The GRACE mission detects changes in Earth's gravity field by monitoring the changes in distance between the two satellites as they orbit Earth. The drawing is not to scale; the trailing spacecraft would actually be about 220 kilometers behind the lead spacecraft.
Gravity. What is it? You can't see it. You can't smell it. You can't touch it. But, it's there. In fact it's everywhere. We are familiar with gravity because we live with its effects every day. We know that when we drop an object, it falls to the floor, and we know gravity is the reason. While the force of gravity is weak compared with other forces in nature, such as electricity and magnetism, its effects are the most far-reaching and dramatic. Gravity controls everything from the motion of the ocean tides to the expansion of the entire universe.
One of the NASA Earth Science Enterprise's focus areas is Earth Surface and Interior studies, which includes studying the gravity field. The Gravity Recovery and Climate Experiment (GRACE), launched by NASA on March 17, 2002, is revealing more detail about the gravity field than has ever been available before. Data provided by GRACE are substantially improving our knowledge of Earth's gravity and of a number of important aspects of global change.
How does GRACE really work? How is it possible for a satellite in space to make such a precise measurement of gravity from so far away? It seems like something only an expert in gravity studies could understand, and we might think the details are beyond our comprehension. Perhaps, however, if we take another look at how this familiar force really works, we can begin to better understand how GRACE measures gravity from space.
The Gravity Recovery and Climate Experiment (GRACE) was launched from Plesetsk Cosmodrome in Russia on March 17, 2002. Shown here is the ROCKOT launch vehicle as it lifts off the pad carrying GRACE.
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