Earth’s center of gravity can be located! We talked a few months ago about measuring the force of gravity surrounding the Moon, and that the way we do this measurement is by having twin satellites, and calculating the difference in gravitational pull on each satellite. As the satellites go over high density regions, the one of them will feel an increased pull before the other, and the distance between the two satellites will change. These tiny changes in the distance between the two satellites allow us to map out the density of the ground below, but it's fundamentally a measure of the strength of the gravitational pull of the ground below the satellites.
We can do the exact same thing for pairs of satellites around the Earth, and we have! The Gravity Recovery and Climate Experiment (GRACE) is a NASA mission to do precisely this. It was a pair of satellites, launched in 2002, which bounced microwaves back and forth between them, very precisely measuring the distance between them, to a sensitivity of about a micron (many times smaller than the width of a human hair.) By additionally communicating with GPS satellites, the GRACE satellites were able to precisely communicate both their absolute positions in orbit around the Earth (to a precision of about a centimeter), and their motions relative to each other. Any deviations in their relative distances should be due to something down below, on Earth.
ESA has also had one of these earth-measuring satellites, called GOCE (Gravity field and steady-state Ocean Circulation Explorer) which operated between 2009 and 2013. Instead of having two independent satellites, it had two sets of accelerometers at opposite ends of one, long, tubelike satellite, which each measured gravity at their end of the satellite.
Both experiments were able to generate maps of the Earth’s gravitational field strength from their locations in orbit. In practice, these are often reported back as a “geoid”, which is a way of deforming the Earth’s sphere so that any point on its surface would have an equal gravitational strength. Anything built up into a lump outwards indicates that there’s extra mass there, and anything sunken inwards indicates that there’s less mass. GOCE managed to map the gravitational strength of the Earth beneath it to a precision of 10^–5 m/s2. While we commonly quote the gravitational force as 9.81 m/s^2, this satellite was measuring it out to 0.00001.
You’ll notice that both of these experiments have another facet to their names - GOCE also says it’s monitoring the ocean circulations, and GRACE is also a climate experiment. That’s because these very precise gravitational measurements can also track the motion of water around our planet. Not just the locations of surface water, or the amount of water in the oceans versus at the poles, but underground water, in reservoirs. Water is a relatively dense material, and so its presence or absence in a certain location will alter the average density of the planet underneath either of these observatories.
GRACE has a follow-up mission, intended for launch this year - GRACE-FO. The FO stands for Follow-On, and is intended to increase the accuracy of the GRACE experiment dramatically, by using laser beams to check the distances between the satellites, instead of microwaves. GRACE-FO will also help us continually monitor our fragile world’s water supplies as the original GRACE satellites age. Not every satellite lasts 15 years, after all.
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