About 61.3 milliarcseconds per year – that is how far the orbits of two metal spheres circling Earth drift, because our planet drags spacetime along as it turns. An international team led by the physicist Ignazio Ciufolini of Sapienza University in Rome has now pinned that figure down more precisely than anyone before, confirming one of the subtlest predictions of general relativity. The measurement appeared in the journal Nature.
The effect is frame-dragging, also known as the Lense-Thirring effect. Einstein's theory describes gravity as the curving of spacetime by mass. If that mass also rotates, it twists its surroundings slightly – rather like a spoon dragging thick honey with it. Around rotating black holes and pulsars, the effect was detected long ago. Around comparatively lightweight Earth, however, it is tiny and buried under interference: our planet is no perfect sphere, its gravitational field resembles a dented potato, and the Sun, Moon and neighbouring planets tug at it as well.
Two spheres, 200,000 laser shots
The answer lies in two unusual satellites: NASA's LAGEOS and LARES-2, launched by the Italian space agency in 2022. Both are passive spheres with no solar panels and no moving parts, their surfaces studded with reflectors. "LAGEOS is elegantly simple – just a ball covered in mirrored prisms," says Stephen Merkowitz of NASA's Goddard Space Flight Center. Ground stations fire laser pulses at the spheres; the echo reveals their position to the millimetre. Because the satellites are compact and heavy, non-gravitational forces barely get a grip on them.
Ciufolini's team analysed some 200,000 such laser measurements taken between 17 July 2022 and 1 June 2025. The trick: LAGEOS and LARES-2 follow near-identical orbits that are tilted against each other, and they circle Earth in opposite directions. Comparing the two orbits allowed the tidal effects of the Sun and Moon to be subtracted – what remained was frame-dragging. According to Phys.org, data from NASA's GRACE mission fed in as well.
The result matches Einstein's prediction, with a relative uncertainty of roughly 0.2 percent. That is about an order of magnitude better than previous determinations within the solar system.
Why it matters: any alternative theory of gravity must now squeeze into a far tighter corset. And the analysis has a side benefit – it also sharpens models of the lunisolar tides, and with them our knowledge of Earth itself.