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GRACE Follow-On

As the name suggests, GRACE Follow-On (GRACE-FO) is a successor to the original GRACE (Gravity Recovery And Climate Experiment) mission, which began orbiting Earth on March 17, 2002. GRACE-FO will carry on the extremely successful work of its predecessor while testing a new technology designed to dramatically improve the already remarkable precision of its measurement system.

Earth's gravity field is lumpy. Gravity is slightly stronger over places with more mass than over places with less mass.

Earth's gravity field is lumpy. Gravity is slightly stronger over places with more mass than over places with less mass.

The GRACE mission measures variations in gravity over Earth's surface, producing a new map of the gravity field every 30 days. Thus, GRACE shows how the planet's gravity differs not only from one location to another, but also from one period of time to another. "Time-variable gravity is the thing you want to measure," said JPL Director Mike Watkins, the original Project Scientist. That's because gravitational differences over time are due largely to movement of Earth's water, both in liquid and ice forms. "The changes that we want to observe in the Earth system, like sea-level rise, polar ice-cap mass loss in Greenland and Antarctica, or large-scale water storage," he said, "are actually quite different from year to year and we want to keep tracking that."

How it Works

GRACE, 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 Gap

GRACE-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).

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