Gravity is the force that pulls two masses together. Since the earth has varied features such as mountains, valleys, and underground caverns, the mass is not evenly distributed around the globe. The "lumps" observed in the Earth's gravitational field result from an uneven distribution of mass inside the Earth. The GRACE mission will give us a global map of Earth's gravity and how it changes as the mass distribution shifts. The two satellites will provide scientists from all over the world with an efficient and cost-effective way to map the Earth's gravity field.

The primary goal of the GRACE mission is to map the Earth's gravity field more accurately than has ever been done before. You might ask, how will GRACE do this? Two identical spacecraft will fly about 200 kilometers apart. As the two GRACE satellites orbit the Earth they are pulled by areas of higher or lower gravity and will move in relation to each other. The satellites are located by GPS and the distance between them is measured by microwave signals. The two satellites do not just carry science instruments, they become the science instrument. When mass moves from place to place within the Earth's atmosphere, ocean, land or frozen surface (the "cryosphere"), the gravity field changes.

Here's an example of how it works. The twin GRACE satellites are traveling in space, both 500 kilometers above the earth. As the front satellite approaches an area of higher gravity, it will be pulled toward the area of higher gravity and speed up. This increases the distance between the two satellites. As the satellites straddle the area of higher gravity, the front satellite will slow down and the trailing satellite will speed up. As the trailing satellite passes the area of higher gravity, it will slow down and the lead satellite will not be affected. As the twin satellites move around the Earth, on-board instrumentation measures the distance between them, which is caused by the speeding up and slowing down of the satellites relative to each other. From the measured relative motion of the satellite, scientists are able to create a new map the Earth's gravity field every 30 days.

variety of complex changes to the earth, its air, its water, and its relationship to space, may cause a change in the Earth's gravity field.

Some of these may include:

• Changes due to surface and deep currents in the ocean, for example: the El Nino climate phenomenon.
• Runoff and ground water storage on land masses
• Exchanges between ice sheets, glaciers, and the oceans
• Variations of mass within the Earth, and
• Variations in the distribution of the atmosphere as seen in daily maps of barametric surface pressure.