A laser leaves a station on the ground and strikes a ball in orbit.
Argus et al., Geophysical Journal International, November 2026 issue (online September 14). A team led by Donald Argus of NASA's Jet Propulsion Laboratory, with colleagues at the University of Nevada, the University of Montana and the Helmholtz Centre for Geosciences in Germany, reported a new estimate of how the place that Earth-orbiting satellites circle shifts over the year, relative to the solid Earth, as water and air move between the hemispheres. The team combined laser ranging to satellites with GPS and several low-Earth-orbit satellites. NASA and JPL announced the result on September 16, 2026.
Laser stations on the ground fire short pulses at satellites and time how long the light takes to come back. The satellites circle Earth's center of mass, a reference point for satellite navigation and for measurements of surface elevation. When it moves relative to the crust, the travel times shift by a tiny amount.
It does move. In April, rainwater in the Amazon basin reaches its yearly peak, and NASA reports that this swings Earth's center of mass about 2.2 millimeters toward South America, relative to the planet's geometric center. The same water presses on the crust beneath it, and the crust carries the laser stations.
A travel time cannot say which of the two has moved. If the center of mass shifts by a few millimeters, the time changes. If a station rides crust that rises or sinks under a season's snow or rain, the time changes too.
For years, two ways of weighing the same yearly exchange of water between the hemispheres gave different answers. Along the spin axis, the reference frames of 2008 and 2014 put the swing of the center of mass, relative to the crust, at 11 millimeters, peak to peak. Estimates built on the gravity satellites of the GRACE mission put it at 4.2.
Argus's team drew on GPS and low-orbit satellites as well as the laser satellites, and subtracted the bending that water, ice and air should cause in the crust, taking the loads from weather and ocean models and from GRACE itself. It now puts the yearly movement at "about half of what we believed it to be eight years ago," Argus said. The paper raises an objection: stations corrected with gravity data, then compared with the same gravity data. Its authors maintain that the satellite estimate stays independent. Four estimates now agree to within 2.5 millimeters of one another. Propagating the errors gives the team's estimate an uncertainty of 0.4 millimeter, which the paper calls unrealistically small, since successive positions are correlated; the scatter between single-site estimates gives about three millimeters.
The satellites that have traditionally anchored the reference frame are two metal spheres, each weighing about 400 kilograms. NASA launched the first in 1976; the second, built by the Italian Space Agency, followed in 1992. Both are expected to keep orbiting for millions of years. They are covered in reflectors and hold nothing that measures.