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GRACE -- Gravity Recovery and Climate Experiment

Author: Byron Tapley

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.

This is a three dimensional rendering of the Earth's geoid magnified a thousand times, with the portion due to the Earths' oblateness removed. The magnification highlights smaller scale variations in gravitational potential.
This is a three dimensional rendering of the Earth's geoid magnified a thousand times, with the portion due to the Earths' oblateness removed. The magnification highlights smaller scale variations in gravitational potential.

While gravity is much weaker than other basic forces in nature, such as magnetism and electricity, its effects are ubiquitous and dramatic. Gravity controls everything from the motion of the ocean tides to the expansion of the entire Universe. To learn more about the mysteries of gravity, twin satellites named GRACE -- short for Gravity Recovery and Climate Experiment -- launched in March 2002 to make detailed measurements of Earth's gravity field. This experiment could lead to discoveries about gravity and Earth's natural systems, which could have substantial benefits for society and the world's population.

The GRACE mission is the inaugural flight of NASA's Earth System Science Pathfinder Program (ESSP). A component of NASA's Earth Science Enterprise (ESE), the ESSP missions are intended to address unique, specific, highly focused scientific issues and provide measurements required to support Earth science research.

Why a mission to study gravity?

If Earth were a smooth sphere composed of similar elements or ingredients, there would be no need for a GRACE mission; the assumption made in most introductory physicscourses that the acceleration due to Earth's gravitational field has a constant value would indeed be correct end of story. However, previous observations have clearly demonstrated that our Earth isn't smooth and homogeneous and it really isn't even a sphere! What's more, the images shown above are just instantaneous snapshots from one moment in time.

This is a plot of the Earth's geoid (surface of equal gravitational potential) produced by the Earth Gravitational Model (EGM96), one of many models used for gravity studies.
This is a plot of the Earth's geoid (surface of equal gravitational potential) produced by the Earth Gravitational Model (EGM96), one of many models used for gravity studies.

The reality is that the gravity field is continually changing, mostly due to variations in water content as it cycles between the atmosphere, oceans, continents, glaciers, and polar ice caps. By far the largest "lump" is the flattening observed at the poles -- the Earth's oblateness. The above profiles have removed the portion of the response due to oblateness in order to focus on the smaller anomalies that exist. GRACE will reveal the broad features of the Earth's gravitational field over land and sea; it will also allow for these smaller scale features to the identified and studied with unprecedented accuracy and it will show how the Earth's gravity field varies with time.

GRACE 2002: A Scientific Geodesy

Gravity is the invisible force that pulls two masses together. The branch of science dealing with obtaining precise measurements of the Earth, mapping points on the surface, and studying its gravitational field is known as geodesy.

This schematic diagram illustrates the different forces that redistribute mass on the surface of the Earth.
This schematic diagram illustrates the different forces that redistribute mass on the surface of the Earth.

Producing a precise model of the fluctuations in gravity over the Earth's surface has proven to be a formidable task. Currently, data from several dozen satellites must be combined to produce a model of Earth's gravitational field. These models do a good job at replicating the large-scale features of Earth's gravitational field but cannot resolve finer-scale features or accurately describe the small month-to-month variations associated with the hydrologic cycle.

The unique design of the GRACE mission (twin satellites flying in formation) is expected to lead to an improvement of several orders of magnitude in these gravity measurements and allow much improved resolution of the broad-to-finer-scale features of Earth's gravitational field over both land and sea.

The distribution of mass over the Earth is non-uniform. GRACE will determine this uneven mass distribution by measuring changes in Earth's gravity field. The term mass refers to the amount of a substance in a given space, and is directly correlated to the density of that substance. For example, a container filled with a more dense materials, like granite, has more mass than that same container filled with water. Because mass and density are directly related, there is also a direct relationship between density and gravity. An increase in density results in an increase in mass, and an increase in mass, results in an increase in the gravitational force exerted by an object. Density fluctuations on the surface of the Earth and in the underlying mantle are thus reflected in variations in the gravity field. As the twin GRACE satellites orbit the Earth together, these gravity field variations cause infinitesimal changes in the distance between the two. These changes will be measured with unprecedented accuracy by the instruments aboard GRACE leading to a more precise rendering of the gravitational field than has ever been possible to date.

GRACE will do more than just produce a more accurate gravitational field plot, however. The measurements from GRACE have important implications for improving the accuracy of many scientific instruments related to climate change.

This diagram illustrates the flight configuration and ground support for the GRACE mission. Fluctuations in density of the Earth's surface result in small changes in the distance between the two satellites, which are measured with very high precision.
This diagram illustrates the flight configuration and ground support for the GRACE mission. Fluctuations in density of the Earth's surface result in small changes in the distance between the two satellites, which are measured with very high precision.

Substantive advances in the interpretation of satellite altimetry, synthetic aperture radar interferometry, and digital terrain models, covering large land and ice areas used in remote sensing applications and cartography, will result from the improved gravitational field measurements provided by GRACE. These techniques provide critical input to many scientific models used in oceanography, hydrology, geology, and related disciplines, and, for this reason, the Earth Science community eagerly anticipates the results from the GRACE mission.

To measure gravity from space, the two identical GRACE satellites fly in the same orbit -- one 220 km (137 miles) ahead of the other. As the pair circles the Earth, areas of slightly stronger gravity will affect the lead satellite first, pulling it away from the trailing satellite. The uniquely designed Superstar Accelerometer is used to distinguish gravity influences from those of air drag. The K-band ranging instrument is capable of measuring the distance between the satellites with a precision better than the width of a human hair. By monitoring this distance, GRACE will be able to detect fluctuations in the gravitational field and, therefore, differences in the density of the Earth's surface beneath the satellites. The data will be combined with GPS data to produce a map of the gravity field approximately once a month.

Additional information about the Gravity Recovery and Climate Experiment may be obtained at: http://www.csr.utexas.edu/grace.

Last Updated: 2 March 2011

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Last Updated: 2 Mar 2011