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 the force of gravity is weak compared with other basic forces in nature, such as magnetism and electricity, 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.

The two GRACE spacecraft undergo final preparations for launch at Plesetsk Cosmodrome in Russia.

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 far reaching benefits to society and the world's population. More specifically, it is expected that quite a number of disciplines that study aspects of the Earth's changing climate will gain substantial benefits from data retrieved by GRACE.

The ESSP Program
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. These 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.

next: A Scientific Geodesy

Remote Sensing
Introduction
A Scientific Geodesy
Launch, Components, and Systems
Management and Future
Spacecraft Diagrams