General questions

• What does GRACE-FO measure?

  Strictly speaking, GRACE-FO measures the distance between the leading satellite and the trailing satellite as they circle the world. Those separation measurements are combined with knowledge of the satellites’ orbit determined using GPS (Global Positioning System) measurements. When matched up, scientists can use computers to visualize the gravitational pull the satellites are responding to during each orbit. Over time, the gravitational pull over all of Earth’s surface can be mapped because Earth rotates as the satellites trace and re-trace their orbits, while a different part of Earth is beneath them. Over months and then years, changes in Earth’s gravitational field can be measured and visualized.  The gravitational field is an indicator of the amount of material (mass), large and small, affecting GRACE-FO’s orbit.

• Earth’s gravitational field changes?

  Yes. There are very long-term changes as cosmic dust and meteorites accumulate on our planet but these changes are so small they cannot be measured. What can be measured are variations in the density and quantity of materials underneath GRACE-FO’s orbital path.

  Because rock is denser than water, it is easy to visualize that the gravitational pull beneath GRACE-FO would be greater over a continent than over the open ocean. GRACE and GRACE-FO can measure these changes, and even smaller levels of change. They can reveal the increased mass of mountain ranges (which have “roots” beneath the surface matching their presence above the surface). Of greater use to scientists is to look for changes in the gravitational field over periods of months to years. 

• What are some examples of changing gravitational fields under GRACE-FO’s orbital flight path?

  GRACE has revealed seasonal changes (between rainy season and dry season) in the Amazon basin. Long-term changes that have been found include the removal of groundwater in California, India and the Middle East, and the melting of the ice sheets on Greenland and Antarctica.

• How does GRACE-FO use gravity to measure mass?

  Every material object, from an atom to a galaxy, has mass. Isaac Newton showed that the force of gravity between two objects is related to the distance between their centers. With GRACE-FO, scientists use the changing separation of the leader and the follower to figure out the difference in mass between Earth and the leader and between Earth and the follower. (There is gravitational pull between the leader and follower, as well, but it is a minor.) 

  GRACE-FO measurements (and GRACE’s, too), are usually used to look for changes in gravitational pull, and therefore changes in mass, over time. Changes in the seasonal mass of water in the Amazon basin have been observed. The reduction in the mass of ground water in areas around the world has been measured. The reduction in ice mass in Greenland and Antarctica is under continuous watch.

• Why do we need GRACE-FO?

  Scientists hope to have GRACE-FO in orbit at the same time as GRACE to provide continuity in measurements and to help cross-calibrate GRACE-FO. GRACE-FO will continue the measurements of changes in Earth’s gravitational field found around the world. These measurements allow scientists to advise governments about the changes so plans can be made to accommodate or adapt to them.

• Will GRACE-FO just measure gravity?

  No. GRACE-FO is going to test a laser system that will be even more precise than the microwave system that GRACE-FO and GRACE carry to make their measurements. Higher-precision measurements allow smaller areas on Earth to be characterized, which will teach us more about our planet and its behavior. An eventual GRACE 2 mission is expected to use lasers instead of microwaves.

  GRACE has been used for studies that scientists did not anticipate when it was first built. Recently its very-well-characterized orbit has been applied to studies revealing details about Earth’s upper atmosphere, a large volume that is difficult to study: too high for planes and balloons, too low for satellites, needing more measurement time than a rocket passing through on its way up or down.