 In this video, we'll discuss the gravity recovery and climate experiment, known as the GRACE mission. So far, we've been using the acceleration due to gravity, little g, is equal to 9.8 meters per second squared for our gravitational field strength. This has been more than sufficient for our purposes of calculation and prediction. But if you could measure gravity to a much higher degree of precision, you can learn a lot about the Earth and the climate. The GRACE mission involves two satellites up in space. These satellites are in a polar orbit around the Earth and are locked onto one another. The satellites operate by sending a microwave beam between each other and detecting their position from GPS signals. This can then be used to detect changes in the distance between the two satellites of less than 10 micrometers, which is less than 10 to the minus 6 meters. This sensitive measurement of distance allows us to map out the gravity on the surface of the Earth below. This works because if the lead satellite starts to experience a stronger gravitational tug due to additional mass below it, it will drift further away from the trailing satellite. This change in distance is detected, and by calculating the velocity and acceleration of the satellites relative to one another, the gravitational field can be inferred. Being able to measure changes in gravity allows us to measure all sorts of interesting stuff. For example, if there's lots of ice on Antarctica, the extra ice applies, extra gravitational pull. By measuring how the gravity changes over the course of months and years, we can track the ice levels of Antarctica, sea level changes, or global ocean circulation. GRACE even allows us to peer into the Earth itself. The magma within the Earth is slowly flowing, and the changes in the amount of magma changes the strength of the gravitational field enough for GRACE to measure. GRACE was launched as a joint mission of NASA and the German Aerospace Center in 2002. It was initially at an altitude of 500 kilometers above the surface, but at this altitude there's still some residual atmosphere. This atmosphere is slowing the satellites down, and they're falling towards the Earth at a rate of 30 kilometers per year. There's another GRACE mission in the works known as the GRACE follow-on mission, which is expected to launch in 2017 or 2018. GRACE follow-on will use lasers instead of microwaves, and is expected to be at least 20 times as precise as GRACE. On this screen you can see a map of the Earth's gravity as measured by GRACE. Pretty impressive, right?