WE NOW have the most detailed map yet of the Earth's gravitational field - and it shows subtle variations around the globe, including the added pull exerted by mountain ranges and oceanic ridges. Hailed as revolutionary by experts, the map should help reveal the movements of ocean currents and their effect on the climate, and could even predict drought or flood.
Twin satellites orbiting in formation collect the data used to produce the map. Dubbed GRACE (Gravity Recovery and Climate Experiment), the project is a joint effort by NASA and German space agency DLR.
The satellites, launched in March 2002, have been following an orbit that takes them over the poles, but in paths about 220 kilometres apart. As they circle the Earth, the precise speed of each and their distance apart is measured using GPS and a beam of microwaves bounced between them. This reveals their separation to a ...
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