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.

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 the Gravity Recovery and Climate Experiment - are being launched 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 will be 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.

The ESSP missions are an integral part of a dynamic and versatile program consisting of multiple Earth system science space flights. The ESSP program is characterized by relatively low - to moderate-cost, small-to medium-sized missions that are capable of being built, tested and launched in short-time intervals. Each mission is led by a Principal Investigator (PI), who is responsible for all elements of the mission, from ensuring the science accuracy to making sure the mission stays on budget and on time. ESSP missions are capable of supporting a variety of scientific objectives related to Earth science,including the atmosphere, oceans, land surface, polar ice regions and solid Earth. Investigations include development and operation of remote sensing instruments and conducting research using data returned from these missions. Subsequent satellite launches are planned over the next few years, all of them focusing on the atmospheric sciences.

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