GRACE: Weighing the Earth with Two Chasing Satellites
🌐 Measuring the Earth from space
Question
How do we map the Earth's gravity field from space?
GRACE (2002-2017) and its follow-on GRACE-FO (2018-) fly two satellites ~220 km apart in a near-polar orbit (~500 km, 89° inclination). When the leading satellite passes over a mass concentration - an ice sheet, a groundwater aquifer, a mountain range - it is pulled slightly forward; the trailing satellite is pulled slightly less. The distance between them changes, and that change is the gravity signal.
The math
The ranging system is sensitive enough to detect separation changes as small as 10 micrometres - about one-tenth the width of a human hair - over a distance of 220 km. The twin satellites circle the globe ~15 times a day.
Why it matters
GRACE mapped the thinning of the great ice sheets, the depletion of groundwater aquifers, and shifts in the crust. Gravity varies across the Earth in ways only possible on a rotating oblate spheroid; flat-earth gravity models cannot produce these patterns. One of the most beautiful experiments humans have ever done.