NASA-led team measures Earth's center of mass shift with new method

Earth's center of mass does not stay perfectly still, according to ScienceDaily. As water, ice and air move around the planet with the seasons, they redistribute enough weight to push the center of mass back and forth by fractions of an inch relative to Earth's geometric center. NASA scientists track this motion because the center of mass is a reference point for satellite navigation and for measurements of surface elevation, the report said.

Researchers led by NASA's Jet Propulsion Laboratory in Southern California developed a method designed to measure these seasonal shifts with high precision, according to ScienceDaily. Their technique and results are described in a study published in Geophysical Journal International. The research shows how melting snow, moving ocean water and dense winter air redistribute large amounts of mass across Earth's surface.

Determining the exact size of the movement has proven difficult. The two most recent international estimates, produced in 2017 and 2023, differ by 0.27 inches (7 millimeters), roughly the height of three nickels stacked together, according to the report. That disagreement is nearly as large as the movement scientists are attempting to measure.

Scientists have long used satellites to locate Earth's center of mass. Two dense metal satellites launched in 1976 and 1992, the Laser Geodynamics Satellites LAGEOS 1 and 2, were built specifically for that purpose, the report said. They resemble 900-pound (408-kilogram) disco balls covered with reflective prisms, and ground stations spread across more than 20 countries fire lasers at them and measure the returning signals.

One challenge with this method is that the laser tracking stations are not evenly distributed around the globe. The new approach combines the laser measurements with GPS tracking and orbital information from several satellites in low Earth orbit, giving researchers a wider range of objects to follow, according to ScienceDaily. It also accounts for how the changing weight of water and ice bends Earth's crust, which means the ground stations themselves move slightly as the surface beneath them shifts.

Argus developed the technique with scientists from JPL's satellite orbit determination team, the University of Nevada, University of Montana and the Helmholtz Centre for Geosciences in Germany. "We're now estimating the size of the movement of Earth's mass center back and forth each year to be about half of what we believed it to be eight years ago," Argus said.