some of this work. But where gauges are scarce — Alaska, Africa, Asian headwaters — these numbers are inaccurate or unknown. The answer holds implications for flood prediction and drought response — even international diplomacy. The vessel for this new knowledge is the Surface Water and Ocean Topography satellite, a joint venture between NASA and the French space agency Centre National d’Études Spatial, with contributions from the Canadian Space Agency and the UK Space Agency. Planned for nearly two decades, the mission is scheduled to launch on December 12 from Vandenberg Space Force Base, in California. | 
