The Gravity Gap: How Two Tandem Satellites Measure Water and Ice From Orbit
The measurements come from the GRACE (Gravity Recovery and Climate Experiment) and GRACE-FO (GRACE Follow-On) missions, each of which consists of two satellites flying in tandem. The map depicts changes in terrestrial water storage from April 2002 to November 2025.
As the lead satellite passes over an area with greater mass, the slight change in gravity tugs at it, and instruments aboard the two spacecraft measure the resulting change in the distance between them. Because water and ice carry weight and move around constantly, those paired readings let scientists infer where mass was gained and where it was lost.
AI-generated discussion · Different perspectives
Nothing in this instrument is really a water detector. It watches the gap between two satellites and lets gravity do the reporting, and the water and the ice are inferences drawn from one very careful number.
That number is careful, but it arrives at a coarse grain: the tandem data are typically processed into monthly maps that show broad patterns of water storage. So what you get is a monthly summary rather than a close-up, and that is the level any argument about such a map should be held to.