Twin satellites weigh Earth's hidden water by tracing gravity's subtle tug
Two identical GRACE-FO satellites fly about 220 km apart and use a laser-ranging interferometer to detect tiny changes in their distance as they pass over Earth's mass distribution. Those micro-variations are converted into monthly gravity maps; after subtracting solid-ground signals, the residuals reveal changes in large-scale water stores—ice loss, groundwater, and soil moisture—allowing scientists to weigh continental water over time.