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grace in space a pair of satellites map subtle variations in earth?s
gravitational field, revealing secret craters, undersea mountains, and the
impact of climate change. by sam flamsteed
if the reverend nevil maskelyne came back to life, the 18th-century
astronomer royal of great britain would probably have no trouble grasping the
idea behind nasa?s remote sensing grace mission. maskelyne proposed a remarkably
similar experiment himself in a presentation to the royal society in 1772. ?if
the attraction of gravity be exerted, as sir isaac newton supposes, not only
between the large bodies of the universe, but between the minutest particles of
which these bodies are composed . . . it will necessarily follow, that every
hill must, by its attraction, alter the direction of gravitation in heavy bodies
in its neighbourhood . . . .?
that?s exactly what grace, the gravity recovery and climate experiment,
detects. every 94 minutes or so, twin satellites whip once around earth at an
altitude of 310 miles, taking 30 days to cover the planet?s entire surface, then
they do it again and again, sensing variations in local gravity. grace maps
local variations in the force of gravity over earth?s surface, revealing
mountain ranges and ocean trenches as well as underground watersheds and other
hidden concentrations of mass. a joint venture by nasa and the dlr (deutsches
zentrum f r luft- und raumfahrt, or german aerospace center), grace looks right
past the familiar oceans, continents, and clouds, showing our planet in a fresh
light?as a knobby, blobby globe of gravitational ups and downs.
among other things, grace may have found a crater deep under the antarctic ice
that may mark an asteroid impact greater than the one that doomed the dinosaurs,
measured the seafloor displacement that triggered the tsunami of 2004, and
quantified changes in subsurface water in the amazon and congo river basins.
?this is really an entirely new kind of remote sensing,? says project scientist
michael watkins, of nasa?s jet propulsion laboratory. ?it?s like when radar or
photography was first invented?you start realizing that it can be applied in all
sorts of unanticipated ways. we?re still discovering them.?


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