How it WorksGRACE, GRACE-FO and the lunar GRAIL mission all use the same method to map gravitational fields. Each mission consists of two nearly identical satellites. One follows the other along the same orbit as both continually measure the distance between them by means of microwave ranging instruments.
As the lead satellite approaches a region of greater gravity (for example, a mountain or--in the case of Earth--a large mass of ice or pool of underground water), it is pulled a little bit farther ahead of the trailing satellite, slightly increasing the distance between them. Then, as the lead satellite flies past the high-gravity area, it gets pulled slightly back while the trailing satellite--which is now approaching the gravitational mass--is pulled slightly ahead, narrowing the gap between the two satellites. Scientists are able to interpret the changes in satellite separation distance to make maps of the gravity field. Though the two GRACE satellites are about 137 miles (220 km) apart, they are able to measure their separation distance to within one micron, about the diameter of a blood cell, enabling them to sense subtle differences in Earth's gravity field from location to location. Flying at an altitude of more than 300 miles (500 km), they are able to detect gravitational differences on the planet's surface equivalent to that of a 300-km disk of water only one centimeter thick.
The two GRACE-FO satellites will use the same kind of microwave ranging system as GRACE, and so can expect to achieve a similar level of precision. But they will also test an experimental instrument using lasers instead of microwaves, which promises to make the measurement of their separation distance at least 20 times more precise.
Mind the GapGRACE-FO is scheduled for launch in 2017. How long the original GRACE spacecraft pair will remain operational is unknown, but it is hoped that they will continue to function at least through 2015. "We are doing the best we can to extend the life of GRACE and get GRACE Follow-On up," said JPL Director Mike Watkins, the original Project Scientist. If GRACE does not last until its follow-on mission takes over, he said, "I think it's safe to say that it will certainly leave a gap in our ability to track large-scale changes in polar ice mass and water storage."
GRACE-FO is a partnership between NASA and the German Research Centre for Geosciences (GFZ).