The first image released from the Gravity Recovery and Climate Experiment (GRACE), a joint NASA-German Aerospace Center mission, graphically illustrates the sensitivity of the mission's twin spacecraft to changes in Earth's gravity. Color gradations in the image measure changes in the distance between the GRACE spacecraft as they orbit overhead approximately 220 kilometers (137 miles) apart. Such variations are caused as the spacecraft fly over Earth's uneven gravity field, for example, when GRACE travels over mountain ranges or undersea trenches. Earth's largest spatial features (those covering the largest areas of Earth) have been removed from this image so that such smaller features can be highlighted. GRACE's extremely sensitive microwave ranging instrumentation is capable of measuring variations at the micron, or millionth of a meter, level. The data for this and similar images will be processed by the GRACE science team to produce precise maps of Earth's gravity field. Preliminary maps of Earth's geoid, or mean gravity field, are expected to be available to researchers in the spring of 2003.
