>> mike watkins: so, "grace" stands for "gravity recovery and climate experiment." >> stahl: mike watkinsad of the science division at nasa's jet propulsion laboratory in pasadena. he was the mission manager for the latest mars rover mission, and he is the project scientist for grace. >> watkins: so, the way grace works is it's... it's two satellites. >> stahl: two? >> watkins: they're actually measuring each other's orbit very, very accurately. >> stahl: what affects that orbit is gravity. >> watkins: as the first one comes up on some extra mass, an area of higher gravity, it gets pulled away... >> stahl: it goes faster. >> watkins: ...from the second spacecraft. >> stahl: and that's where water comes in. since water has mass, it affects the pull of gravity. so after the first grace satellite approaches an area that's had lots of heavy rain, for example, and is pulled ahead, the second one gets there, feels the pull, and catches up. the instruments are constantly measuring the distance between the two. >> watkins: their changes in separation, their changes in their orbit are a little dif