LARES-2 delivers most precise Earth frame-dragging measurement

Earth’s frame-dragging effect has been measured with an uncertainty of about 0.2% using a dense, electronics-free satellite tracked by lasers from the ground, according to Ars Technica’s report on the work. The result comes from LARES-2, a sphere covered in retroreflectors that lets researchers time laser pulses to pinpoint its position to roughly millimetre precision. The experiment targets a subtle prediction of Albert Einstein’s general theory of relativity: that a rotating body like Earth slightly twists spacetime around it.

The measurement is a test of a phenomenon known as the Lense–Thirring effect, which is easier to see near massive, rapidly spinning objects such as black holes but much harder around Earth. The obstacle is not a lack of theory but a surplus of ordinary physics: Earth is not a perfect sphere, and its equatorial bulge produces Newtonian perturbations in satellite orbits that are far larger than the relativistic signal. That means the experiment is as much about engineering away unwanted forces as it is about measuring gravity.

LARES-2 is designed to behave as close as possible to an ideal “test particle”, with its motion governed almost entirely by gravity. Ars Technica notes that it is a solid Inconel sphere with no thrusters, solar panels, or onboard electronics, and it has a very low area-to-mass ratio to reduce non-gravitational effects such as photon pressure. Researchers then use ground-based lasers to collect returns from the satellite’s corner-cube retroreflectors, building a long time series of precise orbital data. The dataset described in the report spans roughly three years, with on the order of hundreds of thousands of laser observations.

The analysis also leans on geometry. Ignazio Ciufolini and physicist John Archibald Wheeler previously proposed using two satellites in supplementary orbital inclinations so that key Newtonian effects cancel out, isolating the frame-dragging signature that slowly shifts the orientation of an orbital plane. With Earth’s gravity field and its imperfections doing most of the “noise-making,” the experiment becomes a contest between how well the satellite approximates a passive mass and how well the modelling subtracts everything that is not relativity.

LARES-2 was built by the Italian Space Agency and launched on a Vega-C rocket, but the lead team reported by Ars Technica is based at the Wuhan Institute of Physics and Mathematics in China. In a field where the signal is tiny and the instrumentation is expensive, the practical unit of progress is often not a new equation but another year of clean tracking data from an object engineered to be boring.

LARES-2 is a polished sphere in medium-Earth orbit, and the most important part of the experiment happens after launch: a ground station fires a laser, and the satellite answers with the same pulse, over and over.