The GRACE satellites are really very cool. Imagine: You’ve got two trapezoidal-looking satellites, chasing each other constantly above Earth’s poles. They’re about 130 miles apart from each other, but can measure their precise separation down to the precision of a single micron. GRACE is all about gravitational anomalies; big masses that shift around on Earth’s surface exhibit subtle pulls on the satellites. And slight changes in distance between the two reveal how the masses are moving down below.

Those masses include water. Water is heavy, Jay Famiglietti of the University of California, Irvine, told the meeting today. Since its 2002 launch, GRACE has watched monsoon rains pile up in some regions of the world, like the MIssissippi river basin, and dry out in others, such as the Congo basin.

Water shifting around, as seen from space – that’s just cool. Nature subscribers can read more in a News & Views article published here.