The Tohoku earthquake that rattled Japan on 11 March changed Earth's gravitational field enough to affect the orbits of satellites. The satellites' altered courses suggest that the earthquake was stronger and deeper than instruments on Earth indicated.
These weren't just any satellites: they are the twin spacecraft of the Gravity Recovery and Climate Experiment (GRACE), which fly 220 kilometres apart in a polar orbit about 500 kilometres above Earth.
GRACE's job is to map the Earth's gravity field, and it does this by monitoring the effect of minute variations in the field on the trajectories of the satellites and the changing distance between them.
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